Changing tire size programming

[Crunchy]

A few months ago I changed the stock tires on my truck for something slightly different.  Noticed my gas mileage has gone down.  Figured it might have something to do with the change of tire size and the computer not reading correctly.  Does a dealership have to update the new tire size or can I do it some other way?

[trophyhunt]

Tag!! 

[netcoyote]

You have two issues. One, if you went to a larger size tire without an equivalent change in gearing you are making it harder on the drivetrain and engine because you've lost mechanical advantage that you had with the stock tires. A few tire sizes up usually don't hurt that much but it may affect mileage. Two, depends on how you have determined the change in mileage. Are you going by the on-board calculator or are you calculating actual fuel used between fill ups? Your on board calculator doesn't know how to compensate for the change in tire diameter.
You can get aftermarket devices that plug into the OBD port to flash the computer to compensate for change in tire size. It will recalculate shift point also.

[Crunchy]

Nothing dramatic on size, I went from a 295/65/20 to a 285/70/20

[jackelope]

Dodge and Ford experience here. The only thing the dealer can program to is another factory tire size. Not sure about GM, Toyota, etc but I assume the same holds true.

[jackelope]

How much has your mpg dropped? You sure it’s not just winter? Winterized fuel, colder weather, using defrost more could all ding you a little in the fuel economy dept. 

[netcoyote]

Nothing dramatic on size, I went from a 295/65/20 to a 285/70/20

Yea, that's not much of a change at all. Only works out to about 10 revolutions per mile less. Hardly enough to make any noticeable difference.  Could be your truck is more sensitive than some or, like someone else mentioned, maybe the shift in fuel blend.

[Crunchy]

Yeah could be winter.  Never had a diesel before so this is its first winter.  Find out in a few months when it warms up.  Dropped maybe 2-3 miles per gallon.  Part is probably warming it up in the AM when it is frozen outside.

[trophyhunt]

I bought my 06 gmc already lifted, stock tires to 35”, I really need a chip programmer for my truck!  There is a lot of different brands, expensive as well.

[Special T]

Fuel filters are cheap. Lots of crappy fuel out there. I keep an extra stock spare in the truck.

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[trophyhunt]

Fuel filters are cheap. Lots of crappy fuel out there. I keep an extra stock spare in the truck.

Sent from my SAMSUNG-SM-G930A using Tapatalk

I change my fuel filter every other oil change, and coach told me to add fuel additive with every tank. 

[Special T]

A fuel additive with anti gel, and some extra lubricant  is good.

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[Boss .300 winmag]

I have Smarty programmer that enables me to change tire size in the brain/computer of the truck.

I’m sure all the programmers have this option.

[jackelope]

99 times out of 100 when people complain about fuel mileage dropping, there's nothing wrong with the car. It's perception or it's the person not actually calculating it and only going off what the vehicle is telling them, which will only be reasonably accurate.
Check tire pressures, check air filter, if it's a diesel, replace fuel filters religiously every 12k-15k miles. Winter will drop your mpg in a diesel a bit for sure. I betcha that's your only problem. How many miles on the truck? Did you add anything to it other than the tires?

[Alchase]

Larger tires are heavier and have a higher rolling resistance.
Any increase in either weight or resistance will decrease your MPG, and performance.