Tire Type and MPG

[asanelli]

I am wondering if anyone can help to determine if a 265/70/17 AT (all-terrain) tire will give me worse MPG and than a HT (highway) tire.

I recently put AT 255/75/17s on my truck which are bigger than the aforementioned 265/70/17, but my MPG has dropped about 15%.

I wondering if smaller AT tires will do that same? Or just less> Or have i answer my own question?

What is your mileage with 265/70/17 ATs?

 

[RonJR]

4 things affect your apparent mileage when you put on AT LT-rated tires:
- Your new tires are bigger than the old ones, by about 3% (32 vs 31 inch). That will cause your computed mileage to be lower by 3% unless you correct for the odometer error. So, unless you've taken this into account, your 15% mileage hit is actually about 12%.
- LT tires, particularly load range E, have much stiffer sidewalls. For the same tire pressure, that translates to more sidewall heating and consequent energy loss - thus lower mileage. This is probably the largest single factor causing actual reduced mileage with LT tires. You can offset that by running the tires at higher pressures. Running at least 40psi will help.
- LT tires weigh more, and since it's rolling mass, the extra weight has about 2x the effect of dead weight cargo. So, if your new tires weigh 10 lbs more each, it's like adding 2x10x4=80 more pounds of cargo that you're carrying all the time. If you do a lot of stop and go driving, this can have a noticeable effect.
- The aggressive tread construction on an AT tire causes more air friction losses at high speeds.

 

[asanelli]

So how you calculate true MPG with large tires?

 

[RonJR]

You have to figure out what your odometer error is first. It's easiest with a GPS. Go out and drive 10 miles on your odometer, and see what the GPS says. If your GPS shows 10.5 miles and your odo 10, then you have a 5% odometer error - your odometer will show 5% less miles than you actually have gone. So, to compute gas mileage, take your odometer reading, multiply it by 1.05, and then calculate your miles per gallon.

To summarize:
Odometer Factor = OF = (GPS reading)/(odometer reading). Write that down.

At the pump:
MPG = (Odometer reading * OF)/(Gallons pumped)

 

[gdvan01]

FWIW...

2017 SR5 Premium

I put Bilstein 5100's on all corners, front @ .85 and swapped out the stock Bridgestones for Cooper Discoverer AT3's, E-rated, LT265/70/17's. Inflation is at 44.

This was 2500 miles ago and when I got home today I was at 19.8 MPG. My daily drive is about 32 miles roundtrip predominantly at highway speeds. I have made 2 NoVa trips in this 2500 miles as well, that's about 300 up and back.

Before switching the above I think I was about 19.6, bone stock. Maybe the added highway trips contributed to the current MPG but still...if you don't drive like a maniac your combined HWY/CTY average is decent.

.02