I'm confused as hell. All this time I thought I had 4.88s and I pulled my differential out today and turns out I've got 4.10s. I've got 31" tires and an R150F transmission. The reason I'm confused is because at about 2700 rpm in fifth gear I'm going 60mph but according to every online vehicle speed calculator plus my own math done by hand I should be going like 70mph. Both my speedometer and tachometer are correct. Are those the same numbers everyone with my setup is getting too?

 

pull out a tape measure and measure the height of your tire with the weight of the truck sitting on it. Just because your sidewall says 31" doesn't mean your tire is actually that size, even more so when you put the weight of the truck on it and squish it down.

Also how are you determining your rpm? a factory gauge may not be accurate...or even close.

 

Your math appears to be correct. A few things to double check.
- How did you determine you have 4.10 gears? You should have 10 teeth on the pinion and 41 on the ring gear. 4.88's have 9 and 44 teeth, respectively.

- Have you checked the speed with a GPS or similar? Your speedometer will show the same speed given a fixed rpm and gear regardless of the rear end ratio. You may be hauling along at 10 over the speed limit and not know it.

- Your numbers work out for 4.10s and 4th gear (1:1 gear ratio). Are you really in 5th when you measure the speed? Do you really have an R150F with the 0.838:1 overdrive?

Basically, at least one of five things is in error:
- Your tach
- Your speedometer vs. actual ground speed
- Your transmission ratio
- Your rear end ratio
- Your tire size.

All you have to do is figure out which one.

 

I counted my differential teeth and yes I have 41 and 10. I used one of those police radars to check, people pass me all the time on the freeway, and I've peered into one of those Honda Civics with the large digital speedometer and my speed is correct. I know I have an R150F and that I'm not in fourth. I measured my tire height and because of the load of the truck and wear it's at 29 inches but still I should be going about 65.

I can't think of any reason it would be off so much.

 

I don't have quite the same configuration. I have 4.56 gears with 33" tires, which is about the same as 4.30 gears with 31" tires. With that setup, 2400 rpm gives me about 60 mph. 2700 rpm is about 67 mph.

Can't really explain your discrepancy, but as I said, it's some combination of the 5 things I listed. There are no other variables and the math doesn't lie. Have you independently verified the tach?

The advice to check your rolling tire diameter is good. Best way is to measure from the center of the axle to the ground and double it. You'll likely find the actual rolling diameter of your 31" tires is slightly less than 30", especially if they have some miles on them.

Could be an unfortunate combination of things. You have about a 12-15% error. Take a 3% error in tire diameter, a 5% error in the tach, and a 5% error in the speedometer, and there you are. None of those are big enough to be highly noticeable by themselves, but added up they can explain the discrepancy.