Did a search on all the speedo posts, didn't see any quite like this one.
Mine is reading about 90 mph when going about 40mph. Everything else seems ok. Odometer even seems correct. Speed sensor going bad? What do you think? Thanks.
94 Ram 1500 5.9, 2WD, auto.

 

There is a speed sensor that measures the rotation of the driveshaft (number of revolutions) but more than likely its in the PCM and a scantool from the dealer will have to check and set the pinion. My truck reads too slow and I have to get it calibrated properly. I have a connection at a dealership who will help me out.

 

I should have also pointed out that it is intermittent, comes and goes with no predictability. Besides the PCM, could a failing speed sensor create this? I'd like to avoid the dealer if possible, I'm poor enough. Also I checked the trip/odometer with GPS and they're good even with the needle way off. Wierd.

 

Could be a dirty sensor. If you download the FSM from the Merry Christmas Thread, you'll be able to locate it. It couldn't hurt to remove it and clean it up with some Brake Kleen. Check all of the connections as well and probably use some di-electric grease on them.

 

i think your 94 has a traditional plastic gear in the transmission and a cable between speedo and trans that spins on the inside. if so, i used to have to remove that cable occasionally on my old chevy and grease it. also i replaced the plastic gear once where it attached to the trans. find out what your setup is, and watch for diff between later models like 01 and your older 94.

 

Did you just replace the tires to a bigger size?

 

40 to 90 from bigger tire size? and being intermittant??...doesn't sound like a tire problem...but i could be wrong...but 50 mph difference in tire size is quite a bit....like 4 to 9 is understandable....maybe even 10.....doesthe speedoshake at 90 or is it steady? when does the problem happen...early in the morning, at startup, on cold days, ect. if you think it's a speed senser, you could just replace it and find out, they aren't expensive.....could very well be a speed senser, but i think it would be jumpy...i could also be a ground somewhere that is screwing it up...

 

"Did you just replace the tires to a bigger size?"
It's had 285/75/16 Goodyear MTRs for about two years now, they actually cause about a 10% decreaseon speedo.
"doesthe speedoshake at 90 or is it steady?"
Smooth as silk.
"when does the problem happen...early in the morning, at startup, on cold days?"
Not many cold days in Fl. in June(all year for that matter). It seems to be after a few miles, engine, tranny etc. up to normal operating temps.
Like I said before the odometer/trip meter are registering properly, I thought all that stuff ran together, like a clock?
This is not a major issue like some folks have on this site, just an annoying one.
dhvaughanbrought upa good point, does this thing have a traditional gear/cable or electrical sensor/sending unit?
Thanks again for responses everybody.