whoa !! spedo

[85-m1028]

When I got my m1028 from the military It had the newer type 7digit milage odometer, which read 11,500. I have noticed that even after putting 35" tires on the spedo still reads about 10-15 mph faster than my actual speed, has this happened to anyone else or do you know how I can fix it??

 

[Prerunner1982]

that happens to anyone who puts larger tires on a vehicle. to change it you need to change the speedo gear in the tcase i believe.

 

[Elwenil]

Larger tires makes the speedometer run slower than actual speed, not faster. That's what makes his problem sort of weird. Might be an internal speedo problem. Does the needle bounce any?

 

[brgmchevy]

Possibly different axle gearing than the trans was originally mated to? Speedo gearing is matched to axle ratio. If different trans or axles, may cause higher or lower speedo readings? ie; 4.56 ratio axles in originally 3.08 ratio truck, trans from 3.08 truck put into 4.56 truck. Speedo thinks truck is going faster, than it is, cause drivetrain is spinning faster with higher(numerical) gearing.<img src="emoticons/icon_smile_shrug.gif" alt="Shrug">

 

[85-m1028]

I know there is gear adapter that is bolted to the t-case which the spedo cable end screws on. but I'm not sure what the internal parts are supposed to look like, I'm thinking that maybe there is a reduction gear missing from it?? or one got swapped in from a blazer?? It does look like the entire drivetrain was pulled at one time, the top end of the engine was painted black and the tag on the valve cover says 1987 HO35696 but the tag on the t-case says 1985 to match the vin, but the spedo in dash is different so it's hard to say

 

[dilvoy]

If I'm not mistaken you have one of the CUCV 1 1/4 ton pick up trucks. They all had 456 gearing as far as I know. The CUCV Blazers had 307 gearing. Usually it is the drivetrain or most likley the broken 208 transfer case swap that makes for these speedo problems. A pick up with a blazer transfer will read slower on the speedo and a blazer with a truck transfer case will read faster at the speedo. The solution is to change the gear that is held into the transfer case with one small bolt and a "U" shaped retainer. I just replaced a broken rear cover usind a parts tcase and checked to see what the speedo gear difference was between the truck and Blazer and found that the plastic gear that is attached to the output shaft inside the tcase is the same for both and the easy to change gear is where the difference is. That adaptor on your truck may have been used on another vehicle and not needed on your truck. Just pull it out and plug the speedo cable into the tcase and see what happens. It can't hurt.

 

[steelsoldiers]

I went to 3.73 gears in my M1009 so my speed was about 10-15 too fast which accounted for the roughly 20% change in gearing. I pulled the little gear box off of the speedo drive and hooked the cable directly. It ended up being about 5mph slow which was where it was before because I was running 33" tires instead of 31"s. So like dilvoy said, yank it and see what happens<img src="emoticons/icon_smile_approve.gif" alt="Approve">

 

[85-m1028]

How did you manage to hook the spedo up without the gearbox?? I pulled mine off and there was a metal shaft through the plastic gear, the end that sticks out of the t-case housing is round with a single spline, this fits into the gearbox and turns the gears, the other end of the gearbox has the square hole the spedo cable fits into. I'm not really sure how the spedo cable is supposed to fit without the gearbox?? the shaft in the t-case holds the gear in place?? so the gearbox is like a cupling ->)=(<-