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.

[Crunchy]

The truck has 9500 on it. I will look into some kind of fuel additive. Fuel filter has another 22% before the comp says they are needing to be changed.

[Alchase]

The truck has 9500 on it. I will look into some kind of fuel additive. Fuel filter has another 22% before the comp says they are needing to be changed.

Crunchy, what vehicle and motor?

Only 9500 miles, and you saw a noticeable MPG decrease?

Shouldn't it be under warranty? I would have them check it out.

[Crunchy]

Ram 2500 with the cummings.  if I had to guess it would be a combo of the tire size, cold weather with more warm up time idling, and little more city driving than usual.

[Alchase]

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.

For a Ram 2500 with the Cummings, I would bet the tire change was insignificant to the issue.

[Crunchy]

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. 

which additive did he recommend?

[Special T]

I went up 8% in tire size and saw  a decrease in Hwy milage...
I would bet that that 22% sat is based off mileage, just like the oil change stuff is. OR  idling your your Cummins to warm it up is having an effect on your Emmisions garbage.
 The fuel filter on my dodge is less than $25. Any noticeable difference in mileage pays to put in a new one.

All that computer calculated output is just bells and whistles. Unless there is some kind of fuel drag/resistance sensors I'm 99% certain it's based on mileage, just like how check engine light tend to come on during service intervals. 2c

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[jackelope]

The truck has 9500 on it. I will look into some kind of fuel additive. Fuel filter has another 22% before the comp says they are needing to be changed.

Crunchy, what vehicle and motor?

Only 9500 miles, and you saw a noticeable MPG decrease?

Shouldn't it be under warranty? I would have them check it out.

He put bigger tires on it and it's winter.
There's nothing wrong with the truck.

[jackelope]

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. 

which additive did he recommend?

Maybe a quality cetane booster?

[Woodchuck]

I run white bottle Power Service in my truck, every tank. fuel filter once a year.

[jackelope]

I went up 8% in tire size and saw  a decrease in Hwy milage...
I would bet that that 22% sat is based off mileage, just like the oil change stuff is. OR  idling your your Cummins to warm it up is having an effect on your Emmisions garbage.
 The fuel filter on my dodge is less than $25. Any noticeable difference in mileage pays to put in a new one.

All that computer calculated output is just bells and whistles. Unless there is some kind of fuel drag/resistance sensors I'm 99% certain it's based on mileage, just like how check engine light tend to come on during service intervals. 2c

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That fuel filter % is based off of a combo of miles, engine hours and engine work load.

Same way as our oil change intervals work now.

[trophyhunt]

With my truck going from stock to a 6” lift with 35”x 12.50x20”, do you guys feel it’s pretty important to buy a programmer ??

[jackelope]

Your speedo is definitely not accurate.  I would assume you're losing some performance because of it too. Shift points will be off, etc.

[trophyhunt]

Your speedo is definitely not accurate.  I would assume you're losing some performance because of it too. Shift points will be off, etc.

yes, Speedo is off, it still shifts nice but I’m positive it could be better. My diesel mileage sucks, about 11.5 per gallon pulling a loaded trailer or not.

[Crunchy]

Your speedo is definitely not accurate.  I would assume you're losing some performance because of it too. Shift points will be off, etc.

Dealership job, or other options?  When I first got the truck I was averaging about 16, after it broke in I was getting nearly 20.  Now it has shot down to about 16 again.  Other than tires, and an oil change nothing has changed.

[horsehunter509]

The truck has 9500 on it. I will look into some kind of fuel additive. Fuel filter has another 22% before the comp says they are needing to be changed.

I never run mine to 0%. Normaly do fuel filter between 20-25%. Not worth pushing it any further for me.

[jackelope]

Your speedo is definitely not accurate.  I would assume you're losing some performance because of it too. Shift points will be off, etc.

Dealership job, or other options?  When I first got the truck I was averaging about 16, after it broke in I was getting nearly 20.  Now it has shot down to about 16 again.  Other than tires, and an oil change nothing has changed.

As far as programming for tire size, as far as I know, a dealer can’t program for an oversized tire like that. Only another factory tire size. I can tell you for sure that Dodge and Ford dealers can’t do it. I assume the same for a GM dealer.

[jackelope]

Your speedo is definitely not accurate.  I would assume you're losing some performance because of it too. Shift points will be off, etc.

Dealership job, or other options?  When I first got the truck I was averaging about 16, after it broke in I was getting nearly 20.  Now it has shot down to about 16 again.  Other than tires, and an oil change nothing has changed.

Are you actually calculating it or going off what the truck is telling you?