[JSeery]

Rick, engine rpm, gear ratios and tire sizes are all related. The easiest way to reduce engine rpm is to increase tire size. In the rpm formula tire size is in diameter. Mart is giving you a tire size (the numbers on the side of the tire) that is equal to a 29 inch diameter tire.

Smaller tires will increase the engine rpm, larger diameter tires will reduce engine rpms. A 29 inch tire is a common diameter for these vehicles.

You can play with the numbers and see what you like, here is the RPM formula:

RPM = (MPH X Trans Ratio X Rear Axle Ratio X 336) / Tire Dia

In high gear in a 1:1 ratio transmission the Trans Ratio is 1, so:

RPM = (MPH X Rear Axle Ratio X 336) / Tire Dia

Now on the question of destroying a flathead engine with RPM, most likely not going to happen. 4000 rpms is very doable by about any flathead.

The MPH formula is:

MPH = (RPM X Tire Dia) / (Rear Axle Ratio X 336)

(4000 RPM X 29 inch Tire) / 4.11 Axle Ratio X 336) = MPH

So, MPH = (116,000) / (1380.96) = 84

At 3000 RPM the MPH would be 63

At 2500 RPM it would be 52 MPH.

Most people are not accustom to the sound of an engine running at higher RPMs because most modern engine are geared to run at lower RPMs. The design of the valve in head engine is very sturdy and the engine will be limited by the breathing capability of anything near stock engine. The engine is not going to be capable of high enough rpm to do any damage.