Engine/Transmission (1994 - 1998) Speedometer 5mph fast

[DRFriend]

My speedometer consistantly shows 5mph faster than I'm actually going. Bothersome because the truck shifts into overdrive and lockup early because it thinks its going faster than it actually is. I have a new speed sensor (haven't installed it yet), but before I start just replacing stuff I wanted to check if there's any solid troubleshooting for this. Anyone have any info?

 

[cerberusiam]

Tire size not OE? You will have to chnage the gear on the speedometer adapter to change the reading if the sensor doesn't fix it.

 

[DRFriend]

Unfortunatley, the tires are OE size. Gears are stock too.

 

[Bob4x4]

Did it just now become inaccurate or did you just now notice it?

 

[cerberusiam]

Could be the speedo if the sensor does not fix it. Is the odo off also or just speedo?

 

[DRFriend]

I just noticed it because they recently put speed reading, speed limit signs around where I live. Checked the signs against my other car and they're dead on. OD works as it should.

 

[CRodi]

My '97 does the the exact same thing. I noticed it when I started using my Garmin GPS. Which one is accurate? The speedometer or the GPS? I would put my money on the GPS. I'm running stock wheels and OEM tires (LT 245/75R16). I never thought much of it, until you mentioned the lock up and overdrive occuring prematurely. Are you concerned that your engine is lugging? This is my first diesel, so it's hard for me to tell if it's lugging or not. I've experienced lugging and pinging in gas engines and based on that experience, I've noticed no adverse effects on my '97 diesel.

 

[Bob4x4]

The gps is what I use when I dial in a speedo.In years gone by our speedos were always off a bit.Now they tend to be very accurate.I did a 2012 the other day with 285.s.My verification test drive showed the speedo dead nuts on.verified by 2 gps units I had in the truck.If your truck has been untouched and has not changed recently.change the gear at the transfer case.

 

[DRFriend]

Lugging.... No weight and on level ground at 45(indicated) it's ok, but it will a little depending on the situation. I can definitely tell it would be happier going into lockup 5-10 mph/few RPMs later. My thing is that it's meant to hit lockup at a certain speed which equals a certain rpm which is already kind of low. There's even a TSB out about replacing the PCM with one that can be flashed to bring the lockup speed to around 50+Mph. I could rig a lockup switch, but I'd rather things work stock before doing stuff like that. Anyway, I'd like to get it dialed in the way it is. I think I could do without a PCM upgrade and spend a little less time hitting the OD off button.

I was afraid someone might bring up that speedo pinion gear. Where can you find those/the right one?
Tried the GPS btw, still 5 off.

 

[DRFriend]

That TSB is for 94 and 95 only btw.

 

[hsmith]

Mine was off also from day one. Switched from 215's to 235's and now it is dead on with the gps. The reason I switched was not for speedo correction, I wanted more rubber on the road. Two birds, one stone.

 

[LandShark]

My speed was also off since truck was new. Since I switched tires from the stock 245's to 265's the speedometer is now within 2 mph of correct.

 

[Bob4x4]

A quick search of the web for ram pinion chart brought up a nice chart. Finding the part should be just as easy

 

[Cummins12V98]

My new to me 98 had the stock size tires on it and the speedo was right on. I think the previous owner went from 3:54 to 4:10's not sure if that had anything to do with it. Since I went with the stock size 3rd gen BFG's I am now 5% slow on the odo reading. So I multiply my miles driven by 1.05 to get the correct miles driven then divide by gallons used to get accurate mpg.