I've got a 2004 V6 Mustang. I purchased the car through Carmax last year and was getting pretty good milage per tank... about 300 miles per tank. Lately, I noticed I'm only getting 200 miles per tank. That's a huge difference I think.

I talked to a mechanic at Carmax and he asked what type of gasoline I use. I told him I've been filling it up with regular unleaded (87) gasoline at Unocal 76, which is where I've always filled it up. He said, "Oh, 76 has a reputation for having a lot of alcohol in their fuel and if you try a different gas station, ie. Chevron, Mobile, Shell, etc., you'll see a huge difference. He also mentioned I should try 89 gasoline.... but I know the user manual says the car should run on 87.

Everyone I know fills up at Unocal 76 and has never had this kind of problem. Even friends and family with older cars.

Carmax looked at everything possible to determine the problem and have found nothing wrong. All systems are normal, they say.

Anyone have any suggestions on what might be the problem? Thanks for your help in advance.

AK

 

If your tune is for 87, 89 won't do crap.
How many miles are on the car. Try doing a tune up? Plugs, wires, air filter, O2 sensors current? What's your tire pressure? Tire size is still stock? And what is your actual mpg?

 

We just had the 30,000 mile service done in early November through Carmax. They did all kinds of stuff, then we tested the milage with 3 tanks of gasoline (non-unocal 76) and we had the same result, about 200 miles per tank. I'm not sure if the tires are stock size, but I know the rims aren't stock (chrome). We've had the same size tires since purchasing the car in August '07. According to the manual, we should be getting 17 city mpg, but we're getting approx. 12.5 mpg right now.

 

Is the car running well otherwise? Sounds ok,, idles good?

I'd re-check the spark plugs etc,, see if there is any signs of trouble there. And yea check the air pressure in the tires

 

If you are getting 200 miles on 12 gallons of fuel, you're at 16+ mpg. "Per tank" is a hard call since some people only drive down to 1/4 tank before filling and some run it on fumes to the station. Top it off, reset the trip odometer and drive the tank close to empty. Fill it all the way. Tank the number of miles driven and divide it by the number of gallons it took to fill it up all the way.

What kind of rims are on there? What size tire? HEavy rims can kill mpg. Also wrong sized tires will cause the odometer to read wrong, making it look like you travelled less or more than you really did. Low air pressure will also cause a lot of drag on the road, hurting you mpg as well.

I'd look at replacing the O2 sensors to make sure you aren't running too rich/lean. If one of the four is bad, the cars computer will jack up your air / fuel mix and waste a lot of gas.

One other thing to look at is the brakes. If one of your calipers isn't letting go all the way when you take your foot off the brake, you are going to be driving against it the whole time. I've seen that happen as well.

 

When testing MPG per tank of gas, we were filling it up and running it pretty close to empty; until the orange "Low Fuel" light would turn on. I guess you're right though. We were putting about 13 gallons every time we fill up, so that breaks down to almost 15.4 MPG. Not too far off of the 17 MPG the car manual says it should be getting. I just recall getting more milage the first year I had the car. I'll continue to watch it and check those few things you guys had mentioned.

Thanks again for all your help!

 

Try making sure your tires are fully pumped, you've had recent oil changes & your air intakes are in tip top shape. There could be an underlying problem here.

 

You should still be seeing more. closer to upper 20s or even low 30s is common.