Technical Your RPMs at 70mph

3300 lb truck, 250 ci inline six, 3.08 gear, 28" tall tire, 70 mph @ 2450 ish rpm. Will do this all day long. High winds kill the fuel mileage due to the tall cab and flat windshield.

 

Ah, the memory's..Back in the 70's I drove all over with 5.14 gears in my shoebox..Speed limit was 55 mph and that was not quite 3400 rpm..Even at 45 mph it was 2700 rpm..Drove to the drag strip, raced, drove home..An evolution since then..5spd with 4.11's [no od] and a 2950 @ 60 mph which was not bad but eventually everyone is going 70 mph because the speed limit raised to 65 mph so I'm back at 3450! Kept the 4.11 and went to 6spd od which made the 4.11 into 3.12's which is 2650 or so at 70..I figured I could live with that but now 80 mph is more common than not and we back to 3000 rpm.!.So point being that its been around 3000 rpm for forty years [50k miles at least] and the engine is still together so I would not fear rpm! If you want less noise then drop the gear but 2300 +/- would be about min at 70 mph...You may get better gas mileage but I wouldn't count on it..

 

You guys talking about RPM and gearing got me wondering.

Have a $8 small digital tach/hour meter that sez it works with "all gasoline engines" just wrap the cable around a spark plug wire. Works great for setting the governor on single jug lawnmowers. Will it work on V8?? V6? I don't see how it could work with "all" engines but I haven't had my fill of coffee yet.

1. (SPARK PLUG FIRING REVOLUTION)
   
   Press and hold the S1 button for 4 seconds. The LCD will display 01(spark plug fires twice per revolution), 02 (once per revolution), 03 (every other revolution), 04 (hours only, will not display RPM).
   

 

Been thinking about this lately-- my 51 I don't think would currently hit 70.. I believe it has 5.38s, might be 5.13s. I ran about 8 miles doing 62-65 and it was screaming. I now have a set of 4.11s that'll help a bunch.

As an aside-- anyone know a tach that'll work for 6 cylinder 6 volt positive ground? Can I use a 6v tach hooked up backwards for the positive ground?

I can, and have checked what my rpm are using a calculator but I'd like to know all the time.


Sent from my iPhone using The H.A.M.B. mobile app

 

it has been quite a few years since I had a working tach, but mine revs pretty high. seems like 3,100 or 3,200 at 70ish. 410 gears and 750 tires.

 

This and beaners posts are probably the most relavent to your real quandary. A sbc and a flathead 8 stock or otherwise are probably going to be happy at different Rpms.

I think stock three speed with a flathead 8 is closer to the question your really asking.

And that's why I think jeff's post gives you the best insight. His drivetrain is probably as close to yours as your going to find at a glance and you know he's driven it to Austin and Detroit from Omaha like a hoodlum so you know it's not "well if your real nice to it" miles of driving.

Add your slightly more highway friendly gearing to the mix and it sounds like you aught to be ok. Probably noisy but ok

 

You hit the nail, square on the head with that one!

 

Back in the day everyone wanted a 4:11 or better yet a 4:56, engines screamed at 65 MPH. The big difference was you didn't go to a car show 4-5 hours away because there wasn't any to go to. We stayed in our neighborhoods and went to the local strips. I sure do like my SBC running 1900 RPM's at 75 MPH with the Tremec 5 speed in fifth gear.

Gary

 

LOL in '71 we drove a '67 Chevelle 327 4 speed car with 4.11:1 from Portland Oregon to Eldon Missouri. No 55 speed limit yet. it didn't mind the revs.

The deal is if your car is stock and it was built to be drive when the speed limit was X and you are driving it X you shouldn't be hurting it one bit. Doesn't matter what gear you got or OD or not it was built to be driven X.

In about the summer of '69 a friend and I drove from Portland Oregon to San Francisco, then over to Reno then we wound our way up to Idaho and across into Washington and back to Portland. '53 Merc Monterrey 3 speed OD original motor with aluminum heads and a 2x2. Granted we babied it because we were young. it made the trip just fine.

 

Funny timing on this question. On Monday I was substitute-teaching at the local high school. I gave the classes a bonus question: My hotrod has an overdrive automatic with a .75 ratio to the differential, which has 5.14 gears and 32--inch-diameter tires. What will be my engine rpm at 75 mph? The four students in Calculus and one student in Algebra II got the answer. Then I got home and measured the tire's actual diameter: 31.5, so I guess I was close enough. Also, I've forgotten whether a Ford AOD is .70 or .75. Anybody? Anybody? Buehler? Buehler?

 

Nothing like them tremecs tho

Sent from my Z981 using The H.A.M.B. mobile app

 

Thanks for all the feedback guys. There is some good info in all the comments.

The crossroads I'm at is with my '48 Ford. As much as a love flatheads, I'm not going to put one back in. I'm going to use a 289. My deliberation is over the trans and rear end. The 2 options I'm considering are
- Use the original 3spd and banjo rear with 3.25 gears. I have the bell adapter I need and a trans ready to go. Calculated RPMs would be about 2650 at 70mph.
Or
- Use a 3spd w/ OD (Tremec/Ford T170 top shift and a 9" rear. I have all the parts needed. Calculated RPMs would be around 2100 at 70mph with the ring/pinion I have.

Based off of some experiences above, I'd be ok using option 1, but option 2 may be more comfortable.

 

235 six with 3 speed OD in a 55 Chevy with 205/75-15s and 4.11s. I have no clue what it revs at 70 mph,since I don´t have a tach in it,but this has been my daily for 22k miles and it never missed a beat-

 

Well on that note I'd leave the stock three speed and groove on down the road.

 

My 37 usually makes an out of state trip every year. It has an 8ba with the original drivetrain. The rear end has the original Ford highway gears.
I run 65-70 on the interstate as long as it takes. This engine probably has 40,000 miles on it now.
No problems, no complaints.
It would always be nice to have an extra gear on these old fords.

 

When they were new, people rarely exceeded 60.

 

32 all steel sedan, (flatty )39 motor & trains with stock rear , 28 tall tall tire , around 2,800 RPM's
Sound like it's going to grenade !! but hasn't yet

 

This all depend on how you want it to run. And all you can afford. Cheaper way is find a rear end and if you run higher numbers the higher it will Rev and lower numbers the lower it will Rev. but will either be torquey or fast depend on you. Tire size as well. Taller tires means less revolutions but you can keep what you have and toss a overdrive in and be good. There is a thread on this some were on how to calculate it all.

Sent from my Z981 using The H.A.M.B. mobile app

 

Piston speed has a lot to do with what rpm an engine is happy with. A 239 flathead @ 2280 and a 289 @ 3000 have about the same piston speed.

 

This is a good reference site for RPM and other calculations.
https://www.ringpinion.com/calculators/Calc_RPM.aspx

 

3:54 gears on 7.50x16"s will put me at 2800 give or take.


Sent from my iPhone using The H.A.M.B. mobile app

 

'40 coupe, 283, stock trans and 3:78 banjo rear. 28" rear tire. About 3100 @ 70. I prefer to run about 65 on long road trips, but have run 75 for hours on end without any problems so far. (about 75000 miles)

 

You know my car. With my gps speedo at 75 mph I'm running 3,000 rpm. I'll hopefully be driving it to the Hillclimb in September... ill probably run it 60-65mph most of the time.

Sent from my SM-N900P using The H.A.M.B. mobile app

 

What are you going to run in your '46?

 

T&A,
To get back to 2500 RPM @ 70, you would need to switch to a 3.28 R&P. (Closest set is 3.25) or you could go to a Quickchange.
My old oval track racer buddy and mentor told me that as a driver he couldn't tell much difference in gear changes of less than .25. Which is exactly what that switch would be.

 

Thanks. I'm okay with the set up I have in my Model A. No plans to change ring&pinion in that car. My conundrum is how to build my current 48 ford project that I plan to use as a long distance cruiser. A few posts earlier I mentioned the options I'm considering.

 

That is a great question! Honestly I don't know. I was trying to figure it out today... where I'm at right now is probably not a flathead... probably the 289 I have. Transmission? It's between 3. Stock 3 speed wit the adapter I got from you, the mustang toploader 3 speed I have linked to column shift or the 5 speed. I REALLY don't want a floor shift in it but I want to drive it comfortably. My tudor is 302 c4 with 3.00 gears and has plenty of low end power to pull around town no issue. So I don't think overdrive is a must with a high enough rear gear.

Sent from my SM-N900P using The H.A.M.B. mobile app

 

On my 32 tudor, Flathead powered, I have 4.11:1 axle. 29 1/2 " inch tires, 4 speed toploader with a Gear Vendors OD behind. In OD, final ratio is 3,21. 70 mph is about 2550 rpm. Engine is very happy at that speed

 

Yes, I understood that you weren't necessarily trying to change the Model A. I was just using your 70 mph example to show the ratio change needed to obtain the RPM drop back to 2500.
As to your new build, unless you have some benchmark using the engine/transmission/rear end combination, all the opinions are just educated guesses. That's not all bad, but until you put it together, you will not be able to dial it in.
Therein lies the beauty of the Quickchange.
From your previous post, I would start with option 1. JMHO

 

My cam likes RPM, 3000 to 3500 in 4th makes it happy. The only OG parts in my engine is the crank and block.
I need to get my door seals in soon, and my rear package tray. This would quiet down the ride a lot. Also need to do the trunk seal, the lid bangs on rough roads.