PDA

View Full Version : RPM's vs gear ratio?


JeffB
04-13-2004, 04:40 PM
Can anyone tell me how many rpms your turning with 4.10 gears in fifth?
I'm looking to modify my 96 and was leaning towards the 3.73's because of some significant highway driving.
thanks

Firme
04-13-2004, 05:21 PM
your question should be what rpm at a certain speed or what speed at a certain rpm.. your question lacks one of the crutial variable.

4.10's in 5th @ 6800 rpm will net you 189.1 mph
4.10's in 5th @ 5000 rpm will net you 139 mph
4.10's in 5th @ 4000 rpm will net you 111.25 mph
4.10's in 5th @ 3000 rpm will net you 83.44 mph
4.10's in 5th @ 2000 rpm will net you 55 mph

4.10's in 4th @ 6800 rpm will net you 126.1 mph

Bartman01
04-13-2004, 06:03 PM
Assuming 245 /45R17 BFG KDW's (diameter 25.7)
Cobra 2001 Trans Final Drive Ratio (.67) - don't know yours.
Given highway speed concerns 3.73 vs 4.10:

55 - 1797 vs 1975
65 - 2124 vs 2334
70 - 2287 vs 2514
75 - 2450 vs 2694

All in all about 200 rpm difference given my numbers.

To calc for your car - get your tire diameter (best to measure, but you can cheat like I do and just use the mfgs spec sheet), your 5th gear transmission ratio, and google a calculator (like http://www.slowgt.com/Calc1.htm#RPMInGear).

JeffB
04-14-2004, 12:29 AM
your question should be what rpm at a certain speed or what speed at a certain rpm.. your question lacks one of the crutial variable.

4.10's in 5th @ 6800 rpm will net you 189.1 mph
4.10's in 5th @ 5000 rpm will net you 139 mph
4.10's in 5th @ 4000 rpm will net you 111.25 mph
4.10's in 5th @ 3000 rpm will net you 83.44 mph
4.10's in 5th @ 2000 rpm will net you 55 mph

4.10's in 4th @ 6800 rpm will net you 126.1 mph
How foolish of me .....60 MPH was the number I was looking for.

JeffB
04-14-2004, 12:33 AM
Assuming 245 /45R17 BFG KDW's (diameter 25.7)
Cobra 2001 Trans Final Drive Ratio (.67) - don't know yours.
Given highway speed concerns 3.73 vs 4.10:

55 - 1797 vs 1975
65 - 2124 vs 2334
70 - 2287 vs 2514
75 - 2450 vs 2694

All in all about 200 rpm difference given my numbers.

To calc for your car - get your tire diameter (best to measure, but you can cheat like I do and just use the mfgs spec sheet), your 5th gear transmission ratio, and google a calculator (like http://www.slowgt.com/Calc1.htm#RPMInGear).


Thanks, I used the calculator and the final drive ratio needs to be ~.8 to work out right based on my observed tachometer readings at 60 which was 2000 rpms. I'll have to look upthe final drive ratio somewhere to vewrifdy it.

tcrews
04-14-2004, 08:55 AM
Thanks, I used the calculator and the final drive ratio needs to be ~.8 to work out right based on my observed tachometer readings at 60 which was 2000 rpms. I'll have to look upthe final drive ratio somewhere to vewrifdy it.

The gear ratios for '96-'01 are the same (1st, 2nd, 3rd, 4th, 5th).
There is about a 200rpm difference between speed with 3.73 gears and speed with 4.10 gears.

4.10s are decent all around gears, 4.30s are definitely better and 4.56 or 4.88 if you are big into drag racing.

I'd go with 4.30s if I were you (highway driving or not) because the car will be much more fun to drive and you'll still get very close to the same gas mileage.

JeffB
04-14-2004, 09:49 PM
The gear ratios for '96-'01 are the same (1st, 2nd, 3rd, 4th, 5th).
There is about a 200rpm difference between speed with 3.73 gears and speed with 4.10 gears.

4.10s are decent all around gears, 4.30s are definitely better and 4.56 or 4.88 if you are big into drag racing.

I'd go with 4.30s if I were you (highway driving or not) because the car will be much more fun to drive and you'll still get very close to the same gas mileage.


Thanks for the info. Looks like at least 4:10s are ok if theres only 200 rpm difference. I drive it quite a bit between Maine and New hampshire and didnt want the thing winding on the highway and ruin a nice highway cruise.