Final drive ratio.

I am working on a "Toy" truck and I am trying to figure out the best final drive ratio for economical, empty highway driving at 65. A 4.56 rear with a .74 OD trans and my 16r20 tires will give me 65 at 1400 rpms, which also happens to be where the peak torque of the engine is. I am not on my home computer at the moment, but there are other ratios that would put 65 closer to 1200 rpms with 70 around 1300... Would this be too low? Any towing would be done in a lower gear and the transfercase would basically give 2 extra lower gears around 18 and 30 to 1. Also is there any noticeable difference in economy between a direct drive and an OD trans when empty? I understand that empty isn't a realm that many deal with, but I appreciate the help.

 

Sorry for the lack of truck info, was focused on the gears lol. By "toy" I meant, not for hauling/ just for the fun of a big truck. I am re-powering a 64 International R-195 (tandem rear,24k gvw). It will be going to a single rear axle and 4x4 with full air suspension, Built DT466 with an eaton 14710, 14713, or 8ll. The 16R20 Michelin tires are 52 inches with 20 inch wheel.

 

Is 1200 rpms OK as long as it has the torque to push it? The land around here is flat except for overpasses, and that would be 12k rpms less per hour vs the 1400rpm, or 10 extra minutes worth of rpms at a 1200 cruse. If that would be ok, I could use a RTLO 14613B with a 3.42 diff ratio... That would get me 65 at 2750, 1970, and 1417 rpms in 6,7, and 8 direct for more power, and a 1204 rpm in 8 Hi. Sound OK, or is the 1200 idea a bad one, and I should stick with 1400 rpms for crusing?

 

peake torque on a stock DT466 is at 1400 rpms. I am going to be shooting for about 500HP