Gear change / rpm

[sjansen]

I'm thinking of changing the rear end gears on my 01 cobra. Can anyone tell me what the RPM will be in 4th gear at 70 MPH with 3.73 and 4.10 gears

Thanks
stan

[Mike Kievit]

3400 is 69.66 mph with 3.73's and 245/45-17 tires.

3800 is 70.83 mph with 4.10's and 245/45-17 tires.

I get my formulas from the "Auto Math Hand Book."

[Ken 01-Cobra]

I can't help you with those two, but with 4.30's, the RPM is right at 3000 at 82 MPH, so it would be about 2800 or so at 70. IIRC, it is around 2200 for the stock 3.27's, that would make the 3.73's around 2400 and the 4.10's around 2600. These are 5th gear RPM's.

There was a calculator at www.prestage.com that did all of this, but he is changing the site around, and hasn't put this calculator back online yet.

EDIT: Unlike Mike and his book, these are totally off the top of my head and may be way off. Take them with a grain of salt!

[Mike Kievit]

Ken,

Top of head or not. I failed to mention my equations were for 4th gear.

5th gear is .67

2300 RPM is 70.28 with 3.73's

formula is RPM X tire diameter 2300 X 25.66= 59,018
then you multiply the gear ratio of 5th X final gear X 336 .67X3.73X336= 839.7

Finally you divide. 59018/839.7=70.28

You do the 4.10's.lol

[Mike Kievit]

fyi The "Auto Math Handbook" is from HPBooks.( The Berkley Publishing Group.

It is written by John Lawlor.



Good investment.

[Ken 01-Cobra]

Heheh, 100 RPM off the top of my head is close enough for me! I have 4.30's, and I am totally happy with them!

[Ken 01-Cobra]

prestage.com has their calculators back online.

Just FYI...in 4th gear the MPH for 3700 RPM for 4.10's vs. 4.30's is 70 vs. 67. Get the 4.30's!