MoneyShot

Well starting out I was getting up to 26 MPG on my stock, worn 16" wheels and tires, I put some 2011 GT wheels on and instantly it dropped to 20 MPG, sometimes lower. I wrote it off as normal and drove on them for a few months. After getting tired of the bad gas mileage, I put new, wider (255/55) rubber on my rears and re-installed the 16"s. I figured with wider rubber I would land somewhere between 20 - 26, Nope, same +/-20 MPGs. Last week i again grew tired of the bad gas mileage and did the following in an attempt to restore my MPG: Oil change, cleaned the air filter, cleaned the mass air flow sensor, check my spark plugs (a little worn but good), cleaned the PCV, sea foamed everything and reset the battery and let the ECU relearn my tune ( Bama 87), I then filled up and put lots of highway miles on her. No change.

I calculate MPGs by dividing miles driven by gas put in per fill up. According to the ol' tire size calc stock, Gt, and 255/55 rubber put the speedo within a percentage point of each other. Fuel filter and plugs were replaced and throttle body cleaned about 30k ago. I might be looking to sell my car. My mustang runs like a champ and I love her but the wife's Focus is very peppy, has over 125K and gets 30 MPGs all day long.

The only other thing I can think of is there was some white film in the oil cap and a little in the filler tube, Is that a sign of water in your crankcase? I have not seen the coolant level drop a bit since I replaced the water pump a few years ago. What about plugs and ignition coil? She runs smooth with no hesitation. Anyone ever have a gas leak?

Help a brother out, is there something I am missing?

akdoggie

I know when I switched from the 215/65R16's and went to my GT wheels (235/55R17's) and my gas mileage went down 3 mpg. I attribute that to the wider tire and more drag that they create. With my tune and CAI, I get my best mpg, using 235/50R18's now, and it is just over 25 on the highway. I haven't driven it in town in a while, so I have no idea on how it would do in city driving.

waykooljr

What tire pressure are you running? Have you tried adjusting tire pressure?

I know this will impact MPG a lot, I had used a pressure guage at a service station, then not long after my MPG dropped drastically. I bought a analog tire guage and checked my pressures, they were at something like 28 when I know I inflated to 34 at the station, so I don't trust the station guages anymore. Inflated them using my new guage, MPG went back to what I normally get around town.

MoneyShot

Hmm.. So 25 MPG + 18" wheels is possible with the 4.0. @akdoggie, what tune are you running and did your "S/D Coil w/Live Wires" make any change what so ever.

@waykooljr good idea, I have not checked since they were put on about a month ago. I hadn't seen the tire pressure light so I haven't really worried about it.

waykooljr

I didn't think about you having TPMS. Curious, how does the TPMS monitor? Is it by psi or does it go by vehicle load or something? If it's by psi, the TPMS might need to be re-calibrated for the 18's.

I went from the recommended 32 stock pressure for the stock 17's to 34 for my 18's, I had read somewhere that +2 psi/inch increase from stock wheel size is a good starting point. I've been running my 18's at that pressure since I put them on the car, I've always had good tire wear and reasonable mpg for a performance car.

Rudolph Hucker

mine is mint factory condition and has a little white sludge under the oil filler cap..
it's normal by counts due to the plastic cam covers

tigercrazy718

THe TPMS will tell you when you're pretty low on tire pressure, but if you fill it up to something like 34, it won't tell you when you drop down to say 25 psi. I know this because when I track my car I used to drop down the tires to around there and no light. Try filling the tires up to what waykool said and see if that changes anything. A psi over the factory specs decreases rolling resistance, which increases MPG (in theory)

AMDanBailer

This doesn't sound to far off. It's pretty common for your car to drop 2-4 MPG when putting on a bigger and wider rim. This happens because the added weight is all rotating mass! So it seems the difference is only about 1 MPG, that small of a number can be +/- based on your driving habits. Maybe you Shifted at a different RPM than usual, didn't do as much highway driving etc..

This is just a few things that come to mind. Have you thought about a Tuner yet? One of the great benefits to a Bama tuner is the increase in Gas mileage. Obviously it's going to be at the Mercy of your right foot, which may be difficult to resist with all the new found power and torque you'll have! However, you can gain that 1-5 MPG right back with a Bama Custom tune

Just a few things to think about! I hope this helps. Let me know if you have any questions or need some assistance!

-Dan

MoneyShot

Thank you for your response Dan, however if you carefully re-read my post you will see that:
1) I already have a tune from AM/Bama (and have had it during all this)
2) I went from up to 26 MPG w/ 16" -> 20 MPG w/ 18" -> 20 MPG w/ 16" (still) -> 20 MPG w/ 16" after tune up & reset
And THAT is the problem, I still am getting 20 MPG with 16", sometimes less

@Rudolph Hucker you are correct, it can be normal. After a little research, Cold weather and short trips can account for the emultion.

@tigercrazy718 good info I will try that, Thanks

Siber Express

2010 Pony Package 4.0 with 18X8 rims running 235/50 18's before my tune I was always getting between 25-26 mixed city and highway, all highway I was getting almost 30 (29.8 average on 2 tanks)