Trouble Calibrating Speedometer for 33" Tires

maplemale

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I bought my 02 Excursion with 35" class D junk tires. They were about 50% worn and the speedometer was about 5% off. I tried fixing this previously by adjusting my tires to 32" using AutoEngenuity. Odd, it made things worse, not better regardless of which direction in size I went. If I went to 32", it would make it read faster. If I went to 34", it would still read faster. Feeling I didn't know what I was doing, I left it alone and lived with the 5% for a year.



I just switched to some nice Toyo Extreme 33" tires (new rims too! :naughty: ). Now of course it's about 15% off which is a major problem. Most annoying is how the odometer is now suffering. So, I started in again trying to fix the issue using AE. No dice. Same problem as before, only worse. I read about TrueSpeed and other inline modules. So, I tore the dash off looking for that. I took the driver's side off and looked everywhere all around the gauge cluster and back to the firewall. I couldn't find any sign of said device. I feel like there's got to be something installed somewhere from previous owner? Why else would it be stuck on 35" regardless of what I do?


Where else could said device exist? I've done a lot of work to this truck and have never seen anything like the TrueSpeed installed. I've looked around the driver's side battery, all around the wheel well and everywhere near where the computer goes thru the firewall. I've taken the driver's side dash off and radio out. I did not unbolt the gauge cluster, but I feel pretty confident there's nothing back there that I can feel/see. How do I find the stupid thing? Or, is there something else that could be going on here?
 

maplemale

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There's nothing physically on the VSS. Are you saying maybe it's an aftermarket/altered VSS?

So, I did some more testing tonight. I took my tire size all the way down to the minimum with AE and it did seem to have a tiny effect. Before, it was exactly 10mph off at around 60. Now, it's 8mph off, the the speedo says i'm going 70 when i'm going 62. That's a tiny change for bumping my tire size down 2 full inches in AE.

Here's the other thing that seems odd to me. If I pull up my "right front wheel sensor" in AE and drive, it is also off by a different measure. For example, i'm getting three different readings with the speedo at 70MPH

1. Speedometer: 70
2. Speed Sensor: 66.5
3. GPS: 62

:cursing:
:wtf:
 

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