Tire size to speedometer

[Rich Johnson]

I have a problem...

Logically, if you put larger tires on a vehicle the speedometer will read slower than your acctual speed...correct?

My 1008 has LT-285 tires,not the stock smaller LT235 size.

I decided to figure out my speedometer callibration by parrelelling my wifes 2000 mustang which still has stock tires on it. She was going 65mph, and I was showing 72 to keep uo with her.

My speedometer is reacting backwards to logic here. If I was doing 65pmh, shouldnt the speedometer show slower than 65? Not to mention, its at least 7mph off.

No wonder all the time I thought I was going 65 and everybody else was flying past me on the freeway, I was probably going 55 or something.

 

[rnd-motorsports]

even the mustang could be off the best way I find to check speedo is to get a navigator and drive with it they work off sat signal a lot more accurate as to you m1008 the transfer could have been changed at some point or rebuilt and a dif speedo drive installed dif brands of tires even same size could be dif just got a m1028 had generals on front 285/85/16 and goodyears in back 285/85/16 tires even being the same size the good years were 1.5 '' shorter in overall hight

 

[Barrman]

There is a reduction box on the speedometer drive off the t-case. That might have been swapped out for the wrong one some time in the past 25 years. Mine reads about 3 mph fast at all speeds on my M1009. I plan to go junk yarding next week to collect some more boxes and play with them until I find the right number for my truck.

I know I have seen post on here about the proper box and gears for each tire/axle combination. Try searching for it.

 

[Csm Davis]

check it with a gps most 1009s i have driven are off 5-10 mph

 

[Barrman]

I did use a gps. I have gotten so spoiled with the gps that I pretty much use it instead of the speedometer in any vehicle I drive now. However, there are times when you won't be able to lock it up and it needs to stay home. For that reason, I want to get my speedo as close as I can. Actually, to tie into the original post. I plan to add 33 tires to mine and3-5 mph fast on stock tires will be just about perfect once I get the bigger tires on.

 

[blitzengine]

There is a reduction box on the speedometer drive off the t-case. That might have been swapped out for the wrong one some time in the past 25 years. Mine reads about 3 mph fast at all speeds on my M1009. I plan to go junk yarding next week to collect some more boxes and play with them until I find the right number for my truck.

I know I have seen post on here about the proper box and gears for each tire/axle combination. Try searching for it.

I am running 31's on my m1009 and my speedo is also off be three miles acording to my GPS. Sounds normal to me, I think

 

[Croatan_Kid]

In stock form, mine read way too fast. I ran 11.00R-16s for a while and it was actually a lot closer and now with 40s on it, it's just about right.

Go figure.


Also, I've never seem any sort of reduction box...just the standard plastic drive gears on the tailshaft of the t-case.

 

[doghead]

When I re calibrated the speedometer on my M1008(after a lift and 37" radials), I discussed the best way to correct the speedometer. I was told by a couple transmission parts shops, NOT to use the gear reduction/induction boxes that go in-line on the speedo cable. They explained that they make much more strain on the plastic gear(main drive gear) that is in the transmission. That one gear is not cheap or easy to replace,

I simply confirmed my speedo error and calculated the correct driven gear needed(less than $20). Also in my case, I needed to change the driven gear's housing(they have a range they will work within), I tried one, and found I was still off(about 3 mph) so I ordered another, now I read within 1 mph correct.

 

[wallew]

try this gear ratio calculator...

YOU put in the tire size, etc you want to use...

Gear Ratio Calculator

realize that gearing and tire size determine your speed AND that your SPEEDO could be wrong (NOT related to anything else).

You can purchase new ones from LMC trucks, though they are six digit speedos - not the five digit OEM speedo - at least the ones I've seen.

Getting mine rebuilt means that my speed is dead bang on. My dually was 10 mph faster than it was going and my Blazer was 10 mph slower than it was going - NOW they are both correct within 1 mph.

Find a speedo shop and have yours checked. Mine was about to fall apart, so I got mine rebuilt just in time. They don't jerk or bounce any more, nor do I feel like they are squealing any more. And I thought it was my speedo cable making all the noise. It had already been replaced at some point in the past, so it was my speedo that needed the rebuild/replace.

 

[hovenga67]

I put a 6in lift and 315 75 r16 on my m1008. It is dead on now. I think they must have been off from the factory.
Brent

 

[big bad blazer]

yea m1009 tires are of because its on 33s when it says im going 45 im really going 55