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
ENGINE RPM	2,899	2,943	2,988	3,032	3,076	3,121	3,165	3,209
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
ENGINE RPM	2,899	3,020	3,151	3,294	3,451
DELTA RPM FROM BASE	---	121	252	395	552
DELTA % FROM BASE	---	4%	9%	14%	19%