Driveshaft RPMs should be greater than rear wheel RPMs. If not, you divided by the rear axle ratio rather than multiplying.
For example, LT235/85R16 tires have a circumference of 99.67 inches. (Slight variation is likely ... plus people doing the calculations for online sites aren't very good with millimeter to inches conversion and simple mathematics.)
63360 (inches per mile) divided by 99.67 (inches) is 635.7 revolutions per mile.
635.7 (rear wheel revolutions per mile) times 55 miles (at 55 mph) divided by 60 (minutes per hour) is 582.725 rear wheel RPMs. The same calculation at 65 mph is 688.675 rear wheel RPMs.
With a 4.10 rear axle ratio, driveshaft RPMs are 2389 RPMs at 55 mph and 2824 RPMs at 65 mph.
With a 4.56 rear axle ratio, driveshaft RPMs are 2657 RPMs at 55 mph and 3140 RPMs at 65 mph.
