I've seen some threads about the effects of tire size, O/D, etc on cruising RPM. I used to have a '62 Willys utility wagon with a low-range transfer case and a Warn O/D (total of 12 ratios available!), and needed this calculator (attached) to figure out what would happen with mud tires, etc.
By the way, for anyone using it, it is important to measure your tires' rolling radius. You can't assume a diameter from the aspect ratio and the width. Also, if you have an automatic, you should fudge in about 10-15% slip (i.e., higher RPM) at cruising speeds (steady throttle).
See if you find it useful, it's an Excel spreadsheet....
Example only, click link to open functional file in MS Excel . . MPHRPM.xls
CHANGING GEAR RATIOS (in .06:1 increments):
(only enter data in RED fields)
|CHANGING GEAR RATIOS (in .06:1 increments):|
|(only enter data in RED fields)|
|SPEED IN MPH||55||55||55||55||55||55||55||55|
|TIRE DIAMETER, INCHES||25||25||25||25||25||25||25||25|
|FINAL DRIVE RATIO||3.92||3.98||4.04||4.1||4.16||4.22||4.28||4.34|
|TOP GEAR RATIO||1||1||1||1||1||1||1||1|
|DELTA RPM FROM BASE||---||44||89||133||177||222||266||311|
|DELTA % FROM BASE||---||2%||3%||5%||6%||8%||9%||11%|
|CHANGING TIRE SIZE (1-inch increments):|
|SPEED IN MPH||55||55||55||55||55|
|TIRE DIAMETER, INCHES||25||24||23||22||21|
|FINAL DRIVE RATIO||3.92||3.92||3.92||3.92||3.92|
|TOP GEAR RATIO||1||1||1||1||1|
|DELTA RPM FROM BASE||---||121||252||395||552|
|DELTA % FROM BASE||---||4%||9%||14%||19%|