So I want to do either 3.73's or 4.10's in the Camaro. Obviously you guys all know it's a 3.4 with a 5-speed. I want to know what my rpm's will be at 65mph with 3.73's and with 4.10's. With 3.42's I was running 70mph at 2500rpms. Help is greatly appreciated!

 

Try using this:

Engine RPM Calculator

The manual entry in teh bottom part should give you more accurate number as it asks for the transmission gear ratio

See if you can find out what your 5th gear ratio is; its probably between .5 and .75, closer to .75 i bet.

 

final drive is .62 in the t5

 

^ That's the 3rd gen 305 V8 version.
The 4th gen T5 has a .73 final drive.

 

These are ratios so basic math applies.
X/3.42 =70
X=239.4

Use 239.4 as you constant To figure out the speed at 2500 RPMs
239.4/3.73= 64.2 MPH
239.4/4.10=58.4 MPH

Yes you can calculate everything long hand but the diameter of the tire plays a big part in the calculation. Since you ready know all the information in the 3.42 rear 2500 RPM = 70 MPH and we know the only thing that is changing is the rear. SO….

3.73
2500 RPM= 64.2 MPH
2500/64.2 = 38.94 RPM/MPH
38.94 * 70 = 2725.86 RPMs @ 70 MPH

4.10
2500 RPM= 58.4 MPH
2500/58.4 = 42.81 RPM/MPH
42.81 * 70 = 2993.7 RPMs @ 70 MPH

I am sure there is an easier formula this just off the top of my head.

Edit:

Duh Easier way
X * 3.42 = 2500
2500/3.42=730.99
We get our constant 730.99
730.99 * 3.73= 2726.6 RPMs
730.99 * 4.1= 2997.06 RPMs
Just times any ratio times your constant.

 

The formula for figuring rpm's at any given speed is:
mph x transmission final ratio x rear gear x 336, divided by tire diameter.
So.... 70 mph x .73 x 3.73 x 336 = 64042.608. Divided by a 26" tire (as an example) = 2463.17 rpm's @ 70 mph.
Adjust accordingly for your tire diameter and gears.

 

The problem with that calulation is it is sensitive to tire diameter. Since he is not asking for accurate speed he is asking for a comparison using the information he gave is more accurate as we are just talking about the speedometers and RPM ratio. If his speedo read 70 MPH and Tach reads 2500 RPMs. After in installs 3.73 rear when his speedo reads 70 MPH his tack will read 2725 RPMs (assuming he get the PCM reprogammed)

I kind of assumed he never flashed the PCM from the last rear change. That would make the "real" car speed moot in the first place.

I am sure its a wheel size issue but your caculation is show lower engine RPMs with the 3.73's then he is stating with the 3.42.

 

The problem with starting with gauge readings as a baseline, is those numbers can be arbitrary based on how someone sees the gauge at a certain speed, if it's accurate to begin with. Math doesn't lie, "estimations" can. Looking at his profile, he has 245/50 16 tires. Unless he's changed tire sizes, his tires are 25.7" tall. That puts the final rpm at 70 with 3.73's at 2492, and 2746 rpm with 4.11's. Based on that, something is different or wrong on his end, because he says he's at 2500 rpm's with 3.42 gears.

 

Can't flash my PCM. I've tried everywhere to get the speedo calibrated and no one claims to be able to do it. And yes I've talked to Solomon.

Also I have 17" LE SS wheels so I'm running 275/40/17's now. I measured them as they sit to be about 24.75" - 25"...

I knew the real speed of the car because after my last gear swap, I was driving next to my girlfriends brand new Scion TC and her speedometer read 70 when mine read 75. That was with the 3.42 gears. With the 2.73's hers reads 75 and mine read 68-69.

 

As tires "sit", they squat. Height specs on those tires are also 25.7". Your speed/rpm numbers for those given rear gears would be what I put above.