Odo vs GPS / Speedo vs GPS

I went from 245's to 265/70r18's knowing my speedo and odo would be out by around 6.5% according to the math. The other day I had my Garmin GPS in the truck and at 100kph, the GPS showed I was actually doing around 106-107kph. 'Very good', I figured. That was the expected difference. Yesterday I did a highway run and decided to compare the odo. After 10km on the odo, I did 11 on the GPS. After watching it for 50+kms, I confirmed the difference was 10%. But the speedo difference was still ~6.5%.

Could the odo and speedo in the truck be out of sync? If indeed I am getting 10% extra distance versus 6.5%, my gas mileage is not as bad as I thought it was.

 

The speedo's are off from the factory, so your problem might be an extension of the original issue. There's some posts about the speedo if you search for them.

 

I found almost the same thing when I put on my 265/75 tires. The odo was accurate with the OE tires and the speedo was 'optimistic' (read faster than I was actually going). With the new tires, the odo is off and the speedo is almost dead accurate, as calibrated against GPS.

Oh and do GPS distance traveled over a fairly long trip with few or no tight turns to reduce inherent GPS inaccuracy.

http://www.gps-forums.net/accuracy-gps-measuring-travelled-distance-t8177.html

 

Yep, from factory odo is accurate and speedo reads fast. Here's a link to the acceptable speedo error on the Tacoma from the FSM:
http://www.toyotanation.com/forum/showthread.php?t=356029

 

I figured my speedo is normally about 3 mph to fast.

 

Mine is as much as 5 mph off up around 65-70 mph. The problem is that it's not linear...i.e. it's accurate around 30 mph, off by about 2 mph at 45, and off as much as 5 around 65 mph. I actually complained about it under warranty but they said it was within limits...I assume their specification is at like 50 mph where it is in fact within spec.