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Calculating Optimal Highway Cruising RPM

Discussion in 'The Hokey Ass Message Board' started by D-Russ, Nov 25, 2008.

  1. I was reading the article "Hitting the Sweet Spot" in the latest issue of Rod & Custom. It's a helpful write up about optimizing highway cruising RPM with rear gear selection and overdrive. The article provides a formula to calculate RPM at any given MPH. It states that about 2200 RPM @ 75 MPH is a good target. Everything I've found on the HAMB says to shoot for 1800 to 2000 RPM. Here's the formula provided in R&C:

    MPH x Rear Gear Ratio x 336 / Tire Height x Overdrive = Cruise RPM

    To begin, here's a brief description of my drive train: I have a stock 283 short block, a mild Comp Cam that makes power from low to mid range (specs below), rebuilt 3890462 heads, three Rochesters w/progressive linkage, a V8 Camaro T5 (1st=2.95, 2nd=1.94, 3rd=1.34, 4th=1.00, 5th=.63 overdrive), 9 inch Ford rear with 3:50 gears and currently 820-15 rear tires (29.56" diameter). I may go to 890-15s in the future (31" diameter).

    Cam Part Number: 12-234-2
    Basic Operating RPM Range: 1,000-5,200
    Intake Duration at 050 inch Lift: 212
    Exhaust Duration at 050 inch Lift: 218
    Duration at 050 inch Lift: 212 int./218 exh.
    Advertised Duration: 256 int./268 exh.
    Valve Lift with Factory Rocker Arm Ratio: 0.447 int./0.454 exh. lift

    For reference, the car that my T5 came out of was an 87 Z28 with a 305 V8. I've done some digging around on the internet (I know, I know) and found the overdrive is .63, the rear gear was 3:08 and the stock rear tire diameter was 26.1 (235-60R-15). Chevy apparently agrees with the HAMB on cruising RPM:
    75 x 3.08 x 336 / 26.1 x .63 = 1873 RPM

    If I plug in the numbers from my current combo, here's what I come up with:
    75 x 3.50 x 336 / 29.56 x .63 = 1880 RPM
    That's right in line with the V8 Camaro the tranny came out of.

    If I change my rear tires to the before mentioned 890s, it works out like this:
    75 x 3.50 x 336 / 31 x .63 = 1792 RPM

    I've read that cruising at an RPM that's too low, and then accelerating rapidly can potentially damage the cam – which I would like to avoid and that acceleration is sluggish if the engine is not operating in it's optimal power band.

    All this leads up to my questions:
    1. Should I shoot for 1800 RPM @ 75 MPH or 2200 RPM @ 75 MPH? (The latter would require a gear change to 4:11 or 4:33 gears depending on tire height.)

    2. Will the first through fourth transmission gear ratios (above) provide good acceleration from a stop with the above mentioned tire heights and a 3:50 rear gear? A 4:11 rear gear? A 4:33?

    Thanks in advance for all the help.
    EDIT: I changed the title of this thread to hopefully get more input.

    How much RPM?

    60 x 3.50 x 336 / 29.56 x .63 = 1503 RPM

    60 x 4.56 x 336 / 29.56 x .63 = 1959 RPM

    60 x 4.71 x 336 / 29.56 x .63 = 2023 RPM

    60 x 4.86 x 336 / 29.56 x .63 = 2088 RPM


    EDIT: New specs for consideration.

    75 x 3.70 x 336 / 30.4 x .63 = 1932
    65 x 3.70 x 336 / 30.4 x .63 = 1675

    75 x 3.90 x 336 / 30.4 x .63 = 2037
    65 x 3.90 x 336 / 30.4 x .63 = 1765

    75 x 4.11 x 336 / 30.4 x .63 = 2146
    65 x 4.11 x 336 / 30.4 x .63 = 1860

    75 x 4.33 x 336 / 30.4 x .63 = 2261
    65 x 4.33 x 336 / 30.4 x .63 = 1960

    75 x 4.56 x 336 / 30.4 x .63 = 2381
    65 x 4.56 x 336 / 30.4 x .63 = 2064
     
    Last edited: Dec 18, 2013
  2. coupster
    Joined: May 9, 2006
    Posts: 856

    coupster
    Member
    from Oscoda Mi

    I would sure like to hear the theory on how you could damage a cam in your above discribed condition. Pistons I could believe. I personally shoot for 2000RPM at 60mph as a good all around cruise gearing. You should be Ok with the tranny gearing you list for a street driven car.
     
  3. I run a 4:11 with a 31 " tire
    Just don't put it in 5th til yer over 85 mph
     
  4. RacerRick
    Joined: May 16, 2005
    Posts: 2,742

    RacerRick
    Member

    My car has an OD and cruises at 70mph at about 2100rpm. I was getting about 22-23mpg with a rich jetted 3310 Holley on a 305. I am going to try a 1850 Holley to see if I can get a few more MPG out of it with better throttle response.
     
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  5. BTW
    This is behind a 377 SBC hi winder
    and gets 23 - 26 mpg on long trips, runnin 75 to 90 on the interstate.
     
  6. belair
    Joined: Jul 10, 2006
    Posts: 4,886

    belair
    Member
    1. H.A.M.B. Chapel

    A 283 might like a little more rpm to accelerate from 60 mph. And it's pistons-spark knock-predeonation hurts pistons, rods and mains.
     
  7. Aman
    Joined: Dec 28, 2005
    Posts: 2,525

    Aman
    Member
    from Texas

    Good thread here. There is a place on the internet (yeah, ok) that has a formula you fill in and get the data you need. I can't remember shit (CRS) but I'll bet in a few minutes another member will post it.
     
  8. hotroddon
    Joined: Sep 22, 2007
    Posts: 12,130

    hotroddon
    Member

    Don't know if this is the one you were thinking of, but I use it.
    http://www.richmondgear.com/101032.html
     
  9. bobwop
    Joined: Jan 13, 2008
    Posts: 5,501

    bobwop
    ALLIANCE MEMBER

    I wouldn't change a thing. 283's like rpm. cruising 75 at 1880 rpm seems sweet enough to me. remember, you have FIVE gears. 75 mph in fourth gear is still only 3000 rpm. 60 mph in fourth gear is 2400 rpm. I wouldn't change a thing. I think you have good range and plenty of gear selection to work with. Give it a try with the parts and tires you have. Changing it later is easy. But I like where you are at.
     
  10. HotrodHill
    Joined: Dec 22, 2007
    Posts: 31

    HotrodHill
    Member
    from northeast

    The new Rod and Custom rag has an article on exactly what you are looking for, it has the calculation right in there. You should be looking for 2200-2400 cruising RPM at around 70-75 MPH, at least a 3:73 gear, but check out the article, Hotrodhill.
     
  11. BigChief
    Joined: Jan 14, 2003
    Posts: 1,886

    BigChief
    Member

    The 283 is gonna like the extra gearing with the taller tires....especially since your using the 462 heads.....small cam or not. I'd try the 4.11's and see where that gets you. It'll be a happy medium for the strip or street. Run the tall tires and OD for the highway trips and use the short tires and keep it in fourth for the digs. I think You'll find that you'll have a good compromise.

    As for your last question, any numerical increase in gear ratio is going to give your motor more mechanical advantage thus it'll rev noticable quicker when jumping from 3.50's to the 4.11s....and if can shift fast enough to keep up with the motor it should only help acceleration providing you can get it hooked up.

    -Bigchief.
     
  12. phat rat
    Joined: Mar 18, 2001
    Posts: 3,516

    phat rat
    Member

    A few years ago I went to a clinic put on by Inland Empire Driveline they stated that they were seeing a lot of driveline problems because of guys driving at too low an rpm and lugging the motor. This lugging was causing trans and u-joint problems. My 41 run a 454/700R4/3.42 with 255/60 X 15 tire. It runs 2100rpm at 70mph and I have pulled as high as 19.6 mpg running at 62-65 mph. I think this kind of mileage is pretty darn good for a 454.
     
  13. On the subject, what RPM should i be aiming for at 62mph (100km/h round here)
    Model A with 29" rear tires.
    351C, mild cam, 700 holley and FMX trans (no overdrive, not sure, any ideas).
     
  14. BigChief
    Joined: Jan 14, 2003
    Posts: 1,886

    BigChief
    Member


    There is so much more to all of this that what folks here are pulling out of thier ass. To just drop figures out is just asking for more confusion. Get figures based on prior experience, backed up by data then you'll have something to chew on.

    Basically its all in the combination and the compromise - just like with anything else motor related. Best cruising RPM relies heavily on cam timing, port efficiency, exhaust scavenging, pumping losses, BSFC/SFC, engine torque characteristics, aerodynamics...the list goes on and on and on.

    That being said, the '70 351C 4V motor (10:1, big ports, small chambers, single plane intake, 650 double pumper) I used to run was OK with its 3.25 gears and a cruising RPM at 60MPH of about 2400RPM....I got mid teens with it for mileage. It could have been much quicker with more gearing. My 428SCJ on the other hand got 14mph with 3.91's and 11.5-12 mpg with 3.50 rear gears....but it also had a largish cam in it (.588/.588-236/244@.050"lift) and the motor was better tuned to running higher RPMs. Although it would run 2600-2700 at 60MPH it was still happier spinning a few hundred RPM up the scale with the 3.91's.

    With your combination shoot for 2300-2500 if its a 2V motor with a mild cam. No need to lug the crap out of it.

    -Bigchief.
     
  15. blown green t
    Joined: Nov 18, 2008
    Posts: 144

    blown green t
    Member

    I agree with big chief, rev it up a little. The cam is a big factor in your rpm. Most cams run good in their advertised range. If your cam card says 2500-5500 rmp then you want at least 2500 rpm on the road. I had a 289 with a big cam in a '66 Mustang. I played around with it on a road trip from Georgia to Arizona. At 65 mph it turned about 2000 rpm and got 15 mpg. When I picked up the pace to 75-80 mph it was turning 2500-2600 rpm and got 17 mpg. (plus I got home quicker!)
    One way to figure out what rpm your engine pulls best if you have a manual trans, use 2nd gear at about 1500 rpm push down on the throttle and watch the tach. You can usually feel when the engine starts pulling strong. Whenit pulls strong it is also more efficient.
     
  16. 29nash
    Joined: Nov 6, 2008
    Posts: 4,546

    29nash
    BANNED
    from colorado

    What he said;
    Saying it a different way, you don't have to go into 5th gear just because it's there. In traffic, tied up at 55-60, optimum in my mind would be 4th gear. It might just be my way but I never go into high gear unless I'm out in traffic over about 55 mph.

    As far as fuel economy goes, MP is going to affect gas mileage as much as rpm. Lugging at 1800-2000 is going to suck just as much gas as loafing at 2200-2400. You probably wouldn't see much, if any, improvement by gearing up.

    OT, but it makes my point, with one car, at least. I had a 92 Laser that got 33.5 mpg in 5th at 60-75 all across Colorado thru Texas on the I-Highways. Made that trip from Colorado to San Antone, me and my wife and back seat/hatch full of gear, several times to visit our kids. One time, I was curious enough, I left it in 4th the entire trip, got 32.1 mpg. It was hard to keep my eye off the tach, but I did it.
     
  17. super plus
    Joined: Dec 14, 2006
    Posts: 566

    super plus
    BANNED

    my stock Z06 Corvette runs just over 1800 RPM @ 80 MPH
     
  18. Parts48
    Joined: Mar 28, 2008
    Posts: 1,222

    Parts48
    Member
    from Dry Heat
    1. Hot Rod Veterans

    The Heretic Jag runs a 3.54 and Super T-10. Real busy about 3000 at 60..
    Gets about 17 MPG (weighs 2400) mixed driving..
    If it wasn't so much fun as a whips and chain car..I might do a 5spd..
     
  19. To me, the "sweet spot" is achieving the best possible MPG at my desired "cruise" speed.

    To do this, hook a vacuum guage to manifold vacuum and while cruising try to aim for the highest reading possible.

    When you obtain the highest reading, note the RPM of the engine. It will probably be in the 1800-2400 range but could be higher.

    Now enter your desired speed/ RPM/ trans ratio/ tire size into the calculator and you'll find what rear gear you need.
     
  20. As LowKat said, get a vacuum gauge installed in the car and drive by that.
    It's a good on the road trouble shooting instrument as well.
    As is a fuel pressure gauge - use a pressure isolater for safety.

    My 88 Mustang GT got better mileage running the desert highways in 4th than it did 5th.
    That due to a lower vacuum level in 4th.
    A one mpg improvement over the same highway.


    Keep in mind that if you're running an automatic you need to think about the converter stall speed.
    A high stall speed won't lock up - except at light throttle settings - with high gears.

    A light car with big engine running the streets doesn't need much stall.
    Some guys install a high stall converter to help with the creep factor in traffic.
    My 2400 converter creeps about the same as a stocker even with the low rpm attained at idle with both the big cam & dual quads and smaller cam with single quad.
    Both setups idle at 600 rpm.

    A lot of cars are built with an eye toward running them at the drag strip.
    Truth is, few of them ever see a strip.

    A high stall converter hits the tires pretty hard at the launch and it works better for a street runner to do the roll the throttle on about half-way then roll into it keeping tire spin to a minimum.

    So, below is the last bit of the recent Differential Swap from Hell I posted.
    The proof of the pudding part anyway.

    The whole damn fooforaw fwiw.
    http://www.jalopyjournal.com/forum/showthread.php?t=311829

    I'll have some more info on vacuum levels and mpg after a planned 200 mile round trip run.

    What's below is the latest info I have.

    Swapped from a 3.70 to a 3.00 and the tires are 30" tall.


    ~~~~~~~~~~~~~~~~~~~~~~~~~~

    "Car is a little quieter, but the wind noise (roadster mit top) seems louder.

    Runs right at 2400 rpm at 70 - with a 75 per speed limit on the local freeway.

    I have a 2400 rpm stall converter in the car and thought it would give a few probs heat-wise, but it's running like it did before.
    Light throttle settings at cruise seem to lock up ok at lower rpms so not a prob there.

    I made a gas mileage run down the hill - 12 miles down and then back up the steep grade - to Laughlin just for the hell of it.

    90 mile round trip including about 20 miles in town at 25-35 mph.

    Got 15 mpg which bodes well for level ground running.
    Probably give that a shot next week after the rain storms are gone.

    That trip usually returns 13-14 mpg.

    The level ground trip returned 16, almost 17 mpg running 70-80 per with the 3.70's last December and as noted I'm hoping to kick the mileage up about 2 mpg.

    The mpg gain is nice, but my main interest was in getting the engine down to reasonable rpms in highway driving.
    3300 rpm @ 80 is a little high imo.

    A lot of guys have suggested I simply go to an overdrive and truth to tell I wouldn't mind one, but the car still has lots of performance (462" & 2400#) with the 3.00 diff and 30" tall tires.
    Since I already have a built T-400 I didn't want to change.

    I'll be thinking a little harder about an OD in the 31.
    It has the same power train the 32 does with an additional 40 HP.

    Gearing on it is 3.25 with 28" tall tires.
    Which figures out to be about 5% higher than the 3.70 30" tall tires the 32 had.
    The 31 should come in at 2200# as compared to the 32's 2400#.

    The 3.70's and 30" tires worked out great in Sunny California.
    Most times speeds were 65 mph and down.
    Out here, speeds on the 75 per freeways seem to hover 75 - 80 mph.

    Gotta keep up with traffic....;) "<!-- / message --><!-- sig -->
     
    Last edited: Nov 26, 2008
  21. Weasel
    Joined: Dec 30, 2007
    Posts: 6,717

    Weasel
    Member

    All the math is done for you www.currieenterprises.com - click on the center logo and then click on "calculators" on the right hand column. Any way you want to calculate, you can find it there.
     
  22. Jalopy Jim
    Joined: Aug 3, 2005
    Posts: 1,871

    Jalopy Jim
    Member

    During the switch to the 55 MPH national speed limit BMW did an SAE paper on that subject . The summary was that an engine is most efficient at about its peak torque. So running at highway speeds at about that area would get you the best performance/ MPG. It also pointed out that the national fleet would use more gas at 55MPG that at 65 because of this factor.
    It also carried no weight with the politicians we were stuck at 55.

    jim h
     
  23. 1919 c cab
    Joined: Nov 20, 2008
    Posts: 3

    1919 c cab
    Member
    from n.j.

    You guys kill me, (1) before you can say what gear to run ya gotta know what's in the motor for parts. (2) Who cares about M.P.G. ? There HOT RODS for crying out loud!
     
  24. phat rat
    Joined: Mar 18, 2001
    Posts: 3,516

    phat rat
    Member

    Yes they are hot rods but if you drive long distance decent gas mileage is nice. When I drove the cpe to Fl. in early Oct and gas was as high as $4.75 a gal. I liked every bit of mileage I was getting. In Oct of 08 I put 8,000 mi on it in one month so mileage is important to me. I think that most who build their cars to drive long distance also want mileage along with the power. But if a 50-100 mi run with the hot rod is a long distance drive then of course mileage doesn't matter
     

  25. My diff swap was done mainly to get the engine rpms down to a reasonable level at highway speeds, but I gotta agree with Phat Rat on the mpg bit.
    A little improvement helps.

    Granted, mpg doesn't mean much to the guy who goes to the local Saturday night rod run and just hangs out.

    What kind of mileage does your hot rod get?

    Perhaps a better question is, how far do you drive it?
     
  26. Moonglow2
    Joined: Feb 4, 2007
    Posts: 646

    Moonglow2
    Member

    As stated above the 283 likes more rpm because of its short stroke (3 inches as I recall) and low torque output at lower rpm. I am convinced Chevy went to the 305 small bore but retaining the 3.48 inch stroke of the 350 in order to handle the torque requirements of 1700 rpm highway cruising.
     
  27. Rich Rogers
    Joined: Apr 8, 2006
    Posts: 2,019

    Rich Rogers
    Member

    using the calculator the 55 came up with 4:10 gear so at 55-60 mpg the rpms are right on target from a performance standpoint with 2800-3000 r's. If I try to drop the mph to slightly lower to say 52-53mph and drop the rpms to 2600 at cruise the mpg changes from 14 to 17. Not bad for a sbc tunnelram running 2 edelbrocks full time but to say the least not much fun to drive on a trip. I don't have a chance to drive the car alot so mileage doesn't matter that much. The puzzeling? fact is that this rear was from a 81 Camaro Z-28 with probably a 25 inch tire and a 4 speed. I put my m-22 behind the small block figuring not much would change in rpms. How the hell with a 29 inch tire and this rear combo end up running more rpms than it did in the Camaro unless something internal was changed in the trans. to give 4th gear a higher numerical ratio than 1 to 1?
     
  28. My original post said absolutely nothing about gas mileage. I'm interested in finding a combination that will provide good performance, both off the line and at cruising speeds (for passing of course).

    Gas mileage is also important to me, so if putting together a combo that keeps the engine in it's best performance range also results in better gas mileage, that's icing on the cake. And this car will be driven long distances.

    Oh, and I did detail the specific parts I'm currently running and I got some very good feedback based on those specs. Thanks to all who have shared their knowledge.

    C9, I'm going to try the vacuum gauge method. Can I get a good read with the car just sitting in my driveway, or do I have to be driving down the road?
     
    Last edited: Nov 27, 2008

  29. Both . . . but - you knew this part was coming - you're looking at different things in each case.

    In the driveway, on startup @ low ambient temps if the engine wants to die you'll see it on the vacuum gauge before you hear it and also before you catch the change in rpm.

    My 32 fires off with elec choke engaged on cool and cold mornings (Edelbrock 750 carb) and most times the vac gauge will start to back down a bit, but comes right back up with just a tiny bit of heat in the combustion chambers from the few seconds of running.
    Real cold mornings it may back down and die unless you tap the throttle lightly which dumps a little accel pump fuel in and then let the throttle back off to the high idle cam which is controlled by the elec choke.

    The 32 idles at 18.0 - 18.5 inches vacuum at 350' altitude which also indicates what a mild cam the engine has.
    You'll lose about 1" vacuum per 1000' altitude and here at 3300' the engine idles about 16".

    Vacuum figures at 1200 or so rpm & up will be the same at both altitudes.

    (The present cam has 260 and 266* advertised duration I&E.)
    With the bigger cam installed it idles 11.0 - 12.0" at idle @ 350' altitude.
    (284 and 292*.) - (Both cams with a 112* lobe center.)

    You're looking over the health of the engine - valves, rings, good head gasket seal, tune-up etc.



    When you're running the highway, a lower gear will raise the vacuum level . . . assuming the same level road etc. and with no changes other than the gear swap - 5th to 4th for example - and enough throttle to maintain the same speed in 4th that you did in 5th.
    The lower gears require less throttle therefore the higher vacuum level.
    My 88 Mustang GT with 5.0 liter and 5-speed was running 12-13" vacuum at 70 mph or so in 5th and vacuum went to 15-16" when shifted to 4th on the long and subtle desert uphills.


    Since my 32 is a fairly light car, with the 3.70 diff it ran about 17" - 17.5" at 65 mph.
    Note that it has the aerodynamic characteristics of a brick.

    The new 3.00 diff looks like it's gonna run 16.0" - 16.5" and maybe to 17" at the same speed.

    I need a little more time with this setup, as noted in the Differential Swap from Hell post a few days back, the first try with the new diff - other than chasing parts in town - was running down the hill to Laughlin.
    Several hills involved and the worst is the 12 miles long 5-6% grade coming up from the 500' altitude at the River to the 3300' altitude in Kingman where I live.

    Raining now, but after the weekend I'm looking forward to an about 140 - 200 mile round trip on fairly level ground to see how it does mileage-wise.

    One of my big concerns was in-town driving performance-wise and that hasn't been harmed at all.

    Pretty good on the highway as well.
    I don't foresee any problems passing and using high gear all the way - in fact, I don't have a kickdown switch installed, that due to if Sweetie drives the car I don't want it go out from under her if she tromps on the throttle.
    She already did that one in front of the gang at the muffler shop.:eek:
    She handled it ok, scared me a touch, but the gang loved it.:cool:

    A lot of the vacuum gauge stuff is learning about the gauge, what you can monitor and how you use it.
    The other part is learning how it reads in your car with your combo so you can easily tell when somethings off a touch.

    My dad ran vacuum gauges in his car and I've always had one in my cars.

    Pic circa December 1956 across the street from Ventura High School.
    [​IMG]
    Just to give you an idea of how sold I am on vacuum gauges.
     
    Last edited: Nov 27, 2008

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