my new motorhome with Garage project gearing question

[mekanic]

I found a good truck, A 94 Ford F-700 12V BT 5.9 Cummins(190HP) , 6 speed trans
The Truck now has a 6sp (direct 6th gear) 5.29 gears and 11R22.5 tires.
And the tires I have now have dry rottted side walls and MUST be replaced.
THe box I just mounted is 1 inch above the frame so It looks like I must go to lo pro 22.5 tires. The brake drum on the air brakes are way too large to let me go to a smaller wheel. This thing is spinning right at 2800 at 65mph or so right now.
What I have now: 65mph X 5.29 X 336/ 41.3 = 2797 RPM
Tell me if I'm going too far:
36.5 tire + 3.90 gear @65MPH = 2333 RPM ?
Or should I go to a 3.70 gear? 65= 2247 RPM ?
The formula I'm using is
MPH X GEAR X 336 / overall tire height = RPM

[rustyrim]

Have you thought about bigger tires and boxing the bed?

[mekanic]

Originally Posted by rustyrim

Have you thought about bigger tires and boxing the bed?

The tires on it are the tallest 22.5 tires made.

[ArdcoreChoppers]

Any luck on the gearing? Just curious as I just went through all that with mine.

[mekanic]

Originally Posted by ArdcoreChoppers

Any luck on the gearing? Just curious as I just went through all that with mine.

Haven't changed it yet. So,.... what did you do on yours?
see this thread about my rear axle:
Need help ID ing rear axle - Ford Truck Enthusiasts Forums
Is this what yours looks like also?

[mekanic]

After spending a few hours of online searching
I found it.
Its a 22 series (99% sure its a #22121)

this page has the info for Eaton rear axles

(Go to the last link at the bottom of the list.!!)

LiteratureCenterResults

[ArdcoreChoppers]

I wound up just changing the whole rearend. I found it cheaper that way. I bought the whole thing drum to drum for $350.00 with a 3.90 gear. When I called about a new r&p they quoted me $700-$800 "if" they could get the right ratio (was looking in the 3.70 range) and truck parts places wanted the same for a used 3rd member. My intention was just swap the 3rd member as the rear I bought was out of a Top Kick (same as mine). I didn't check i.d. tags as I figured they would all be the same. The rear I got was a lighter duty (smaller diameter axle) and the 3rd member wouldn't interchange. Since we are not really loading this thing anywhere near capacity I figured it would be ok with the lighter rear. The only difference installing it was the axle pads on the housing. The lighter duty rear had a shorter pad length so I just tacked a shim to it so it would fit the saddle on the spring without worrying about it ever moving. After getting it driving I still need more gear. I'm just gonna go ahead and bite the bullet and get one of the gear vendors overdrive units, my goal is to get this running about 1,900 rpm at cruising speed. My thought is getting just above the peak tourqe it should still give me plenty left and I really don't wanna be spinning this motor at near peak rpm for hours at a time. Please keep in mind I have never claimed to be a smart guy and this may be way off base but I'm gonna give it a try.