Kleinsmith

Wow, looks like I picked the wrong day to join the forum! Looks like someone's really messing with the place.

I'm sure this has been discussed a lot and as I get familiar with the place I'll figure out where to look so bare with me.

I recently switched to a slightly larger tire size (LT 285/70 17's) they seem to be about an inch taller than factory. My dealer in The Dalles, Or. told me that the computers on these newer trucks can't be changed to reflect different tire sizes. Is this true?

Weird thing is that the speedo is dead on compared to a gps. I'm not sure about the odometer yet but figured it must be off a little. I also noticed that my computer is showing me at about 1-2 mpg lower on average ever since the tire change. Could this be related?

Thanks

TJPlatinumEB

Just about an inch taller than factory, maybe a bit less. The computers can absolutely be programmed for whatever size tire you want it to be set to. Ford dealers are limited to only what size factory tires they trucks came with.

What do you mean it is dead on compared to a GPS? Do you mean you have a GPS running and it matches? If so, then great, no worries. If it doesn't match the GPS, then you need to find a way to have it reprogrammed or else everything will be off.

Lastly, larger and heavier tires will always kill mileage. Most see anywhere from 1-2 when they do that. I lifted mine up 4" from factory 2wd height and put 75lb 33" tires on it and lost about 1.5

Kleinsmith

Thanks for the reply. Yes I compared my speedo to an app on my iphone and at 70mph it was a match. So if I'm a match now then my truck must have been off with the stock tire size, which would have given me a false mpg reading as well. Looks like I'm fine and don't need to change my computer settings.

It was my impression that the dealer could get into the computer to change the tire size setting. My dealer service guy also told me the only way to change that setting was to buy the aftermarket performance programmer. But warned me about using one to change performance settings. The on board computer records all those changes and even if it was set back to stock mapping the computer keeps track of any changes and Ford can go in and look at it, which would void the warranty. I was interested in the Livernois tuner http://www.livernoismotorsports.com/product/LPP631102 but now I'm rethinking that for fear of loosing the warranty.

Thanks

Wiggum

I can't recall the allowable percentage, but there IS an allowable percentage that the speedo can be "off" on a vehicle. So the truth of the matter is, you may drive a brand new vehicle off the lot, and while you're going a TRUE 60mph, your speedo may read anywhere from about 57mph to 63mph, that's within the allowable tolerances.

But if you have a GPS, and trust it ... and it shows you going 70mph when your speedo says 70mph, then your tire change did you a favor.

130428

I believe that for the 15 & up, the dealers can NOT change the stock tire size, so your service guy could be right.