MPG loss with bigger tires

[muzzymaniac]

I recently put Grabber AT2's (255/70/16) on my Ex. I was expecting a dip in mileage but I'm seeing 3+ mpg loss! What are you guys seeing?
Now I'm faced with trading them in or sucking it up. I can still trade for 100% value towards different tires. I'm torn. I'm thinking of going with the HTS model since I rarely need off road - if ever. More like dirt road. The HTS have a nice low rolling resistance tread so then I'm wondering if I should say screw it and go back to stock or stay with the bigger size. Like I said, I was expecting a little loss.
So back to my original question - How much did you guys lose?

 

[rileyrs]

the tread will impact mileage very little.
size has a much bigger impact.

 

[Erik88]

I too recently upgraded from my 245/65/17's to Grabber AT2 245/70/17's(an inch larger diameter). I thought the same thing you just said, that there was a large mpg difference. So I started thinking about it a little more:

I don't know if i got 3mpg drop, but noticed about a 2 mpg drop. this is partially due to the tire size. Also, my speedometer, as I'm sure all Ex's, read abut 2-3 mph fast at road speed. Due to the gearing changes, my current tires read about perfect. The larger tire diameter "slows" the speedometer, and odometer, so it appears to be a larger drop in fuel economy... at least if you use gallons divided by miles driven, like me. (the computer would probably read off also)

Basically, the smaller tires probably got better fuel economy, but also the smaller size made it appear that you were getting better mileage than you really were. So if you lost 3 mpg, about 1-1.5 of that should be from the gearing changes affecting yourr odometer and mpg calculations. The rest is probably from the actual size(and weight) increase.

Hope that's not too confusing.

 

[corkey]

if the speedometer is wrong, than the odometer that gets it's reading from the same sensor has to be wrong too,,
so the data would be all skewed anyways,,

 

[KPSquared]

Bigger tires will screw up your mileage. Just make sure you are doing the calculation with the different final drive ratio that you've created by changing the tire size.... If your truck thinks you've gone 10 miles when you've really gone 5, you're gonna calculate WAY worse mileage.

First figure out how far off your speedometer is off, THEN figure your mileage.

 

[muzzymaniac]

Eriks got a good point and it was something that crossed my mind a while back but I forgot about it. My speedo was 3 mph fast with stockers. I'm dead on with the 255's at around 60 now.

 

[waskly]

just get a tune for bigger tires so the speedo works correctly.