Gear Ratio Education

[Ray C.]

Hi!

I need to be educated on transmission and differential combinations. I have a 1961 4-speed with a 270HP and a 3:70 differential. When I am at 55 to 60 miles per hour the engine is at 2200 to 2300 RPMs. The 1963 340HP 4-speed turns 2900 to 3000 RPMs at the same speed. The differential is coded to 3:55. I spun the rear tire on the 63 this morning to confirm the 3:55 differential and it seems to be correct. One revolution of the tire and about 3.5 revolutions of the drive shaft. I confirmed the 3:70 differential in the 61 many years past.

Thanks for the education

Ray

[Harry Sadlock]

Re: Gear Ratio Education

Ray, are you sure you were going the same speed? Maybe the speedo gears were changed. Try it again with a pace car and have them drive at 60 and you check your rpm's and note your speed.

[Michael H.]

Re: Gear Ratio Education

Ray,

I think Harry is correct. It sounds like both cars have the incorrect speedo drive/driven gears. Guessing, I would say thr 61 with a 3.70 should be in the range of 2700 RPM at 60 MPH. The 63 sounds like it has speedo gears that would be used with a 4.11.

If this is the case, it may be more involved than just changing the driven gear on the 63. Not sure but I believe the drive gear would also be different for a 3.55.

Do you happen to have the tooth count or color code of of the driven gears for either car? That may help determine the drive gear that would have been used with it.

[Ray C.]

Re: Gear Ratio Education

Thanks! All

I have use a trailing or lead car to confirm the speeds and will do it again with in the next few days. I had the 63 out for its first road trip since restoration (over 100 miles) last evening and was following a new Z06 and the driver confirmed the speeds. I rebuilt the transmission on the 61 and must not have installed the speedometer gear far enough on the shaft because my speedometer does not work, (I feel like Buba on this one) I have been using the Tach to judge speed. I do not have the tooth count on the speedometer gears for either cars.

Thanks for any help!

Ray

[Michael H.]

Re: Gear Ratio Education

Ray,

Let us know what you find. In the meantime, I'll dig out the original part numbers and tooth count for the drive/driven gears that you would need for the diff ratios that you have.

[Robert S.]

Re: Gear Ratio Education

You might want to play with these calculators too!

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

[Bob R.]

Re: Gear Ratio Education

It is also possible that the tach is not calibrated correctly. I watched the dyno at Corvettes at Carlisle. They hook up an electronic tach and compare to the mechanical one on the dash. The operator told me many tachs were off by as much as 500 rpm.

[Joe R.]

Re: Gear Ratio Education

Another variable in the mix here is the tire diameter (or more accurately, the circumference). Most of the on-line RPM calculators use diameter, but I find it difficult to measure diameter with good accuracy.

Jack a rear wheel up off the ground and use a tape measure to measure the circumference of the tire. The best kind of tape measure is one that does not have an arched cross section, such as a cloth tape or a 100 foot steel tape. These will lie flat on the surface of the tire when you wrap them around it.

Once you have an accurate circumference measurement, divide that by 3.1416 to get the diameter.

If you know the rear axle ratio and the tire diameter, the calculated speed of the car is a direct function of engine speed when you are in the 1:1 4th gear of a Corvette 4-speed. There are no other variables for determining the true vehicle speed.

So, if the calculated speed does not match the indicated speed, the problem is with the speedometer (typically the speedometer drive gear in the transmission) and/or the tach.