MPG just went down big

[Raque Thomas]

I get about the same mileage with the same tires and a K&N drop in filter - 13.2 on the last tank. Found out (the hard way) that I get 276 miles on a tank of gas. The 285/55/18 tire is actually a little taller than the stock 18" Bridgestones - about 1/2", I believe. I do have a recurring CE light for a lean condition in bank one - not sure if that is contributing or not. Just put in a bottle of injector cleaner on this tankfull - we'll see if that helps.

Just rechecked the tire size calculator - stock tires are 29.6" in diameter, 285/55/18's are 30.3" in diameter, meaning your speedo is 2.6% slow and theoretically, your gas mileage is 2.6% better than what you calculate off the odometer - ie, you are actually getting 256.5 miles per tank instead of 250 (wooohooo!)

[xdfarrx]

Check your intake manifold gaskets, and common chamber gasket too. To me that is what it sounds like. I had the same symptoms, just replaced and things are good again

[PeteVX]

will check the gaskets, i did replace the pcv valve a month ago, the new tires i worked out to be only .5" dia smaller, from the stock 245/60/18 to a 255/55/18 which should be neglible.

[Raque Thomas]

will check the gaskets, i did replace the pcv valve a month ago, the new tires i worked out to be only .5" dia smaller, from the stock 245/60/18 to a 255/55/18 which should be neglible.

Your new tires are actually .7" larger in diameter than the stock tires, which means the miles per hour and odometer will read less than actual by 2.6% (at 65mph registered, you're actually going roughly 67mph - and for every 100 miles registered on your odometer, you have actually traveled 102.6 miles)

[Raque Thomas]

Dimensions from the tire calculator chart at http://www.miata.net/garage/tirecalc.html

245/60/18 285/55/18
sidewall 5.8" 6.2"
Radius 14.8" 15.2"
Diameter 29.6" 30.3"
Circumference 92.9" 95.3"
Revs/mile 682 665

[Triathlete]

Most tire companies exagerate the size of the tire (very common in off road tires). So that calculator (although a usefull tool) is just in general. Would have to actually measure the tires to get the "true" size.

[Raque Thomas]

Ummmmm, that calculator is not brand specific; and I believe from my past studies that the tire sizes as stated on the side of the tire are relative to the size of the tire - the 285 being the cross section width of the tire at its widest point (the sidewall) in millimeters. The 55 refers to the aspect ratio of the tire, or its sidewall height percentage of the cross section (55% of the 285 number). So there should not be a great variation in height, I wouldn't think. Of course, as you stated, the only real way to be sure would be to jack the car up off the tire and measure.

OK - just went to Yokohama's web site, they list inflated height at 30.4", and Bridgestone's web site says 29.7" for the H/T Dueler - still no guarantees without measuring, but interesting how they are both .1" larger than the calculator states. I guess another consideration would be tread wear - a new tire with 12/32nds of tread vs your worn out tires at 5/32nds, adds another 7/32nds of height to the equation in mileage comparisons before and after the tire switch. I'm done........

Oh, and Pete - sorry to hijack your thread and turn it into a discussion of tire dimensions!