top end speed calculation

I did the search for this here but no luck , then I googled it and same . what I am trying to figure out is if I have a 5.14 rear diff , and 37 inch tall tires and the 5th gear in my trans is .83 to 1 , and I then apply the brownie aux trans OD which is .85 . at 1850 rpm , how fast will I be going , how about 2000 rpm ?

37 inches tall tire times 3.1415 (pi) = 116.24 inches circumference
116.24 inches / 12 inches = 9.68 feet.
one mile = 5280 feet / 9.68 feet = 545 revolutions per mile
545 tire revolutions X 5.14 ratio = 2801 engine revolutions per mile or 2801 rpm at 60 mph
fifth gear is .83 X 2801 = 2325 engine revolutions per mile or 2325 rpm at 60 mph
Brownie is .85 X 2325 = 1976 engine revolutions per mile or 1976 rpm at 60 mph
If you insist on having 2000 rpm your speed will be a little over 60 mph.
I hope this is correct. B72 what say you??

Good Morning Nikki ,and thank you for doing that .I'm hoping to be able to get to it next year , but it gives me the motivation to get the garage done so I can begin . I think that it sounds right , at 2750 I had figured 57 mph , but the additional overdrive was confusing to me . Black diamond 308 runs pretty good at 2000 , purrs at 1850 , but I'm not sure that there is enough horse power to move the truck at those speeds even with the gearing .What do you think ? I'm going to follow thru on it and will at least get better use out of the lower gears but these things can only go but so fast with these motors , It is been balanced and is as good a running 308 as can be , but when I'm on the highway to get to 55 is about it and its turning about 2500 , like I'm working against gravity .

If you can fit 9.00x 20 tires you could bring the rev per mile down to 518 and help a whole lot. don't forget that while you are bringing the engine rpm down, you are still spinning the driveshaft much faster.

Horsepower and torque are easier to understand when compared to a garden hose and running water.
Horsepower is the gallons per minute. Torque is the pressure. Torque does the work. Horsepower is how fast the work can be done.
These old engines do not have the extra punch available like a modern engine. I would be surprised if that truck with that driveline would get much better than 4 miles from a gallon of gasoline. Gasoline was delivered to our farm in 45 Imperial gallon barrels (205 liters). For less than $10, Dad told me he paid around $7.50 a barrel for a fuel called distillate. Distillate was the fuel for his 1938 John Deere, model D. Taxed fuel at a service station pump was more costly.

Both my K's with the smaller bore as the 308 but same stroke get close to 10 MPG, one weighs in at 8 K the other 10.5K.
The lighter one sits on 8.25x20 rubber and the heavy one on 9.00x20. Other than that the drivelines are identical. The heavy one will cruise at 55 on the level but requires downshifting more on hills, the lighter one is really wound out at 55 but will hold it well.
I would shoot for a cruising RPM of 2000 to 2400. they will pull down to 1850 or so. These are truck engines and pull like them.