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)

[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

[cascabel3]

Tire pressure would most certainly effect mileage.

The lower the pressure the less mileage.

[Mr. I-MAN]

I guess I don't feel so bad for only getting 12.5 to 14mpg

[Tone]

I just came back from a 500 mile round trip using my new Scan Gauge and it is so cool! Instant and average MPG that really shows how just a little lighter on the pedal can improve mileage. Intake air temp was another cool feature to see how it really does not cool off much at even 100MPH - with a metal intake tube and outside temp at 70, intake air stayer around 85-90. I can offer these for $119 delivered - plugs into ODBII port.

[cascabel3]

Tone,

more info on the scanner please. I am interested. How is it installed and what does it look like?

[scarpezio]

Tone, how do I order one since it's not listed on your site? I have a Davis carchip installed on the port right now but a visible readout would be nice and I'll just switch the carchip to the Vette.