I have a 95Z28 when i bought the car i was told it had a 4:10 gear and was wondering what RPM i should be at when in overdrive running 70MPH? The tranny is a 6oE and the rear tires are 18x9.5

 

Need more info. 18 x 9.5 is a wheel size, not a tire size. You need the tire size, such as 285/35-18 to calculate the tire diameter.

 

Also realize the tach is usually off by a couple hundred RPM

best way is to pull the diff cover and read the ratio off the ring gear
for example you might see 37:9

meaing 37 teeth on the ring gear, 9 on the pinion, giving a 4.11 gear ratio

 

Tire size is 275/40ZR18.

 

Ive never removed a diff cover how do you fill it up with oil?

 

Jack up the rear put a chalk mark on the wheel and the driveshaft. Then spin the wheel on revolution and count how many times the driveshaft spins per that one rotation of the wheel.
If the DS spins just over 4 times per 1 wheel spin then you have 4.10's

 

This method can be unreliable since the other tire will just spin (with an open even limited slip differential). You'd want to spin the driveshaft and count how many times it goes around for the tire(s) to spin once, not the other way around.

 

Use the calculator linked on CamaroZ28com

https://www.camaroz28.com/calcs/rpmmph.html

Your tire diameter is 26.66"

O/D in a 4L60E is 0.70

Answer should be 2,533 RPM.

Read the RPM off the PCM with a scanner. That will be more accurate than the tach.

 

Thanks for all the Time and help it was very useful.