i have a question about gas mileage. when i fill up my tank with 93 octane, the computer reads i should have 398 miles to empty, driving efficiently i only get 230 miles per tank. i put 14 gallons in fuel. give or take a little off or more depending when i fill up. and usually about the same miles per tank.
i just got oil changed, plugs,pcv valve, rotate and balance. also i feel a very very slight misfire at idle.

 

So what is your question?

 

well i hear posts of other fellow 5.5'ers saying they get about 350-400 miles per tank. why dont i?

 

Your driving habits?

I'm usually getting 300 miles

 

im driving pretty easy. i wonder if the very slight misfire has something to do with it.

 

^
If you do a lot of city driving?

 

A misfire will certainly affect fuel economy. There are MANY other factors as well....

It's been winter and if you've let the car warm up (which you should) then that will significantly affect fuel.

As mentioned, lots of city/ stop and go driving will consume more fuel. Even if conservative. Auto tranny can get worse fuel because of this. If a auto tranny is slipping, it can definately show.

Other maintenance issues as well - low tire pressure, seized brake caliper, or the known ebrake cable seizing issue, etc etc.

So, what to do? Ensure all maintenance is addressed. That includes inspecting brakes and making sure none are dragging. You can look at rotors for discoloration or even feel the center of the wheel with the back of your hand - if one wheel feels hotter than the other (left compare to right) then that wheel may be dragging. Don't compare front to rear as it should be expected fronts will be hotter.

If at a long red light, put the car in neutral or park (consumes less fuel). Eliminate any unnecessary weight if you have any. Carrying a toolbox, heavy sub/amp system, etc will pull fuel economy down the tubes.

Most important!! DO NOT measure fuel economy using the on board estimate system. Our tanks hold 18 gallons. If you fill 14 gallons- good - shouldn't run till dry. But, the ONLY way to get an accurate measurement of use is to divide the actual miles driven (means resetting trip each fill up) and dividing those miles by the number of gallons filled. That will give you your miles per gallon.

I get roughly 21mpg during winter. I get 23 in summer. Close to 25mpg on road trips. Consistent driving habits.

I'm sure there's more, but hopefully that's a start.

 

yes i do about 50/50 city/hwy, and my vehicle is a 6 speed. im wondering will a failing coil not throw a check engine light on, and will it just show a slight misfire? im thinking that might be the problem, because i already changed plugs, oil, pcv, tire rot and bal, belts. also my vehicle did not come with an air scoop(inlet) i just purchased one from another member. just wondering will this cause the problem.

 

Nah, people run cone filters and ghetto mods that pull under hood air all the time.

 

so it wouldnt cause a impact with fuel economy.?

 

are you running premium fuel?

 

yes i put 93

today while idling, it sounded like little explosions coming from the back of the car. and every once in a while i would get the infamous p0300

 

P0300 is multiple cylinders mis-firing. This means 2 or more cylinders. The little explosions may be the engine backfiring from too much unburned fuel in the catalytic converter.

Unfortunately P0300 doesn't tell you which cylinders. Most of the time this is caused by the ignition coils failing, but not always.

 

ok i ordered three ignition coils from ebay just to see if will fix the problem, and im thinking if its the coils it will improve my fuel economy as well, right?

bump

 

No need to bump, your question has been answered multiple times.

I'll leave this here too: