What gear ratio?

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cazsper

Well-known member
Joined
May 30, 2012
Messages
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Location
Sunnyvale, CA
My Car
1968 Coupe: 393w, TKO-600, Maier Racing springs, Global West suspension, Currie 9in with forged axles, 3.50 gears, Trutrac, Baer brakes front/rear
1973 Mach-1: 351c 4V, C-6, 3.73:1 gears and a long "To Do" list..
How can I tell what my gear ratio is? When I purchased the car, the previous owner told me it had 373's. All I know is that in the freeway, I'm between 3500-4000rpm. I have a C6 auto with 275/35-17 in the rear. I'm thinking of going with either 3:25's or 3:50. I just want toake sure I don't already have 3.50's.

The previous owner also told me it had a 750 Holley with 2.5" exhaust and it ended up being a 650 with 2.25" exhaust. So I can't just take his word for the gears..

 
If you can determine your exact speed at a given rpm you can calculate your gear ratio.

For example if you assume a 24.58 inch diameter tire at 3500 rpm going 65 mph with a final drive ratio of 1:1 (no OD) your gear ratio calculates to be 3.94.

http://www.summitracing.com/expertadviceandnews/calcsandtools/gear-ratio-calculator

You can also approximate by jacking the car up and counting the revolutions the driveshaft makes in relation to the tire rotation.

 
...All I know is that in the freeway, I'm between 3500-4000rpm....
I agree, we need to know your exact speed and the corresponding rpm at that speed (assuming that both your tach and speedo are correct).

 
Here's a tire diameter calculator: https://robrobinette.com/tire_diameter.htm

Your overall wheel diameter (using that calculator) is: 24.5" (275/35R17)

Using TommyK's calculator on Summit Racing, a wheel diameter of 24.5" at 65 mph pushing 3000 rpm would have an effective gear ratio of 3.37 (3500 rpm=3.96, and 4000 rmp=4.49).

Based on the information you've provided, and the WAG use of these calculators, I'd have to say you're probably running 3.50 gears.

You'll need to nail down some specifics to know for sure, using calculators of this kind.

Hope this helps.

 
Sweet. Thank you everyone..

 
Assuming 70mph

3.25 = 3122rpm

3.50 = 3363rpm

3.73 = 3584rpm

4.11 = 3949rpm

You mentioned 3500 to 4000 rpm would make it at least 3.73. Any lower speed just drops the corresponding rpm's.

So if you are going only 60mph at 3500 the rear gear would be 4.30.

 
Last edited by a moderator:
I believe if your lift the car and turn the rear wheel one revolution and count how many revolutions the driveshaft turns you can tell what the ratio is .

 
Assuming 70mph

3.25 = 3122rpm

3.50 = 3363rpm

3.73 = 3584rpm

4.11 = 3949rpm

You mentioned 3500 to 4000 rpm would make it at least 3.73. Any lower speed just drops the corresponding rpm's.

So if you are going only 60mph at 3500 the rear gear would be 4.30.
What would that be in a 14" tire?

mike

 
.

unfortunately you cant do it that way unless you know your exact speed and you cant trust your speedo.

jack both rear wheels up.

rotsate tire forward whilr looking at the other tire . . if the other tire spins the same direction, you have a posi so do the following

put the valve stem exactly at the bottom.

put a 1" long mark on the side of the drive shaft facing the tire.

rotate tire forward exactly 1 full turn whole counting how many revolutions the white mark makes.

of ot is not a posi, lower 1 wheel to the gtroind then do procedure above.

3 turns = 3.00

3 1/4 turns = 3.25

3 1/2 turns = 3.50 or 3.55

3 3/4 turns = 3.70 or 3.73

 
Thanks.. I'll confirm my speedo and see how it goes. Now I have to figure out what speedo gear I need.. ?

 
Assuming 70mph

3.25 = 3122rpm

3.50 = 3363rpm

3.73 = 3584rpm

4.11 = 3949rpm

You mentioned 3500 to 4000 rpm would make it at least 3.73. Any lower speed just drops the corresponding rpm's.

So if you are going only 60mph at 3500 the rear gear would be 4.30.
What would that be in a 14" tire?

mike
275/35-14 tire dia = 21.5" (not sure if this is an available tire)

Assuming 70mph

3.25 = 3558rpm

3.50 = 3832rpm

3.73 = 4084rpm

4.11 = 4500rpm

275/60 - 14 dia = 26.9" would have 300 lower rpms than the 17" wheel at:

3.50 = 3063rpm

 
Last edited by a moderator:
Assuming 70mph

3.25 = 3122rpm

3.50 = 3363rpm

3.73 = 3584rpm

4.11 = 3949rpm

You mentioned 3500 to 4000 rpm would make it at least 3.73. Any lower speed just drops the corresponding rpm's.

So if you are going only 60mph at 3500 the rear gear would be 4.30.
What would that be in a 14" tire?

mike
275/35-14 tire dia = 21.5" (not sure if this is an available tire)

Assuming 70mph

3.25 = 3558rpm

3.50 = 3832rpm

3.73 = 4084rpm

4.11 = 4500rpm

275/60 - 14 dia = 26.9" would have 300 lower rpms than the 17" wheel at:

3.50 = 3063rpm
These are P235/60R14. Car now has 2.75 rear and trying to decide

if to put in the 3.50 Trac-Lok I got from Don. Don't do much hiway

driving. I would get better snap at the gas pedal I guess.

mike

 
What would that be in a 14" tire?

mike
275/35-14 tire dia = 21.5" (not sure if this is an available tire)

Assuming 70mph

3.25 = 3558rpm

3.50 = 3832rpm

3.73 = 4084rpm

4.11 = 4500rpm

275/60 - 14 dia = 26.9" would have 300 lower rpms than the 17" wheel at:

3.50 = 3063rpm
These are P235/60R14. Car now has 2.75 rear and trying to decide

if to put in the 3.50 Trac-Lok I got from Don. Don't do much hiway

driving. I would get better snap at the gas pedal I guess.

mike
Tire dia is 25"

3.50 = 3295rpm @70

3060 @65

2824 @ 60

2589 @ 55

With 3.50 you'll like the launch

 
I run a 3.5 and it's a good launch but not so great on the freeway. I put in an GV OD and really like it. But due to my mustang sickness I am considering swapping the c6 with a AOD and keeping the GV so I have 8 forward gears and can step up to gears in the 4.x range.

 
Thanks.. I'll confirm my speedo and see how it goes. Now I have to figure out what speedo gear I need.. ?
you need to know your gear ratio or exact rpm to do that.

 
I run a 3.5 and it's a good launch but not so great on the freeway. I put in an GV OD and really like it. But due to my mustang sickness I am considering swapping the c6 with a AOD and keeping the GV so I have 8 forward gears and can step up to gears in the 4.x range.
Yeah, but will a C6 GV work on an AOD? The AOD is pretty much just an FMX with built-in OD.

Which reminds me, "Seeing's how the VP is such a VIP, if word of the PC leaks to the VC, the VP could become MIA, and then we'd all wind up on KP."

Gotta love all the acronyms. rofl

 
So the 275/35-17's give me a 24.57" tire. From that, how can I calculate what speedo gear I need for the c6?

 
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