When planning a project like MLP, far too many people start with the engine and work their way back to the rear wheels. Then, they wonder why things don't turn out the way they'd hoped/wanted.
Recommended (and most effective) way is to start with the rear tires and work your way forward to the engine.
The majority (roughly 80 percent) of MLP's use will be highway miles at 55-65 mph ... so we want to select the combination of engine parts that are most efficient at the engine RPMs that match the tires, gearing, and vehicle speed.
It's important to remember, as I've said sooo often, a motorhome is a heavy truck, not a sports car. Don't expect sports car performance out of heavy truck.
So, started with the circumference of your planned tires -- in my case, LT235/85R16. Divide 63360 (inches in a mile) by tire circumference in inches. This yields tire revolutions per mile. Plug in vehicle speed (55-65 mph) to get rear wheel revolutions per minute (RPMs) at those speeds.
Use the rear wheel RPMs with the rear axle ratio to get the driveshaft RPMs.
Next, take into account the transmission's final (third, fourth, fifth, or what ever gear) ratio. Most older transmissions, such as the A727 automatic, have a 1:1 final ratio. Newer overdrive transmissions, such as the A518 automatic, have a different final ratio.
In the case of transmissions with a 1:1 final ratio (i.e., the A727 I plane to use), the driveshaft RPMs will match the engine RPMs. In the case of overdrive transmissions, the engine RPMs will be lower than the driveshaft RPMs. (If not, you screwed up your calculations.)
You now have the preferred 'sweet spot' around which you (or your engine builder) can plan the engine and select appropriate engine parts.
