OEM Tire size??

[DanMcG]

It might be easier to get yourself a GPS speedometer for 25 bucks and not worry about it.

 

[HotRodPC]

It could have had a gear change at some point in its life?

I'd say that's very possible. People didn't like those 4.10 gears in the 1 tons when the speed limit was raised from 55 to 70. Someone might have found a 3.73 axle for cheap and swapped the whole axle when your originally had 4.10's. I don't a tire change would throw you off 10mph unless it was like a 4-6 inch bigger tire and you'd notice that much of a bigger tire on the truck.

 

[HotRodPC]

It might be easier to get yourself a GPS speedometer for 25 bucks and not worry about it.

I use DigiHUD Speedometer App on my phone. It has trip odometers, speed alarms, fastest speed etc and it don't work well at night, but velcro it to the dash, at night it gives a good Heads up display on the windshield. I checked it for accuracy on my GPS and my Speedo, it's always been right on the nuts accurate. Just go to Play store and search DigiHUD Speedometer.

 

[PrairieDrifter]

I'd say that's very possible. People didn't like those 4.10 gears in the 1 tons when the speed limit was raised from 55 to 70. Someone might have found a 3.73 axle for cheap and swapped the whole axle when your originally had 4.10's. I don't a tire change would throw you off 10mph unless it was like a 4-6 inch bigger tire and you'd notice that much of a bigger tire on the truck.

Yeah for sure, I meant that as in the tire sizes would make small differences, but combined with the other stuff like transmission swaps and unknown history as far as parts changing, it all adds up to that big difference lol

 

[HotRodPC]

Yeah for sure, I meant that as in the tire sizes would make small differences, but combined with the other stuff like transmission swaps and unknown history as far as parts changing, it all adds up to that big difference lol

Very True. Somewhere around here, we have a speedo calibration page with the forumula to figure out which drive gear and which driven gear you'd need in the transmission for your combination.
Do keep in mind depending on your combo you could be up to 5% off because not every combo is available exactly, though some combos are. That's not bad though. So 5%, figure at 100mph, you could be 5mph off. At 50 that's 2.5 mph off and at 25mph thats 1.75mph off, so you can get very close.

 

[PrairieDrifter]

Very True. Somewhere around here, we have a speedo calibration page with the forumula to figure out which drive gear and which driven gear you'd need in the transmission for your combination.
Do keep in mind depending on your combo you could be up to 5% off because not every combo is available exactly, though some combos are. That's not bad though. So 5%, figure at 100mph, you could be 5mph off. At 50 that's 2.5 mph off and at 25mph thats 1.75mph off, so you can get very close.

I’m definitely gonna pull the trigger on a set of Dakota digital gauges and try out that gps speedo setup. My caprice had issues with the speedo bouncing and my burban has intermittent issues and has a broken needle lol. One time it made the most god awful noise and was telling me I was doing 85 when I was going like 30 lol. Sounded like the dash was about to disintegrate. Also have seen many other vehicles with the same issue.

 

[Michael Wood]

Sorry, been extremely busy for awhile lately.
I checked the rear and it still has 4:10's which is what it came with from the factory.
So all I know for sure is:
1. transmission has been swapped from a TH400 to a SM465.
2. The tires are 285/85-16 now vs. the 7.50x16 that were OEM which isn't much of a difference in diameter.
3. I decided to just live with it for now.
I know on the hwy I the speedo is reading 10 mph less than true speed.....