Adjust speedo
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Junior User
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Adjust speedo
I put larger tires on my truck, but only by an inch. Stock tire size was 275-65/18 and I put 275-70/18 on it, according to the online calculator they are the stock width but taller by about an inch. I asked the dealership after they installed them to adjust my speedo. When I went to pick it up they said that they checked it and it was good, although it was stupid of me to trust them. So anyway, I checked it with against a gps and it was only off by 2 miles per hour. When I put enough miles on it to re-check my gas mileage it was extremely bad, now I don't care too much about mileage but I went from averaging 16 mpg to about 11 with only one inch larger tires. This can't be right so I called the dealer back and they told me that they don't have the program to adjust the speedo and referred me to some speed shop out here. What the hell? How does a dealership not have the program? They also told me that my tires weren't taller just wider, am I just dealing with an idiot or what? Is there any other way to adjust the speedo? I had a bad experience with tuners so they are out of the question. Any help is appreciated.
Postmaster
Joined: Jul 2010
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From: Henrico, VA
Well in my experience with speedometer is that if it is off by only a 2 mph then they won't adjust anything because that is considered within normal limits.
My guess is that the mileage calculation you did is off some where. When was the last time you actually check how many miles per gallon you were getting?
And about the dealership yeah you are dealing with an idiot there.
My guess is that the mileage calculation you did is off some where. When was the last time you actually check how many miles per gallon you were getting?
And about the dealership yeah you are dealing with an idiot there.
Elder User
Joined: Jan 2007
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I went to 1" larger diameter tires and actually found my speedo was closer to matching the GPS and local speed indicators. Anyway, if your speedo is off so is your odometer as well as the MPG calculator (assuming built in). A 1" increase does not sound like much, but is a 3% change which is not so insignificant.
While larger tires will tend to degrade your fuel economy, I suspect the change is not as bad as you think. I too found the stealership said it was unable to adjust the speedo which surprised me too. The local 4wheel shop cannot do it either without installing a programmer.
While larger tires will tend to degrade your fuel economy, I suspect the change is not as bad as you think. I too found the stealership said it was unable to adjust the speedo which surprised me too. The local 4wheel shop cannot do it either without installing a programmer.
Posting Guru
Joined: Nov 2009
Posts: 2,016
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From: NC
Here's an online calculator that will show you how much your new tire size affects your speed. Speedometer Calibration Calculator And Information
And the dealer is FOS. The IDS software can change your tire size. However, it might be limited to certain tire sizes for your particular VIN.
There are inexpensive scanners that allow you to change the tire size and GR (Speedometer Calibrator - Part #742003, Performance is our Passion - Hypertech). If required by the programmer, don't forget to account for squish (sidewall flex and tread flattening at the contact point which essentially reduces the overall circumference).
Please do not think I am supporting Hypertech as a tuner/programmer. I think their programmers are junk (and their tech support is worthless), but since you aren't interested in a tuner/programmer, the Hypertech might be the ticket.
And the dealer is FOS. The IDS software can change your tire size. However, it might be limited to certain tire sizes for your particular VIN.
There are inexpensive scanners that allow you to change the tire size and GR (Speedometer Calibrator - Part #742003, Performance is our Passion - Hypertech). If required by the programmer, don't forget to account for squish (sidewall flex and tread flattening at the contact point which essentially reduces the overall circumference).
Please do not think I am supporting Hypertech as a tuner/programmer. I think their programmers are junk (and their tech support is worthless), but since you aren't interested in a tuner/programmer, the Hypertech might be the ticket.
Posting Guru
Joined: Mar 2004
Posts: 1,195
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From: Sunapee NH
Just order up a Gryphon either with custom tunes or without. The tuner will wake up your truck a bit and allow you to adjust for your larger tires. If you do go with this route, 2566 seems to work well for me with my 275/70-18 Silent Armors
Senior User
Joined: May 2009
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From: North GA
Yeah I used my superchips tuner to change my tire size. I went from a 265/60/R18 to a 275/65/R18 and it was only off by about 3mph. Although now the abs and traction control lights come on every time I drive it and I cant figure out why. 
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I appreciate all the fast responses, I used the calculator posted and it said I would only be off by 2 MPH. And to be honest I think the speed is close to accurate, however I am on my second tank of fuel since replacing them and I am just got 180 miles on 3/4 of a tank. That is insane, and I calculate my mileage the old fashioned way with the tripometer and how much gas upon fill up. Now as said before, I understand it is a large truck and I knew the mileage would drop when I put larger tires on it. However, I drive like an old man the truck rarely goes over 2000 rpm's and I used to average 16 mpg. I just hope nothing is wrong that is causing this, the only other thing I did was replace the spark plugs with champions and put a k&n filter in it at the same time as the tires. This just seems insane for mileage. Any thoughts?
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Elder User
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Was that K&N filter or K&N CAI? Again, even using the old fashion calculation method, the odometer is providing erroneous mileage by ~3% (theoretically), thus your numerator is not accurate. That can only be a part of the issue. Perhaps the remainder is a combination of the larger tire (rotational inertia) and the computer needing to relearn your driving habits with the different plugs and new filter/intake. Dunno. Did you disconnect the battery for a period of time to reset the PCM after installing the filter/intake?
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Junior User
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I just went through my next tank of gas after disconnecting the battery to reset the computer and still got 180 miles on 3/4 of a tank. Also, I believe that I have a miss on the left side cylinder but no check engine light comes on. I revved the truck to 2000 rpms and it sounds like a miss from that side, however I am not experiencing a loss of power or drive ability. Maybe I am just over thinking this. I replaced one of the COP boots that was ripped during plug change and am thinking that I should replace them all on that side of the engine. What do you think?
Senior User
Joined: Mar 2008
Posts: 444
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From: Vermont
I think you may be experiencing more than just larger tires. I don't think a tire that size would cause his calculation to go down that much even with the inaccuracies. I'm no expert in this situation as I've never replaced my plugs but I would check each plug and COP to see that they are still in good condition and maybe try to diagnose the problem. I have heard of people losing gas mileage and having engine issues after switching to champions and that could be the case here too.
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Joined: Oct 2002
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From: Denver USA
Just put on some 275/70-18's on my otherwise stock 2006 F150. I haven't really seen any decrease in mpgs. Now I do have a tuner though and adjusted for the tire size. No GPS readings, but by the mph readers around town I'm within 2mph and my mpgs really haven't been hit.
Two quick questions for folks. Not to hijack the thread.
Two quick questions for folks. Not to hijack the thread.
Posting Guru
Joined: Nov 2009
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From: NC
Just put on some 275/70-18's what size were the stock tires (276/65R18)? on my otherwise stock 2006 F150. I haven't really seen any decrease in mpgs. Now I do have a tuner though and adjusted for the tire size. No GPS readings, but by the mph readers around town I'm within 2mph 2 MPH high or low? and my mpgs really haven't been hit.
Two quick questions for folks. Not to hijack the thread.
Two quick questions for folks. Not to hijack the thread.
- Stock were 32.1", new tires 33.2". My tuner has tire settings of 33" or 33.25", which should I select? 33", correct? Both of those are unloaded (no squish). You should multiply the 33.2 by .97, .96, or .95 to determine a loaded diameter. Does your tuner accept revs/mile? How about circumference in cm?
- How positively/negatively will this impact the odometer? If your actual speed is higher than your speedometer, your odometer will read low.
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