Tire size change and Mileage

[Travii]

I haven't seen this posted yet, if so, please direct me:

I was rescently able to get my new set of tires and decided on the BFG ATs moving up to 31" tires. My average mileage before the new tires was about 16 to 17 mpg (thats average with no change in driving style). Now my average mileage is 12 or 13 mpg. I know that going bigger would effect mileage, but didn't expect it to do this much.
I am curious what changes others have noticed after moving up to 31s or even larger.
Just interested in knowing if there might be a problem somewhere or maybe something that could be done to help the milege... I just didn't think that with only an inch of change could lower mileage so much..
I see the pole done earlier showed that most people have gone to 31s and 32s, what happened to your mileage?

[OffroadX]

That is a pretty extreme drop, more than I would expect. However, factor in the tendency to be a little heavier on the throttle to get the same "punch" from the engine to make up for the taller overall gear ratio and you'll get something more than the mathematical mileage change. Also remember that your speedometer is going to read a 3-5 MPH slower at highway speeds and the increase in wind drag from doing 75 at an indicated 70 will eat into your mileage a bit.

I assume you went from 265/70R15 to 31x10.50R15. The old tires are about 93" circumference to about 96.5" for a 3.7% increase. However, remember that your recorded mileage will be about 3.7% less than the distance you actually travel, which will cause mileage calculations to be erroneous, so it's not really as bad as you think. Multiply the miles your odometer logs between fill-ups by 1.037 and use that in your calculations and you'll get a more accurate reflection of your true mileage. Say you log 220 miles and fill up with 17 gallons of gas, that = 13 MPG. Adjust your miles by 1.037 and you get 228 miles, for 13.4 MPG. Obviously not much of a difference, and still too low even when factoring in the two points in the first paragraph. The tires are almost exactly the same width as the 265s, have a pretty good footprint and don't have a soft compound, so rolling resistance shouldn't really be a factor. I'd expect something in the 15-16 MPG average from the change, but 12-13 doesn't add up.

Brent

[kaybeejay]

I went from the stock grabbers to the BFG AT's and I still get about 250-260 miles per tank. I will up at 15 or 16 gallons so that equals about 16 mpg. I pretty much always have gotten this mileage since day one with the Xterra. I try to drive conservatively at all times. Thats odd that you would experience such a large drop in your mileage though. Its a mystery if you ask me.

[XID]

I know this was an old topic, but I wanted to relay my experiences.

When I upgraded my stock Grabbers to Discount Tires Pathfinder (245/75/R16) I noticed my speedo and odo were off by about 10%. At 30MPH I was really do 33, 60 MPH, doing 66. Also my for every 1 mile traveled the odo registered .9 miles. Get it out on the highway and mesure between the mile markers, or better yet, watch a GPS while you're driving, both methods confirmed what my 10% reduction. When I adjust for the 10% my milage really didn't change much.

Hope this helps.

Andy

[rrdstarr]

That is quite the gap in difference! My GPS shows me going 70 mph with 31" tires. The speedo shows 67-68mph

[ChuckH]

I have about a 8% speed differential with 32" M/T's. So if my speedo says 60 I'm really doing 65-66 according tot he GPS. WIth my 31" A/T's on the difference is about 3%. Is a 245/75/16 about the same size as a 32" tire? If it's smaller then your speedo was probably off to begin with.

[XID]

You may be right, the speedo might have been off a little before. I don't seem to remember the GPS reporting a different speed than the speedo when I had the Grabbers, but that was 1.5 years ago and you know how the memory goes....

The 245/75/16 tires should be about a 30.5 inch tire. 245mm = 9.65 Inches * .75 = 7.2375 * 2 = 14.475 + 16 = 30.475.

Did I do my math correct on the tire hight??

Andy

[sis27]

What GPS's shows how fast you are going. My girlfriends car seems to go slower than the spedo says. I clocked it with a stop watch going .9 miles for every one mile. I just want to conform this so she doesn't get pulled over for going to fast. Can anyone suggest a GPS that i could temparly use in her car and later move into my truck.
Thanx
Simon

[sis27]

Forget what I just said, I just looked it up on CNET. Gonna get a Magellan SporTrak or SporTrak Map, unless someone can suggest another good GPS for that price range
Thanx
Simon

[Schludwiller]

Everyone I know uses Garmin, but take it for what it's worth.

[OffroadX]

I think all GPS units will report your speed. I'd be surprised if one didn't. The Delorme Earthmate is the only one I can think of, it is only designed to work when connected to a computer.

Brent