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HELP stall speeds,tranny ratios, diff ratios and finial drive? ratios hummmmm??

Discussion in 'The Hokey Ass Message Board' started by goldhunter_2, Oct 25, 2011.

  1. goldhunter_2
    Joined: Jan 10, 2010
    Posts: 83

    goldhunter_2
    Member

    I can comprehend the engine makes more power at higher rpm, and where ever your converter locks in it transfer power to tranny, and that the tranny has certain gear ratios, and that the differential also has gear ratios ..... what confusing me is combine all these to get a finial drive ratio:eek:

    I thought taller rear end gears and lower OD gears would produce higher speeds and better gas mileage but I just read a article at hemming that says lower final drive ratios is actual bad for gas mileage:confused:

    link to article;
    http://www.hemmings.com/mus/stories/2006/01/01/hmn_feature9.html

    Quote form article;
    Shift ratios for this transmission: 1st 2.74:1; 2nd 1.57:1; 3rd 1.00:1; 4th 0.67:1.
    As a rule of thumb, the taller the rear-end gearing you have in the car, the more gas savings you will realize. Generally, installing a TH200-4R in a passenger car with a gear ratio of 2.73:1 or less will not give you much in the way of fuel economy. This is because when the 0.067:1 overdrive gear is applied to the 2.73:1 differential ratio, this will reduce your final drive ratio to 1.82:1. Final drive numbers that low can actually work out to be worse for your gas mileage than sticking with your stock transmission. So rear-end gearing is something you should check into before considering this conversion.



    my combo on this build I thought was pretty simple but nothing is ever simple is it :D I am using a ls1 corvette motor supposed to be rated 345hp (115hp more then original engine and a few pounds lighter to boot) , a 2004r transmission with a overdrive gear ratio of 0.67 and rear end differential ratio is 3.07 , a tire size is 255/60/15 (27.04's) when I go online to a finial drive calculator it says my final drive is 2.0569 my goal was something I can AFFORD to drive on the highway and get good gas mileage but after reading that article I need some one to explain it to me in simpler dumb redneck English so I can understand it cause I thought the lower final drive would get me good gas mileage ?

    .
     
  2. Black Panther
    Joined: Jan 6, 2010
    Posts: 1,857

    Black Panther
    Member
    from SoCal

    Goldhunter,

    First things first...you are mixing "low" and "tall" and getting them confused. You are also confusing high speed and fuel economy...the article mentions low gears and a "final drive ratio that low"...the last usage of "low" was numerically...as in their example the number "1.82" is low numerically compare to "5.00" for example. I think mixing the term "low" like they did is piss poor writing if you ask me. Low gears in a rearend means 3.73, 4.11. 4.56 like that...tall gears are 2.41, 2.56, 2.73 and even your 3.07 can be considered "tall". What the article is saying is that your rearend is too "tall" ie...your 3.07 and an overdrive trans..you end up with a "final drive ratio" in the order of the 2.0569 you mentioned. You dont need to go to any fancy final drive calculator..if you know your overdrive's 4th gear is .67 then you just multiply that by your rearend ratio...so 3.07 X .67 = 2.0569.

    So is that too "tall"? A 1977 Trans Am with a 403 Oldsmobile motor came regularly with a 2.41 geared rearend and a turbo 350 transmission with 3rd gear being 1:1..so if you multiply 2.41 X 1 = 2.41. You are at 2.0569 so its even "taller" than that. What is the disadvantage of having too tall a gear? Your engine lugs at legal highway cruising speeds...so at 65 mph lets say you might only be doing 1600 rpm or so...the lugging of your engine is what causes mileage to drop...counter intuitive..but its not really...ever driven a manual trans car and be in too high a gear for a low speed? Well that lugging is what your car will be doing at 65 mph...its just not an optimal way to setup the car to run at 65 mph for mileage purposes...now if your intent is to have a 80-90 mph cruising speed and a higher top speed to race at Bonneville...thats a different story!

    All that being said, the weight of your car, size of your tires etc also plays a role. I would say if you were putting that drivetrain in a model a or something light that it might be ok since you are "over powered" for the weight of the vehicle. If you are putting that drivetrain in a truck or something heavier and something with tall tires (that also affects final drive ratio) your 2.0569 will not work, it will be too tall. I would say taking the size of your tires into account..if you end up at about 2.25-2.50 final drive ratio or so that would be pretty optimal since that is what GM did in that Trans Am example from above...

    An interesting question is how in the hell the terms, "low" gears which are higher numerically and "tall" gears which are lower numerically, ever came to be? The only thing I can think of is that if you are in a manual trans car, lets say in 2nd gear..the engine will be roaring and you will be going about 35-45 mph or so..well putting "low" gears in your rearend does exactly the same thing...it will increase rpm in any mph range over the tall gears you replaced them with, which makes the car seem like its in a lower gear....since the car will be revving higher to accomplish the same speed than it did before. So why install "low" gears in your rearend? Torque multiplication...you can get off the line faster if your gears are "low" ...the great thing now if with an overdrive trans you can have low gears and an overdrive and have the best of both worlds. You can run a 3.73 rear gear X .067 = and your still at 2.4991 which is pretty damn good! You can launch off the line and still cruise like a Caddy...
     
  3. goldhunter_2
    Joined: Jan 10, 2010
    Posts: 83

    goldhunter_2
    Member

    The old vettes I believe came form the factory at about 3200lbs so your comparison to the trans am is pretty appropriate. I do tend to drive around the 80 mark so that may have been part of my problem as I was figuring 70-80 mph and trying to match to a lower rpm then say my truck which is 1900-2100 at those speeds and forgot to figure the lugging of engine around town.

    thanks that will help me and give me something to work towards your post helps explain it a little better that article or at least that part really had me confused.

    1st gear 2.74 x 3.07 = 8.41 finial drive ratio
    2nd gear 1.57 x 3.07 = 4.81 final drive ratio
    3rd gear 1.0 x 3.07 = 3.07 finial drive ratio
    OD gear 0.67 x 3.07 = 2.056 finial drive ratio

    so these numbers are basically what I have ( I basically over allowed for fuel mileage and need to go to a bigger rear end gear to get the 2.25 to 2.5 finial drive ratio
     
  4. goldhunter_2
    Joined: Jan 10, 2010
    Posts: 83

    goldhunter_2
    Member

    thank you by the way
     

  5. Jmountainjr
    Joined: Dec 29, 2006
    Posts: 1,443

    Jmountainjr
    ALLIANCE MEMBER

    You are headed in a good direction. Ideally you want the cruise RPM - at whatever MPH you consider "cruise" - to be in the engine's designed "sweet spot" based on the cam, etc. If you are not sure of that RPM, take a look at the torque graph for that engine and see at what reasonable RPM it's making reasonable torque. If you fall below that RPM, mileage will suffer as the article suggested. the engine isn't making enough torque to pull the load. Too far above that RPM, and the mileage results are the same, but for different reasons. So if you think that cruise to you might be 55-70 MPH, pick the middle and based on your tire size use a calculator to determine engine RPM. Play with the final drive ratio until the RPM is about in the middle of the sweet spot.
     
  6. I use the term low or high "numerically"...

    Low means 4.11s and high means 2.56s but low numerically is 2.56s and high numerically is 4.11s..
     
  7. outlaw256
    Joined: Jun 26, 2008
    Posts: 2,023

    outlaw256
    Member

    confusing as hell aint it.lol low for high high for low, who ever thought up this shit should be shot.lol ive been doing this for 40yrs and sometimes i now have to stop and think about it. but black panther but it out there pretty darn good.
     
  8. Ebbsspeed
    Joined: Nov 11, 2005
    Posts: 5,810

    Ebbsspeed
    ALLIANCE MEMBER

    If your primary driving will be on the highway, build it for that use. You can always downshift to a lower gear to drive it "around town".
     
  9. BLAKE
    Joined: Aug 10, 2002
    Posts: 2,778

    BLAKE
    Member

    I think JMOUNTAINJR is on the right track... determine what you want your cruise RPM to be, and then do all the math from there. You'll have to consider tranny ratios, rear gear, tire size, engine power curve, and driving habits to come up with the right combination, but shooting for 2300-2400 RPM at cruise is a reasonable target.

    The key seems to be which of these are more variable to get where you want to be - if you need more RPMs on the freeway, are you more willing to run a smaller tire or swap the rear gear? Once you figure that out, you can come up with a sequential order for putting this together.

    I'm no expert, but I just went thru this when putting the combo together for my RPU. With my blown SBC, 700R4, and 30" tire (all decided - not variable), I determined I should run a 3.70 rear gear to make the truck do what I want. With all that decided, then I picked a lock-up converter to best match my combo, cam choice, etc..
     
  10. squirrel
    Joined: Sep 23, 2004
    Posts: 51,222

    squirrel
    Member

    Assuming you're using the complete engine with it's original fuel injection, stock cam, etc. then you're probably going to want to gear it close to what the car it came from was geared. These modern efi engines have a very wide torque curve, they'll run great at low rpm, way lower than what we considered "lugging" the engine in the old days.

    The way that modern cars get such good mileage (30 mpg from a corvette, for example) is that they run the engine real slow, so the throttle is open quite a bit at cruising speed. Low vacuum, low pumping losses, low BSFC, good mileage.

    I'd guess that the 3.07 gears would be fine.
     
  11. Ebbsspeed
    Joined: Nov 11, 2005
    Posts: 5,810

    Ebbsspeed
    ALLIANCE MEMBER

    BSFC = Brake Specific Fuel Consumption
     
  12. goldhunter_2
    Joined: Jan 10, 2010
    Posts: 83

    goldhunter_2
    Member

    Ebbsspeed,
    I would estimate 85% of my driving would be open 5% crowded highway and 10% town

    squirrel,
    yep the whole motor efi and all is stock I'll have to look into where the sweet spot is for that particular motor . I mite just try the 3.07 gears since there already in there before I go out and buy a ring gear set just to see how they do first and have something to compare to when I do change.

    BLAKE,
    I just have a hard time comprehending that 2300-2500 rpm is what I want for target rpms :eek: I just got home and I wrote down some of my trucks rpms (heavy SUV, 350 ,700r4) and just seems I want lower rpms then it has for better mileage or maybe I just need time for it to sink in to my hard head ...lol

    today drive in truck :
    50mph = 1200-1300 rpm
    60mph = 1500 rpm
    70mph = 1850 rpm
    75mph = 1950 rpm
    80mph = 2150 rpm (just thinking lower rpm meant better gas mileage)

    outlaw256,
    you can't even image how much I agree with that right now ....lol

    Pitts64,
    ya that high low thing just makes things more confusing , its not that its that hard just that I get to thinking about the other stuff then the terms high low tall etc etc just make it confusing



    thanks everyone I try to give it a go form here and see what happens
     
  13. Just changed out 2.73 rear gear in wifes 50 Chevy for 3.73. World of difference. Engine is a ZZ3 with 200 4R tranny. 205/75/15 tires.
     
  14. Keeping the engine in its sweet spot rpm is going to give you the most torque.
    That sweet spot depends on mostly your cam choice.

    Rpms under the sweet spot the engine will consume fuel and make less torque per fuel unit.
    Rpms above and past the sweet spot the engine will consume fuel and make less power per fuel unit.

    OD and 2:xx rear gear will bring Rpms way down.
     
  15. Black Panther
    Joined: Jan 6, 2010
    Posts: 1,857

    Black Panther
    Member
    from SoCal

    Goldhunter....you got some good responses...especially Squirrels...the newer engines will run a car down the road at much lower rpm than old cars used to. I had a 1996 Trans Am which had the LT-1 and a 6 speed and I remember that car going 70 mph at about 1800-1900 rpm or so...so use what everyone said as kind of a guide. You just want to avoid having 70 mph land you in 1500 rpm or so..you would never use your overdrive.

    I think that if you set it up like you said and the rear gears were too "tall"...at worst you can change the gears to change the final drive ratio. Dont be confused by Blakes example...his engine is different from yours. He says he has a blown engine..so he doesnt want to run at cruise speeds at the lower rpms like you want to and can.

    I think that if you are at 1850 rpm at 70 mph...youre pretty close...like you said..at worst just bump up to the next lower gear like a 3.23 or 3.31..but see how that affects your cruising rpm range..you only want to change gears once..
     
  16. goldhunter_2
    Joined: Jan 10, 2010
    Posts: 83

    goldhunter_2
    Member

  17. squirrel
    Joined: Sep 23, 2004
    Posts: 51,222

    squirrel
    Member

    ZZ3 is not an LS engine, it's the old style small block, and has a bit of a cam in it. My guess is it would do great with 3.36 gears, and get just about optimum mileage (if you care about mileage)
     

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