That sounds low to me. My Carryall is 7300 pounds curb weight and loaded for my trip considerable more which included (4) 5 gallon Gerrys cans of D'zl, spare parts and tools. I am running a 4BTA (CPL 857) into a NV-4500/NP-205 turning 4.89 ring and pinions and 36" tires. My fuel use on a 3411 mile trip was 25.9115 mpg. This included 539 miles of primitave gravel road running for hours in lower gears at 20 MPH due to terrible condition of the road. Had I been on good black top for the entire trip, I would have been close to 30 mpg. I ran legal road speeds which typically were in the 50 to 60 mph range.
i just picked up a multimeter that reads rpms....
it was about $60 but you can get cheaper ones...
it read about 650rpms on idle
in gear it read 600rpms
25 mph it read 1200-1300rpms
40mph it read 1400rpms
55mph it read 1500-1550rpm in overdrive
dose this sound about right???
i have the alternator with the pickup terminal.
Feed your data into this calculator then compare. You can use the mileage markers on the highway to check for speedo error. Once you know the correct answer from this calculator you can double-check tachometer readout for accuracy.
If your GPS displays your position, it has acquired the needed satellites. That being the case, its deadly accurate, given your GPS is working properly. Have no fear, you will not drive faster than the GPS can calculate ha ha