Guessing rear gear?

[mcditalia]

Can anyone tell me what gearing I am running in my rear differential just by matching rpm's at a given speed? For example, if I'm at 2700rpms in 4th gear at 65mph can I match it up to a particular ratio? All things considered with a stock 327 L79 and standard muncie 4spd.

Thanks, MC

 

[muddywaters]

Gears

If ratio is unknown try marking rear tire w/chalk at the 12 o'clock position, then mark drive shaft were ever you can, have a buddy rotate rear wheel one complete revolution, eye ball the drive shaft and count the rev's it makes, three and a half times around is in the 3.50 area. Only ball park.

 

[Verle]

It is dependent on tire size:

but a 27" tall tire with a 3:36 rearend yields 2700 RPM at 65 MPH.

Verle

 

[66427-450]

go here and page down: put in tire and RPM, and it'll show speed for the various rear gears


http://www.keislerauto.com/gm/transmissions/corvette_63-67.asp

 

[396 RAT]

http://www.idavette.net/tech/ratioc.htm

And here is one more......

 

[studiog]

"Can anyone tell me what gearing I am running in my rear differential..." From your opening statement I think you are looking for the information given by MUDDYWATERS. I used this same method to determine my rear gear ratio and it can be done with very good accuracy and you can easily tell the difference between a 3.08, 3.36, 3.70, 4.11 etc. without any other considerations. Good luck!

 

[Tom Bryant]

Tool box calculators

Click on the Tech Center tab at the top of the page and then click on Tool Box. All of the calculators you will ever need are right in there.

Tom

 

[mcditalia]

Thanks for the responses, never bothered to look in the tech section of CAC for this calculator. Other sites mentioned are helpful too.

One question though, my tires are firestone deluxe champions, 7.75 on 15 inch rims. Does that equate to a 27 diamater tire? Eyeballing it with a tape measure, it seems smaller than 27".

Thanks, MC

 

[Tom Bryant]

You dwould have to contact Coker Tire or one of the Corvette catalogs like Ecklers to get the exact specs on a reproduction 7.75 tire. It will be around 27 to 27.5 though.

Tom

 

[Vettehead Mikey]

Trying to reverse calculate rear end ratios by using these calculators is less accurate than getting underneath and counting revolutions in/out.

There are indication errors in the tach and the speedo. Your speedo might not be geared correctly to your rear end. The tires run at less than their full sidewall height at normal inflation pressures. You end up with so many compounded errors that it's barely worth doing the excercise.

 

[Ron Miller]

Trying to reverse calculate rear end ratios by using these calculators is less accurate than getting underneath and counting revolutions in/out.

There are indication errors in the tach and the speedo. Your speedo might not be geared correctly to your rear end. The tires run at less than their full sidewall height at normal inflation pressures. You end up with so many compounded errors that it's barely worth doing the excercise.

My thoughts . . . . Go ahead & use one of the calculators to determine your rear end ratio. The differences in rear ratios (i.e., 3.08 to 3.36, 4.11 to 4.56) are widely separated enough that the calculator will put you close enough to your actual ratio to be of use. They work for me on both my cars.

Rlm :cool

 

[Tom Bryant]

Exactly. You have ho way of measuring how much your tires grow in diameter at 65mph but you don't need to know exactly. Just plugging in what you gave and a 27" tire (close to a 7.75) yields a 3.338 ratio rear gear. Obviously given a small amount of tire growth you have a 3.36 rear gear. It isn't off far enough to make you think it could be a 3.70 or 3.08. With stock tires a 3.36 it will be about right on 3000 rpm at 70 mph. Both needles (tach and speedo) run real close together in 4th gear with a 3.36 and point straight up at 70.

If you have doubts as to the accuracy of your speedo just hit the interstate and hold your car ar 60mph. The mile marker posts will be 60 seconds apart at 60 mph (60= 1 mile per minute). If you are more than just a hair faster or slower then you need to try a different driven gear on the bottom of the speedo cable to correct it.

You can also check your tach by connecting a dwell tack with multiple ranges that allow you to compare over a few thousand rpm. An aftermarket tach temporarily hooked up can be used too.

Nothing wrong with turning the drive shaft and counting revolutions either. I've done it many times. But unless you attach a degree wheel to both the driveshaft and axle flange and attach a pointer to zero both at the start of your test you won't be 100% accurate here either. You will be close enought just using a chalk mark to say with some certainty that you have a particular gear and that's what you get with the calculators too.

Tom