New Larger Tires, Fuel Mileage and Computer Calibration Help

ntrimm · December 9, 2013

I searched this forum and google as well before I decided to start a new thread but had no luck finding my answer.

So, I just put on new, larger tires today and a 2" level kit. I went with the Toyo AT 2 XT in 295/55/20 and kept my stock 20" wheels. The stock tires are 31.9' tall and the new Toyo tires are 32.8" tall and have 21 fewer revs per mile. My fuel mileage on my trucks screen has seemed to take a huge hit. I expected to take somewhat of a hit on mileage but not this much. My questions are as follows.... is the mileage on screen inaccurate due to the change in tire size, which would mean Im not really taking that big of a mileage hit?? Where can I get my computer recalibrated to reflect the new tire size?? Thanks in advance for any help you all give me.

geektoad · December 9, 2013

The calculation may be off a bit, but I'm willing to bet your new tires are significantly heavier than your old ones, and heavier tires are right near the top of things that will kill your mileage. Rotational mass is a killer to performance and efficiency.

Jon A · December 9, 2013

Yes, your actual mileage (as accurately as the computer can calculate it, anyway) is whatever the computer says times 652/634 (or 1.03). So at least part of your noted reduction is simply a miscalculation. In order to fix it you need to reprogram the computer via a custom tune, Hypertech, etc.

Stex · December 10, 2013

Uh, check your mpg the old fashion way. Use the same pump and park same and stop when pump clicks off. tire size not that far apart.

juanlpadilla · December 10, 2013

Any rubbing with that size tire?

geektoad · December 10, 2013

Again, it has very little to do with size, and a WHOLE lot to do with weigh. How much do your new tires weigh vs. your old ones?

ntrimm · December 10, 2013

Any rubbing with that size tire?

No rubbing at all. Perfect fit and completely changed the looks of the truck. Much nicer stance.

TJay74 · December 10, 2013

Uh, check your mpg the old fashion way. Use the same pump and park same and stop when pump clicks off. tire size not that far apart.

Hand calculations will do nothing as the mileage will be off on the trip/odometer which is what the hand calculations go off of.

Miles driven / gallons to fill up.

If the new tires are only showing 350 miles on a full tank of gas when the truck actually went 400 miles it would throw the calculation off. Until the tire size is corrected in the ECM there will be no way to tell the mileage in order to get a accurate gas mileage calculation.

TJay74 · December 10, 2013

Here is the error rate for his new setup:

Old - 632 revs per mile

New - 615 revs per mile

2.7% difference

At 60mph on the new setup the truck will only be doing 58.35mph. So that means when he drives 600 miles on the new setup he will only have put 583.5 miles on the truck. So roughly for every 300 miles he is short 9 miles, or 3 miles per every 100 miles driven.

Jon A · December 10, 2013

Again, it has very little to do with size, and a WHOLE lot to do with weigh. How much do your new tires weigh vs. your old ones?

He was specifically asking about errors in the computer calibration. Those are 100% due to the change in height. Will heavier tires also cause a reduction in actual mileage? Of course, but this was not his question. Without first getting this sorted out he won't even be able to calculate his actual mileage.

While this change was relatively small, 3% still means .6 MPG if you're somewhere around 20, which is certainly noticeable. And this same error will still show up if mileage is calculated manually (the odometer is still 3% off).

As for what things affect actual mileage, they are many. Weight is big, especially in city driving. Rolling resistance and aero drag are the big ones on the highway.

kstruckcountry · December 10, 2013

Object in motion remains in motion. IE if you're moving at a constant speed tire weight has no effect. What matters is tire compound, width, and OD. Just my experience going to tires more than double the weight of my original tires. No change in MPG because I went with a similar size.

DanielAcosta · December 11, 2013

My tires are even bigger than yours, so I went with a Hypertech which restored my shifting and AFM operation. My error was 8%, my mileage improved about 3-4% from the Hypertech. Your error is so small you might want to just live with it and realize your mileage is actually 3% higher than you think. If you've got an extra $200 laying around, you'll like the Hypertech. Details in my signature block. Good luck.

KUUG · December 11, 2013

I went with a 305/55r20 and aftermarket wheels. The biggest problem I'm seeing is the truck wont switch over to V4 as often as it use to; this is the main reason I'm looking to get the Hypertech soon.

I ran my Garmin on the drive home yesterday; truck showed 2 miles less on the trip odometer than the Garmin on a 42 mile drive. Every 20 miles I'm clocking 1 miles less than actual miles. Tomorrow will be my first full tank of gas so I will be able to do accurate hand calculations using the trip od on my Garmin

I think the biggest problem is getting these things going with the weight added. I'm just hoping I can get somewhere near 20 mpg again. Right now I'm only at 15.5-16.5 on the highway.

Dnt · February 19, 2014

Gas mileage can be pretty good on these 2014's I have run a couple of tanks of nonethanol 87 octane and have been easing up on the lead foot a bit. I think most people do not buy a truck for the fuel mileage and I did not either but have been impressed with it so far. 2014 GMC Sierra Crew Cab SLT Z71 4x4 5.3 3.42 20in all terrain tires typically around 400 lbs of materials in the bed plus I have a Tonneau cover and Bedrug. First 2000 miles were a lot less due to a couple of trips up North into Ohio where I used the auto 4 wheel drive and bought E85 fuel plus when the interstate cleared I was pushing 80mph most of the time and it seems that there is a HUGE mpg drop with these trucks at anything over 70mph. I think I am going to install a set of LRR Michelins whenever I need a set of tires. I am not a big offroader myself and I like the longevity and smooth ride of the Michelins and I bet they will save some fuel over these Good (Bad) Year Wrangler all terrains.

TxTruckMan · February 20, 2014

All correct. The only "error" is the 2.7% mileage difference. Not worth the money of a tune in my opinion, but maybe the dealer can fix it for free for you if you really want it. Otherwise I wouldn't worry about it.

Your mileage probably sucks because of the heavier tires and lift. The heavier tires are harder to get going and stopped (think city mileage and some minimal brake wear), and the increased lift will slightly hurt aerodynamics (highway mileage).