I bought a 98 durango, stats:
5.9L
awd/4x4
auto(obviously)
147k miles.
Everything is great, no problems yet except the gauges.
So I noticed that I was going way faster then everyone else the day I got the rig and figured that my speedo was a little off so I just slowed down and then got pulled over...
So I called my friend and had him drive 35 and drafted him. My speedo read just over 25...so 10mph off at any speed. We went to 45 and once again just over 35 on my speedo.
So I did the whole push the trip button on the dash while I turn the key and read all the gauges and here is what I got:
Bat-9.5, 16, 18(all good)
Cool-140, 240, 260(only off in the middle, weird?)
Tach-900, 2800, 5750(all off)
Speed-14, 49, 118(all off as well)
Oil-35, 55, 110(all good)
Fuel-just under 1/4, just under 1/3, full(only the first two are off. Dosn't bother me as much as the tach and speedo)
So what do I do?
I am running 31's and the stock gears...I did notice today that my tach has adjusted itself and is now just above 500 after it is warmed up and at idle...

 

are 31's the stock tire size?

 

I am not sure, it does have a factory tow package and factory fender flares so I was told that 31's came standard. The weird thing is everyone I have told asked me if any lights where on, I know that the dash is a common problem but I haven't found anything like this...

 

open your drivers door and read the sticker on the back below the latch, it has your stock tire size. if its still there then you should have a sticker in the glovebox with info but I dont remember what is on that one.

if your tires are larger then stock then you will travel faster then your speedo tells you are going.

 

I am going faster then what my speedo says, which is why I mentioned my tire size. If I was running 33's or something this wouldn't be a problem but ony 31's? Ill have to check tomarrow after I pick up my rig again. Had to take it back so they could install parts they ordered.

 

I thought I just said that. example, my durango has 235/75/15's with 3.55 gears stock, if I put on 31's I will be traveling faster then my speedo says so yes I could be doing 35 when my speedo says 25 if I have to large of a tire. if your door says 235/75/15 and your glove box says 3.55 gears and you have 31's on those stock 3.55 gears your speedo will read slow.

 

Yea I was agreeing with you, lol
I was just saying that switching from a 235 to a 31 isn't a huge jump. It should only be 5% off. But no matter what speed I am going it is exactly the same off. So at 25 I am actually going 35, and at 35 I am actually going 45. If it was just a tire thing then when I was going 25 my speedo would read 23 or something like that...

 

+1 RollerRed

31's while not throw it off that much it could be the speed sensor

 

I thought about that, but wouldn't the speedo just not work? Or start working at a certain mph? I am just going to change them both anyway but...idk. Just really weird to me. I am trying to find somewhere, or someone that has a plug in programer that I could use to just change it myself. lol

 

no the speed sensor could be miss reading the gears, check/change the tranny output speed sensor, and the rear wheel speed sensor (on top of the differential) and pull the diff cover and check the tone ting around the outside (it looks like a big gear around the outside of the big drive gear, if it was missing teeth in it that could be your problem)