Hi all,

I need some help figuring out the math of gear ratios. I had a 68 that was great fun to drive because of the engine/trany/ and gear ratio setup. The rpm vs speed was about 2500 rpm at 70-75 mph. The trans was a close ratio and I could shift to 3rd at 65 mph and hammer it. I have a 73 now with a wide ratio and the rpm vs speed is 3500 rpm at 75 mph, and I can't downshift at anything over 50 mph without grenading the engine. My question is this: What gear ratio do I need to accomplish the same 2500rpm/75mph? (My guess is that the 68 had a 3.08 rear end, however I hessitate to spend the $1k +/- on a guess.) Is the close ratio in the 68 I had unusual? (4-3 downshift)

Can anyone help with the math of the ratios and rpm and tire size to road speed?

My goal is road race and highway speeds, not light to light.

Thanks for the help.

Alex

try this formula
(tire diameter times 3.14 divided by 1056) times
(rpm divided by gear ratio) = mph
Do the calcs in () first and multiply them.
This works out to 67 mph (with a 28 inch tire and
a 3 08 gear @ 2500 rpm)
Hope this helps!
Save The Wave!
Steve

Try this site:
http://www.idavette.net/techno.htm
I have an M21 with a 3.08 rear and I do 2900 rpm at 70. If I remeber correctly, my tires are 26.5 inches.

Thanks,

I am now confident in the setup. I hope to have a report prior to June.

Alex

I'm no expert on this by any means, but the rear ratio doesnt' change (unless you physically have the rear diff rebuilt with a different gear). The relationship between your fastest speed gear and the next one down (your downshifting reference at high RPM) is determined by the transmission gearing - is it not? Point is that the gearing in the tranny also affects the relative RPM for a given speed when you change gears. For example the M-20 and M-21 (wide ratio) transmission had 2 different spacings between the gears (I haven't a clue what they are numerically) where one is wide, with more spacing and lower RPM at top end, more RPM at bottom end in lower gear for launch. Which will let you put a higher ratio rear gear in. http://www.idavette.net/tech/ratioc.htm ~Juliet

|UPDATED|12/22/2002 10:33:30 PM|/UPDATED|

I just have to say that it's quite a shame that there are not more women in the world who can talk about vettes and and related stuff like Juliet. About all my wife knows (or cares to know) is that my vette is more than 30 years old and it's orange. Juliet, your wealth of knowledge proves that there is hope for her and others like her -- your posts are terrific and continue to impress me.
~ Joe

Hi Juliet,

It is my intent to change the ring and pinion in the differential to 3.08. The reason for the tranny change is to have closer ratios between the gears. Thanks all for your input.

Alex

Having driven wide and close ratio trannys, you won't be happy with the 3.08 rear and close ratio tranny. It will require too much clutch slippage to get started, not just for racing, but for everyday use, too. You would be better off keeping your rear end and putting the money towards a 5-speed tranny. Joe

Hi Joe,
I had this exact setup in a 68 roadster some years ago. Close ratio and tall gears. Until Glenn pointed me to a ratio calculator, I was unsure of the rear ratio. Now it is my belief that the car had the 3.08. Although it wouldn't rip the tires(295/50-15 on 10" rims) off in first it had plenty of take off power. Currently my 73 has the ZZ-4 motor rated at 355HP and 405 lb-ft of torque. This should be enough to overcome the inertia of the car. This summer I hope to install the hot cam to push the Hp over 400, and the torque up 30.
Alex