Im a nooobie with buicks and im sure there is a calculator for this but I have not found it yet. Car is a 1967 4dr Wildcat that is all all bone stock with the exception of HEI, Sean Murphy Induction rebuilt orginal carb stage II, 2.5" dual exhausts with no resonators, and a 3:42 posi. Question: What kid of RPM will the car see going down the ghihway at lets say 65mph assuming its a 235/70-15 tie with a tire height of 28".
We are a friendly bunch here. Post your first name. In any case, 336/tire diameter X gear ratio X MPH = RPM So 336/28 X 3.42 X 65 MPH = 2668 RPM. That is without torque converter slip. Add another 150 RPM for that. At 60, figure 2800 or so.
That's a great formula! Thanks for posting it. I need to hook up my tach because I thought my motor was spinning much higher on the freeway than the formula indicates! ( my other oldie:63 C-15 w/3.73 rear and 26.7" tires and saginaw 4spd) Is there a formula using just the tire diameter and rear gearing to get a final drive ratio? I've seen one that uses mph but is there any other way to figure it without mph? Thanks! Rick
I very well could be as this formula does not include torque converter slippage. It is just a general formula helper. NOT TO BE TAKEN FOR EXACT APPLICATION. It will be close enough though. Jim
The gear ratio is indeed affected by tire height. The gear ratios available in a particular model are based upon the stock tire size. For an A body Buick, the stock tire height was approximately 26.6". To figure what the effective gearing was after a tire change, you can take the stock tire height, divided by the new tire height, then multiply by the gear ratio. For instance, with a A body rear using 3.73 gears, if you went to 28" tires 26.6/28 X 3.73 = 3.54. A taller tire will numerically lower the rear ratio. A shorter tire will numerically increase the ratio. If you went to a 25" tire with the same 3.73 gear. 26.6/25 X 3.73 = 3.97
