I know that there is a gear speed calculator somewhere out there, but i am unsure of all the 6 gear ratios and i have never been able to work that thing, so jsut wondering how much is my speedometer off by. I wouldnt think very much since its a small jump, but id like to get a ballpark # thanks!

 

I dont think it would be very much either. If I were you, I would have somebody pace you and tell you, that should let you know where its at until you can get it corrected.

Only idea I have, but its a bump for you.

 

http://www.f-body.org/gears

Click where it says "Make A Selection" and elect 1998 F-body M6 and change the "Axle Ratio" and click calculate.

 

To save you the time... at 2000rpms in 6th gear 3.42 = 89mph and with 3.70s 2000rpms in 6th gear = 83. Now obviously it will be off more at higher speeds and less at lower speeds, this is just ballpark.

 

I went from 3.42 to 3.73 and I noticed about a 5 mph difference.

 

Your speedometer will read (41/11)/(41/12)=12/11 (9%) higher than your actual speed.