Re: Engine, Tach, and Speedster Questions

Posted by Owen_Dyneto On 2009/1/2 15:02:56

Would be helpful to know the tire size and rear axle ratio. Here's some factory data for 1931 with 6.50x19 tires and 4.69 rear axle.

20 mph = 985 rpm
50 mph = 2463 rpm
70 mph = 3448 rpm
90 mph = 4423 rpm.

For the 4.38 rear axle, same tire size:

20 mph = 920 rpm
50 mph = 2302 rpm
70 mph = 3222 rpm
90 mph = 4142 rpm.

By just measuring your tire circumference and knowing the axle ratio you can work out any combination you want. I guess you know that a 5 inch stroke and babbitt bearings aren't conducive to long engine life at much above 2500 rpm or so.