10.1 MPG!!!

You are assuming wrong....Filled up this morning and again when i got to work. Did not calculate difference in mileage with tires. How the hell could I come up with a calculation by just guessing?

 

Even with my 35s I still get about 15-16mpg at 70 and about 18-19 at 60-65mph. I can't see actually getting 10 mpg without towing.

 

OK, I'm guessing you've got a couple things going on.

1. Yeah, you just get less mileage with the bigger heavier tires.

2. The new tires have bigger diameter. As Seth mentioned above, add 6% to your result. For my specific tires, my personal measured correction factor is 7.4%.

3. You're checking after a low-distance fillup. Gas stations vary as to where they "click off" and so if you "topped off" at home and got a short fillup, then "topped off" at work filling up right up to the neck, that can easily be a difference of a full gallon or so. That makes a big difference on a 38 mile trip - not so much after 300 miles.

4. Lead-foot syndrome. Especially if you've got a four-banger, the engine has to work much harder to spin up heavy wheels and tires - accelerating your rotating mass requires more energy than accelerating non-rotating mass carried in e.g. the cab or the bed. Try easing up on the jackrabbit starts.

I can get 10mpgs or less on a single tank when I measure short tanks on a day of wheeling, filling up before and after visiting the OHV park. But never on a complete tank.

My total cumulative MPGs on my 285/70R17 Goodyear MTR/Kevlars is 16.5 corrected, after about 8000 miles.

If you're really getting 10 - I'm guessing as was suggested above that the tire change somehow caused your parking brake to stick. Check rear wheel hub temperatures with your hand and see if they're hot. Warm to borderline hot is OK, but immediately-blister-your-fingers-scorching-hot is NOT OK.