Not trying to beat anything to death here...just trying to learn. Alb, you wisely say above that with 25" tires and the 3:88 and .89 fourth gear a car will go 77.5 mph at 3,500 rpm.
Not trying to pick hairs here either, but does anyone take into account that the tire is not round? ...It really is flat on the bottom where it meets the road and a quick measurement with my setup shows that the radius is actually about 1/2" less than 1/2 the diameter just by the weight of the car and thus the "true" radius is reduced where it is most effective...the center of the hub down to the road. It only accounts for about 4.35% on a 25 inch tire if I'm right, but your rpm might go from 3,500 rpm to 3,652.
Not a big hill of beans for sure but how many times do we hear that lads say their
speedo is off compared to other peoples stated rpm.
So, my situation is this...I just measured the diameter of my tires and they are 24" diameter, yet flat by about 1/2" on the bottom yielding an effective radius of 11.5".
Now, going from Al's reference above with the 3:88 / 89 fourth doing 3,500 rpm hitting 77.5 mph with an effective 12" radius, I'd be doing about 3,652 rpm. But
for now I have the stock 4.125 which would give me 3,882.
Thinking about a new tranny upgrade soon, if I went from the 3:88 down to the
3:44, with the same tires I'd be doing 3,237. Now couple with that my real cruising speed is between 60 -65...call it 62.5, my actual rpm would be about 2,610 rpm.
Sounds like a good choice considering this torque graph or not ? Thanks..