Google or Bing "gm transmission vibrate software" and the vibrate software will tell you the transmission nomenclature, build codes, and the actual ratios of your transmission. Then go to any gear or road caculator and start playing with the numbers. Again, it's best to go to the actual tire manufacture for tire RPM's because like I said earlier each type of tire and tire company measure their tires differently and it throws off the aspect ratio. For example the tread width on my 265-75-16 on my p/u are narrower then my 255-55-16 on my camaro dispite being a 10mm wider....and why both of them are wider then my 295-75-22.5 tires on my semi. Some measure actual tread width, some measure outside casing to outside casing, some measure outside casing to inside caseing, some measure inflated mounted and under load, and some don't. Most of your sport tires will be measured very close to one another..but again you can find two same stamped sizes from two manufacturers, but are actually different sizes. That vibrate software is pretty awesome and I think they have the info for about every transmission ever made. Good luck.

 

Your method worked perfect on my 02 Ws6
i'm running 3.42 and the engine rpm was close to 3500 but a little below at 3rd 75mph.

 

Yes because 3rd is a 1:1 ratio gear.

My 3.46 geared automatic is just over 3400 at 75mph.

 

Thanks guys. I know it worked for me, but I see a lot of new threads on here because the OP doesn't know which gears they have. Seems that such a simple way of figuring it out would be more widely known.

 

Also if it works in 3rd gear in auto's it should work just as well in 4th in a m6.