Speedometer Offset with 31x10.5 Upgrade

[Arctic 4x4]

My 90 T4R 3VZE Auto 4x4 (4.10 ratio) was fitted with 31x10.5's at some point before I bought it. With those larger tires, the speedometer has GOT to be incorrect. That means my ECU is getting the wrong speed reported to it as well, which maybe could cause it to shift at the wrong time? Sure seems like it's confused about what gear to use sometimes. Cruise control is a joke right now... lol it changes gears constantly while driving down the hwy.

I know that on a Ford you can get a replacement speedo-gear for the tranny that corrects the speedometer when you change tire sizes. They have a different color coded speedo-gear for various tire sizes that you may have switched to.

So is there some similar procedure for my T4R? I know that this thing doesnt use a speedo cable - its electronic somehow - so what do I do to make the speedometer read correctly with my 31's?

Thanks in advance. I tried searching the forums for the answer and couldn't find it. Go easy on the n00b.

 

[fourwd1]

1. Are you sure you have 4.10 gearing? The auto trucks are usually geared lower. Check the axle code on the doorjamb tag.

2. Marlin Crawler has speedo parts for the cable driven speedo, he may have something for yours, call them.

 

[Arctic 4x4]

Yes, I checked the door jamb. Already did the breakdown of what it all means, based on some info I found here in the forums. Here's what my door jamb says...

Model Code# VZN130L-GKPGEA
C/TR# 202/HA11
A/TM# G294/A340H

Factory Tire P225/75R15
Current Tire 31x10.50 R15 LT

So from the factory, it should be :
8" Gear
4.100 Gear Ratio
4 Pinion, Open Differential