Speedo calibration
Cargo Master
Joined: Jan 2015
Posts: 2,824
Likes: 39
From: Boise
Rasalas, a member on this forum had his dealer get it done. The 2 dealers in my area could not fix mine. SCT nor 5star has anything yet last I checked. Any dealer should be able to do it if one has. It is very frustrating my dealer can't fix mine.
Senior User
Joined: Aug 2009
Posts: 106
Likes: 0
My understanding, and I'm sure someone will correct me if I'm wrong, is that the dealer and Forscan can only reprogram up to the maximum size available from the factory (or thereabouts) without persistent check engine light.
I'm sure someone will chime in if this is inaccurate
I'm sure someone will chime in if this is inaccurate
Cargo Master
Joined: Jan 2015
Posts: 2,824
Likes: 39
From: Boise
Yeah, you're probably just over 10% off. I had 245's and now 35x12.50x20's, I'm 9.5% off, I was able to use the speedo check station on Interstate 5, near where I live.
Cargo Master
Joined: Jan 2015
Posts: 2,824
Likes: 39
From: Boise
My understanding, and I'm sure someone will correct me if I'm wrong, is that the dealer and Forscan can only reprogram up to the maximum size available from the factory (or thereabouts) without persistent check engine light.
I'm sure someone will chime in if this is inaccurate
I'm sure someone will chime in if this is inaccurate
Senior User
Joined: Mar 2004
Posts: 108
Likes: 0
From: Anahuac, TX
I used the Hypertech Speedometer Calibrator to recalibrate mine. It does require you to remove your instrument cluster. It wasn't difficult to use at all and worked great.
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Post Fiend
Joined: Apr 2004
Posts: 5,158
Likes: 35
From: North of Salt Lake City
Others have tried in FORScan but if you change much at all, the truck throws a code. Maybe you consider it a free extended warranty? 10%? That would make your 36,000 miles warranty a 39,600 mile warranty. Whoohoo! Mine is off by 5% and I just always do the math in my head. As the tires wear they get smaller and the adjustment is less but you can easily mentally recalibrate with a simple Whats-My-GPS-Speed app.
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Cargo Master
Joined: Dec 2003
Posts: 2,289
Likes: 2,036
From: Central MO
Reading 69 at 76 is essentially 10% (just a tad over, but close enough). So, at an indicated 50, you will be doing 55, at an indicated 20, you will be going 22, at an indicated 90, you're doing 99. Just add 10% to your indicated speed and you're good to go.
Ideally though, you would want it calibrated because so many computer-dependent things depend on speed and RPM values.
Cargo Master
Joined: Jan 2015
Posts: 2,824
Likes: 39
From: Boise
Simply using Michelin's tire specs of the 245 and your new 295/65/20, revs/mile are : 660 vs 592. That is 12%.
New User
Joined: Mar 2017
Posts: 8
Likes: 0
Yeah, looking at mine, 2017 Lariat which should have had the stock 275/65R18's on it (there were no upgrades to the tires on the window sticker) and the dealer had installed a 6" lift kit with 37/12.50R20's on it before I bought it. Explains why when I set the cruise control at 65 I feel like a rocket (I passed someone last night with my cruise set at 65 and I saw their digital speedometer said 69 and I was rolling right by them with no effort). According to what I have found online, I'm almost 15% off, yikes. But hey that will greatly extend my warranty (and totally screw with my MPG) :-) Downloaded some GPS based Speedometer apps and confirm today.
Laughing Gas
Joined: Aug 2016
Posts: 1,182
Likes: 176
Dealer did mine and was only able to use a factory available size's, main reason I went for the 295x70x18's instead of the 35x12.50x18's there 34.3 instead of 34.5 and the 265x70x20 are 34.1 tall so .2 is close enough.
Elder User
Joined: Sep 2016
Posts: 781
Likes: 6
From: Edmonton Alberta Canada
6% according to this calculator. I don't know why it's a big deal just go a tad under speed limit. My last truck had 12,000 kms more then speedo said at point of sale. Like money in the bank lol
Cargo Master
Joined: Jan 2015
Posts: 2,824
Likes: 39
From: Boise
