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Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Transmission - Rearend> Gear ratio
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Topic Review (Newest First)
08-13-2004 05:09 PM
trees If you are cruising at high RPM, you don't have a 3.08 rear. You should not be turning more than 2300 RPM at 70 MPH with a 3.08 gear If you have a Tach, then set 60 mph, and record your RPM and then go into the calculator Willys steered you to and compute your rear end ratio (RER). No tach? Then remove your cover or slide out your 3rd member and count the teeth on the ring gear (big gear) and Pinion (small gear) and divide the little number into the big number and the result is your gear ratio. I have never had good luck turning the rear wheels and counting drive shaft turns. Come up with a different answer each time I try it!!!

08-12-2004 06:47 PM
kd5boc OK..."for dummies" so to speak...

1.) You have to rotate both tires at the same time (for 1 revolution) while you count the driveshaft.
2.) If you hold 1 tire and turn the other one, you must turn it twice to get the ratio...I seriously HOPE you don't have 6's...

By the way...if you want them:

Tire size is it's diameter (in case you didn't know. )

mph x gear ratio x 335 / tire size = rpm
(rpm x tire size) / (mph x 336) = gear ratio

08-12-2004 02:38 PM
Neophyte It sounds to me like you've got some small tires at the rear. Tire size on the drive wheels also play a major role in the overall gearing.

With a TH350, 3.08 diff and 235/60/15 tires you'll get 25.3 mph for every 1000 rpm's meaning that at 3500 rpm you should be doing 25.3 * 3.5 = 88.55 mph.

Even with 195/50/15's you should still be getting 77.06 mph.

That therefore tells us one of three things:
1. Your speedo is inaccurate.
2. Your tacho is inaccurate.
3. You don't have a 3.08

It could even be a combination of the above as the 195/50/15's with 3.73 gears will still give 63.6 mph.
08-12-2004 02:27 PM
08-12-2004 01:58 PM
ricks032003 willys36, thanks again I marked one tire and driveshaft and rotated the tire one complete full turn while watching the drive shaft turn a little over three times is how i can up with 3.08 ratio, i read someone else suggested two complete turns on wheel which would have given me 6.16, which is right? also i turn about 3500 rpm's @55mph is this because the 350 transmission instead of the rearend? thanks for your Patience.
08-12-2004 12:41 PM No, you are backward. For highway cruising you want a numerically lower gear ratio like 2.73 (They call that a 'high' gear - really confusing!). For quick acceleration you start going to numerically higher ratios like 3.54, 4.10 etc. Your 3.08 is actually a pretty good all-round compromise, with a bias to the highway cruiser side. Conversely, you would lean to the 3.54 - 3.75 region for a redlight bandit that could still be highway driven w/o screaming the driver out of his mind. 4.10 and on up is for 1/4 mile bursts only! Best of all worlds is to do what I am doing in my '53 Chevy pickup - 3.54 rear gears w/ 200R4 OD tranny w/ 0.71OD final ratio which gives an equivalent 2.51 rear ratio when in OD.
08-12-2004 11:31 AM
ricks032003 Thanks; sounds like i for sure have street gears maybe for street racing quick takeoffs from one red light to another? do you know what would be a good highway gear maybe like an 3.73? what do you suggest?
08-12-2004 11:23 AM
johnsongrass1 Higher numbers mean more acceleration and higher RPM.

Lower numbers mean less acceleration and lower RPM.

Finding a happy middle ground is the trick.

Street gears are typical 2:73 to 3:73

Track gears are typical 4:10 to 7:50
08-12-2004 11:11 AM
ricks032003 Thanks Willis but i still don't understand! is there anything a little easier for dummies?
08-12-2004 09:04 AM Go here.
08-12-2004 08:49 AM
Gear ratio

Can anyone tell me if lower the ratio more RPM'S or higher? I have an streetrod with a 82 mustang rearend 7.5 3.08 limited slip which runs very high rpm's on the highway speeds, 350/350 drive train so what gears is recommed to lower rpm's 2000/2500 rpm's @70mph for highway use? thanks for everyone's help.

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