Thread: Gear ratio question

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If you want to play with the numbers, Cruise RPM = (mph x Gear Ratio x 336 x OD Ratio)/Tire Diameter in inches. For example, if you're looking at 75mph on the highway, (75 x 3.73 x 336 x 0.70) divided by say 28" tells you that you'll be cranking 2,349.9 rpm, or 2350 which is pretty good. If you run a 29" tall tire you'll drop to 2,269 rpm, even better for economy.
Now if you drop in that set of 2.75's in the back, with that 28" tire and 75mph your cruise rpm will drop to 1,733 rpm which may be a bit below your torque curve on the 350.

Roger, I appreciate that formula. Over my head but thx. I know I can figure it out. I have a fairly complex job but all I really do is read prints to install under ground utilities. I follow directions, this I have to figure out. Pretty cool, I hope I don't lose interest, I am a little nervous about messing something up bad, there is very little budget for this build.