MPG just went down big

[PeteVX]

Used to always get 300 miles per tank of gas, now getting 250.

Happened after i did 3 things in one week so i dont know which one caused it, i fitted new tires Yokohama Geolanders 255/55/18, installed a KNR intake filter (filter and bend connected to the flow sensor) and finally had a new final drive assembley installed in the rear diff under warranty (which still whines a bit but its their 5th attempt at fixing it...long story)

I've tried with the power button on and off, doesnt make much difference. 15mpg was bad enough but now getting 12.5 to 13 is hurting the wallet some,

any suggestions?

Pete

[AREA 51]

The shorter tires should make your mileage look better than it actually is.

[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.

[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!

[Hotsauce]

Agressive tread pattern can kill mileage. Also, even in stock tires, a few more pounds of air can give dramatic increase in mileage due to smaller contact patch and less rolling resistance.

If you have illuminated CEL, the engine goes into limp home mode, which is to retard the timing, and set rich mixture in an attemt to not cause detonation, or high EGT from the lean condition it believes you are having.

John C.

[AREA 51]

Raque, he said 255

[Raque Thomas]

Raque, he said 255

Oooops! Better get my glasses checked! Please disregard previous rants!!

[cascabel3]

I put on Nitto Terra Grapplers 285-60-18 and my mileage went to ~14/15MPG.

However, as someone mentioned, due to the increase in tire size the mileage shows less than actual. I also have a Calmini cone filter. Roof rack may cost some also.

[PeteVX]

Lots of interesting info coming out here, would tire pressure also affect the diameter, ie if pressure was low would it have the effect of a smaller tire. It wouldn't would it? it should still have the same circumference right? Its too late in the day for me to get my head around that one!

I think i'll hook up my gps and see how accurate my speedo and milage indicator are. It would have been useful to have done that before the mods though as a comparison.

Pete