the truck has 4.10 axle ratios, nv4500, and 35" tires, gear ratio calculator says i would be at around 1800rpms at 65mph


the day i bought this truck i was amazed at its poor highway performance, it really only wants to do about 73mph if i really get after it. i checked the speedometer with a gps and its about 2mph slow at 50mph, so the margin gets greater as the speed increases im sure but still, couldnt be more then 5mph.

the clutch feels rock solid around town, it doesnt shutter any when i let it out, i can lug a gear and the truck pulls hard, no slipping feeling at all... is it even possible that the tach could be off, could my tires run small for their size, is my speedo off more then i think, i dont know what to think... is this common with a bad clutch?

im getting 14.5mpg around town and maybe 16 highway, starwheel maxed, no plate, is this about right or should i be getting better(indicating a slipping clutch).

 

seems higher then mine but I have an auto wiht 4:10's and ony 285's. My truck at 65 is like 1900rpm

 

the dealer can reflash the speedo to read higher/lower as needed for calibration.
also since all the gauges are electric, a faulty ground/resistor can cause them to read funky.
for comparative purposes, run the truck in 4th (direct drive) and get #s from that, that way, even autos/6spds would be the same.

 

doing a gear tooth count in a moment, i may have 4.56s. the truck is a wiring nighmare so the ground issue is also very possible. ill gps it at the speed im concerned with rather then 50mph like last time, ill also get a 1:1 reading rather then OD.

 

My '95 would only do 70 down a hill before I put in the Govener spring kit in that really woke it up and my speedo has always been a little off

 

ok, counted the teach on the gear set and it is a 4.10 ratio. i ran it again and got 70mph at 2150rpm, that checks out just right on the gear ratio calculator. must just be an intermittent wiring problem with the dash, i wouldnt have thought of that if nobody mentioned it. thanks alot for the input, i was on another forum and had 5 pages of people bashing me and insisting i need to bring the truck to a shop because something is seriously wrong, ect. needles to say ill no longer be visiting that forum for any of my diesel needs .

and i was thinking the same thing with the governor spring, theres really no more power past 2200 rpms, my highway cruise is above 2200 so it make sense that the truck starts to run out of steam on the highway.