I dont have a tach yet, but I know there is a formula to figure out your RPM.
I have a Getrag, 3.5 gears with 235 85 16 tires, i am wanting to know basically what rpm im turning at 70 mph.

Thanks

 

According to my quick calculations, it should be around 2023 RPM. I just made my own formula that goes something like this:

engine rpm= wheel rpm*(rear end gears)*gear ratio(0.77 for 5th in the Getrag)

wheel rpm=((mph*5280)/wheel circumference)/60 (in ft)

wheel circumference (in ft.) =6.28((((23.5*0.85)/2.54)+8)/12)

Im sure I overcomplicated it, but it works. And in case you are wondering, in the wheel circumference formula, the 23.5 is just 235/10 and the 0.85 is just the tire aspect ratio/100. The 8 is just rim diameter/2.

 

The following two sites will help.

http://www.dirtroadmagazine.com/gears.htm
http://www.f-body.org/gears/

 

The f-body.org is the best, it is right in the money!

 

Mine runs right around 2k at 70, your calculations are right on the money.

 

Yes. Approx 2000 rpm at approx 70 mph.