So I have installed this C6 into my 1965 Gal and upon the road test the speedo is off by 10 mph. GPS / satellite tracking shows me at 55 and the speedo is saying i'm doing 65. I know you can change the speedo gear in the trans, but the question is which one do i get? My info tag on the door shows my rear end having 3:05 rear gears. I'm running 245/60/15 tires on 8 in wide rear rims. to me having larger tires would make the odometer show slower not faster. any ideas on what I need to get for a gear? also, is it just the 1 gear in the trans that i need to change ?