I had 205/45-17 on the car and everything was fine I just did not like the way that the car road. You felt every bump in the road. So I put 215/45-17 now its rides a lot better, but now the mileage counter is way off. I was getting 26-28 mpg now its more like 19-21 mpg and I only get about 330 miles to a full tank of gas vs. the 400 I was getting before. I measured the two tires circumference of the 205/45 was 73 inches and he other one is 76 inches. If anything how do I change the mileage counter?

Some tires are simply less efficient than others. I suppose energy is lost in internal friction, flexing, and heat generation. Supposedly, the Michelin "Energy" tires are more efficient than older-formulation tires, but I'm not sure how much of a mileage improvement you get. Remember bias-ply tires? They were grim and radials resulted in an amazing improvement in mileage.

Check to see if your brakes aren't dragging. If they didn't tighten up the lugs properly the rotor may be warped.

I came up with roughly 3% difference in diameter. So that wouldn't throw your displayed economy off anywhere near what you have experienced, maybe 1 mpg tops.

Klaus pointed out the major culprit. Also remember colder weather reduces your economy. Mine regularly went down 2mpg in the winter with my '95 850T.

Don

Brand new tires will get less mpg for the first 1-2K miles because of the deep tread causing squiggles. It will get better.

Your new tires are also wider which incur a penalty for increased grip and drag. They are also larger in diameter, which will throw off your car's calculations.

Klaus
--
Volvoless. Sold the R. There is now a void...

The tire change size in circumferance induced a 3.947% error. Klaus hit it correctly in that new tires have a "squiggle" factor as they break in as the main contributing factor. I've experienced this many times as I've replaced tires. Wait until the tires "set". Pressure is a significant factor, so make sure they are set correctly. You can correct speed and distance by 3.947%. I do not know how to correct for this with your visual display of your gages. This would be the correction for both speed and distance (assuming the initial reading was correct) which is subject to an entirely different thread! For the calulations I used the before 73 inch circumforance to the now 76 inch.

Act Indicated
10 9.6
15 14.4
20 19.2
25 24.0
30 28.8
35 33.6
40 38.4
45 43.2
50 48.0
55 52.8
60 57.6
65 62.4
70 67.2
75 72.0
80 76.8
85 81.6
90 86.4
95 91.3
100 96.1
As you can see, you are off about 2.5 mph at 60.

Hope this helps.

Jerry