Lifter583

I would put it on a dyno and see what your A/F ratio is running...I am really thinking your tire revs. per mile is off and you are really showing much less miles traveled then actual...try a mile marker run and see what you read on the odometer...

modru2004

ya the stealership should be able to do it. no idea what they would charge.

but if your looking for more performance you can always buy a programmer for yourself.

the x3 is popular and runs around $380ish new. whoever provides your custom tune can fix that for you, or you can always do it yourself with the programmer. its a simple setting change. just tell it wheel/tire size and your gear ratio flash it and your good to go.


you can always pick one up used for the $200-300 range. but you have to pay extra if you want a custom tune for more performance.

but if you have no interested in the performance benefits or additional settings you can change yourself it would probably be cheaper just to have the dealer ship change the setting for ya. but if you see yourself changing more things in the future the programmer would be the way to go.

Toxix

From what I've been reading around apparently moving to 18inch wheels to 20inch wheels WITHOUT Recalibration could be part of the issue.
Stocks - 235/50/18R TO Current - 275/35/20 [Rear] 255/35/20 [Front]

But......

The diameter almost remains the same....just about .5 of an inch off, don't if this could be the issue.

Now, referring to my to-do list above, is there anything else you guys can recommend I do, might as well do it all at one time.

- Spark Plugs
- Clean MAF
- Recalibration!!

And as for Spark Plugs - NGK Iridium ones or just go stock Motorcraft? Hearing good things about NGKs, but shops are recommending to go with regular Motorcraft.

HenryinFlorida

The size of the wheel has nothing to do with this. What's important is the total height of the tire and wheel. If you upgrade to a bigger wheel and put a tire on it that has the same diameter as the tire you took off, there will be no change. Afterall, 2a 7 inchestall tire is 27 inches, weather the tire and wheel are 17, 18, 19 or 20 inches.

Art161

Are the brakes binding?

Nuke

Bingo. Unless you went with a 20" tire in a 50 series or taller sidewall, I doubt you drastically changed the tire OD enough to make enough difference to cause a significant MPG loss.

I'd check to see if the brakes are dragging or if you have a bad wheel bearing; ANYTHING that may be causing rolling resistance since everything else sounds like it checks out.

LordRipberger

I was thinking the wheel might be the issue too but after thinking about it, I am not convinced. Going off of Toxix's example above, the 3 inch difference for each rotation during driving is a significant difference in travel over the course of 200 miles. If I did my math right, its just under about an extra 1000ft per mile that a 20" wheel gets over a 17" (assuming the tire size increased as well). Even though the odometer says 200 miles, there is a significant difference in actual miles traveled; the driver should have noticed that it was taking a lot longer between fill ups then before the new wheels.

Maybe the fuel to air mixture is not right.

Maybe the parking brake is on!! haha, just kidding.

modru2004

no kidding! really !! lol....

i used wheel size as a simple example. if you had actually read my post you would notice that i said i was leaving out certain things like tire size to simplify the point i was making because i had no idea what his tire size was at the time.

and yes the wheel size DOES matter. its part of the equation. if you run 235/50/18 and then put on 235/50/20 your diameter just increased. but yes if you go to a smaller profile tire that has the same overall diameter as your previous setup, then ya your right. but generally going to a larger wheel your going to have some increase, even if its only slight. is this true all the time no its not.
but you can't make the statement that everyone always goes with the same over all diameter either.

Toxix

From what I'm driving, no bad experiences with the brakes or anything of the kind. I don't feel the brakes dragging or anything.
AS for wheel bearings, might as well get that checked on the way.

Also, I'm still looking for opinions on Spark Plugs - Iridium NGKs or just stock OEM straight replacements?

I'm going to try what I can in the upcoming week.

Lifter583

Have you tried filling up and driving a known exact distance and then refilling at that point??