Okay, I decided to run some calculations:
The high gear for the NP200 is the same as the NP205, however, there is a difference in noise (NP200=straight cut gears, NP205 = helical gears) and in heat/RPM tolerance due to the difference in gear cut and bearings between the two. Here's some calculations to look at:
Stock configuration transfer case, transmission and diff gears:
Tire diameter = 35.4 (Power King Super Traction NR-50 tires)
Transmission final gear ratio = 1 (T98 stock transmission)
Transfer case high gear ratio = 1
Axle ratio = 5.87
RPMs @ 65mph = ~3621 (motor not happy)
RPMs @ 75mph = ~4178 (motor blown up)
Stock configuration transfer case and transmission but switched to 4.56 diff gears:
Tire diameter = 35.4 (Power King Super Traction NR-50 tires)
Transmission final gear ratio = 1 (T98 stock transmission)
Transfer case high gear ratio = 1
Axle ratio = 4.56
RPMs @ 65mph = ~2813 (motor alright)
RPMs @ 75mph = ~3246 (sort of doable)
Transmission changed out to an NV4500 (again, still don't know if this is possible) but with the stock 5.87 diff gears:
Tire diameter = 35.4 (Power King Super Traction NR-50 tires)
Transmission final gear ratio = .73 (NV4500 5 speed transmission)
Transfer case high gear ratio = 1
Axle ratio = 5.87
RPMs @ 65mph = ~2643 (motor alright)
RPMs @ 75mph = ~3050 (should be okay?)
Transmission changed out to an NV4500 (still trying to figure out if this is possible) and with 4.56 diff gears:
Tire diameter = 35.4 (Power King Super Traction NR-50 tires)
Transmission final gear ratio = .73 (NV4500 5 speed transmission)
Transfer case high gear ratio = 1
Axle ratio = 4.56
RPMs @ 65mph = ~2053 (motor alright)
RPMs @ 75mph = ~2369 (motor alright)
Hope that helps you out. I'm thinking that the NV4500 is a great idea if it's possible to do. I'm already going to get the diff gears swapped out, but that will have to wait for the money to do it. I'm going to do all of the research I can to find out if I can get an NV4500 to mate up to the stock motor. It would be nice to be able to cruise at 75mph if necessary in my truck. At minimum, the diff gears will get me 65mph and with an NP205 and selectros, the truck should be happy at that speed. If there is a way to mate an NV4500 transmission to the stock engine, it looks like I'll have no problems hitting 75mph.
--Wintermute
You spent a lot of time there, but all of your rpms are higher than the stock setup with 35.4 inch tires at 45 mph, which is about all they should run and that is really too hard for extended distances. I get 2507 at 45 mph for the stock setup. Those who are really in the know recognize that is about as hard as the engine should be run for an extended period. Most of the trucks I owned or were around has speed limit 45 painted on the inside of the driver's door. I suspect after the motor pool was swarmed with blown engines.
The stroke is long and the bore is small, they had a fragile rod that was prone to distortion and once they distorted, the bearings failed quickly.
Most people recognize the small block Chevy as a 3,000 cruising engine, which brings the ground speed up to 55 mph. Stepping up to a 38" tire, which is not as easy as it sounds, still runs you over 3,000 at 60.
Do not get me wrong, I love the trucks, I just hate to see people misled into thinking one can be easily adapted to cruise at 65-70, or even 60 for that matter. The military found out they would not survive, fire departments and forestry units across the country found out they would not survive, yet we are here talking speed.
Along this line, has anybody actually been able to buy a higher gear set for the stock axles? They may be a Dana 60 and Dana 70, but they have different dimensions than everything else. Just try buy about anything attached to that axle, seals, brake cylinders, brake drums, even brake shoes. I had to take my old shoes in and have them relined as there are none to be bought. Glen