Those numbers don't work out quite right; all the charts I found on the 245/75R22.5 tires say they're 37.1" to 37.4" in diameter.
What speeds at what rpm's are you getting now?
This is the online calculator I've used for all my computations:
I have a DT466 in my 1979 Blue Bird full size (66 pax) bus. I drove home from Denver to north of Seattle and made three round trips (1400 miles each) to northern California from NW Washington. Lots of hills on all my trips! On the flats I get just a hair over 9 mpg (usually about 9.2) , in the hills I'll drop into the low 7's. On the trips to California I average about 8.5 mpg.
I run at 55 mph at 2400 rpm. Wide open, foot on the floor, hoping for a tailwind speed is about 57 to 58 mph at 2600 rpm.
If you use 2600 as the rpm, 4.10 for the rear-end, 1.1 for the final gear ratio (allowing 10% slip on the AT545), and 37.1 for the tire diameter that comes out to 63.6 mph. You'd be around 2450 rpm for a 60 mph cruise.
To get to your hoped for numbers it seems you'll have to go a little numerically lower for your rear-end. A ratio of 3.70 would give you 60 mph at 2200 rpm and 70 mph at 2600 rpm (is that your governed rpm?).
What's really hurting you are those itty-bitty tires; do you have room for something taller (like 11R22.5)?