I looked up all my RPO codes a while ago. Apparently, the rear gear ratio for my 2000 Z28 M6 is 3:42...

I did the little 80mph in 4th test today. My tach was reading 3.75k rpm. When I bought my wheels and tires, they guy looked up the sidewall and wheel info, we did the math. My speedo w/ 18" wheels and 35 series sidewall is only off a tenth at 60mph.

That 4th @ 80mph, is only a guestamite, but does this sound odd to you??

Also, I bought my car used 4 years ago w/ 19,500 miles on it. It had all the free mods done. Maybe someone threw a set of gears in the 10 bolt?? ..

 

I got 82.3mph in 4th with those wheels and tires and 3.42s. It was ~75 with 3.73s.

 

could be a slight change to 3.55's, idk if u can get 355s in a 7.5" rear to

 

Play with the calculator at www.f-body.org/gears/ It lets you plug in different tire sizes, gears, etc. then recalculate.

 

You also need to tell us the section width of the tires in order to do the calculations.

 

Rears are 285/35ZR18. My stockers were 245/50R16 I believe.

 

How are you reading the RPM? The stock tach sometimes reads 100-200rpm higher than the actual reading. Try reading the RPM off the PCM with a scanner. A 200 RPM error would account for your difference. My T56/gear/tire spreadsheet tells me with a 25.85" tire and 3.42 gears, you would be at 3,557RPM at 80MPH in 4th gear.

Also keep in mind that the tire rolling radius will vary from brand to brand (the 25.85" is a theoretical dimension), with tread wear and with inflaction pressure and vehicle load.

 

Rotate your rear tire one revolution and count the number of times the driveshaft rotates. This will give you your rear axle ratio.

 

^ I've heard of that way. But it, again, is a guestimation.

Ehh.... WHy don't i just save $2200 for the Strange 12B w/ 3.73's and KNOW what gears I have!...