MPG just went down big

[mbeach]

I get the same mileage (about 12.5 to 14 mpg). I run the same (size) tires, and have similar work on the vehicle. I dunno what's going on either, the milage issue is really irritating. I did the PCV thing a few tanks ago, and that seems to have helped a bit. Next I'll clean the EGR (49k miles) and switch over to 0W-30 Mobile 1 (dealer rec'd) and see if that helps.

[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.