Speedo Re-calibration?

[Mrlunchbox]

When I bought my Trac it had 255/70/16 tires on it. I upgraded to 265/75/16 for some time and am now back to 235/70/16 tires. When I had the 265's on the speedo was only about one MPH off from my GPS speed. I just noticed today that I am now almost ten, yes ten MPH off from the GPS speed compared to my speedo. The speedo is reading ten MPH faster than actual travel. I am curious as to why such a significant difference and if I can some how re calibrate it? Thanks!

 

[Phil Frady]

You can have the ECU flashed from the dealer for a price, or you can use a Xcal and do it yourself.

 

[Scott Tanner]

the smaller tires will ball your speedo cal up... your truck came w/ 255's, and a 4.10 rear, right?

 

[Mrlunchbox]

I am assuming it is the 4:10. I should look into that someday to see. It did come with 255 originally but I wouldn't think that would matter. I thought the difference would be from the different height IE going to 75 then back to 70 as even the 255's were 70. Maybe the dealer it is! haha

 

[Scott Tanner]

My '02 3:73 geared Trac had the 235's, and it read 3-4 mph high... When I went to 265's, the speedo was dead on accurate. Now, with LT245's, I reads 1-1/2 high.. I would imagine it matters which brand of tires you get... I prefer the larger tires, as the 235 look tiny in the wheel wells.. See my library for pics of both sizes...

 

[Mrlunchbox]

Ahh yes I see. Actually when I looked at it again my speedo is only 5 to 6mph off so its not too bad. I can live with it. These tires I just put on are used so I only plan on keeping them for a year or so. When I get my new ones i'll go bigger than current as these ones do look way too small! haha

 

[Phil Frady]

Based on what I see from your library pic, you have the package that included 4.10 gear which means your ST came with 255/70-16 tires. The 235's won't hurt and might "feel" a little peppy.