Speedo (suddenly) reading high

[DaveO9]

This is a weird one, my speedo is suddenly reading high. Or at a minimum, I suddenly noticed it. About 14% or so. It'll be reading 50, but according to phone GPS (I have a speedometer app) I'm actually going 44. Similar percentage change across the range. I did a gear ratio swap last summer, 3.08 to 3.42, 11% change, in the right direction. BUT.... I had my tuner that I trust give me a tune for new ratio right after the swap. He had me measure tire OD and put new ratio in as parameters. And that was months and several thousand miles ago. It is possible I'm just now noticing this problem, but I don't think so. I was driving on a 45 mph hour road I regularly drive last week and freaked when I saw I was going 60! But then after slowing down I thought that sure didn't feel like 60. That's when I started checking it against my phone and found the 14% difference. I'm almost sure I would have noticed the difference long ago if it had been doing it since my gear swap.

Could the sensor in the transfer case be going bad or a problem with wiring? Some other problem? The digital speed readout in the DIC matches the analog gauge so it's not just a problem with the mechanical part of the gauge. In the days before noticing this I installed a new backup camera and ran wiring from the tailgate along the passenger side up to the radio. Had my aftermarket Apple CarPlay radio out, and of course to get the radio out you need to move the HVAC controls and the button panel for adjustable pedals, traction control, etc. out of the way. didn't disconnect any of those wires and I didn't touch any wiring on the driver's side.

 

[rdezs]

A bidirectional scanner that shows live data would be useful to see what your ECM says your speed is compared to what's being displayed. If it shows the correct data, I'll be looking at the instrument cluster itself.

But.... With the programming change, that itself raises some questions. There's a lot of bidirectional scanners out there that will let you make adjustments yourself for the tire size.

 

[Marky Dissod]

Obviously, better if your speedo reads faster than actual (slower than actual is potentially very dangerous)
but you need your speedo recalibrated in the ecm for the 3.42 and whatever new tire size you're using.

 

[j91z28d1]

honestly sounds like the
tuner missed the speedo cal. that and measuring tires doesn't make sense to cal for a gear change. it might have adjusted for tires since but not gear ratio.

hptuners makes it very easy since you know your GPS speed. you put your GPS speed in one box, put your dash speed in the other. hit calculate and it spits out the number you need to paste into the tune. no guessing or measuring. I think you should stop by your tuners and have him take a look.