Why is my gas mileage so low? I've looked around and most people on here seem to get 32 mpg average and some people even manage 40 mpg average on the highway. I get like 28 mpg average, sometimes getting 30 mpg on the highway. I get about 270 miles per tank. My Scion xD is a 2009 and has 50k miles (35k when I bought it a year ago). I read somewhere that spark plugs may be the issue but is there any way to know? I drive pretty normal. I don't hypermile but I also don't drive like a maniac.

 

I dunno man, I have an 09 with slightly wider tires on stock steel rims. It’s a 5 speed and I get 25-28 mixed and 29-31 hwy. I have new spark plugs but I’m sure the tires get me. I don’t think the people saying they are getting 32 average and 40 are correct. It just doesn’t seem possible.
my car is tuned up and well maintained except the slightly winder tires. I never even got close to that before the wider tires.

 

I get 41mpg on the highway (instantaneous) but average 32mpg in overall driving. I'd say I'm a good 60% highway 40% city driver, and accelerate quite slowly. People pass me very often while I am accelerating. I have the 4-speed automatic. The biggest thing that can change your gas mileage is how heavy your foot is, but spark plugs can also cause it. Iridium plugs (which are the ones installed on the 2ZR-FE) are meant to last 100k miles, I replaced mine at 116k miles and saw nearly a mile per gallon increase in fuel economy.

Your fuel mileage does not seem all so low for a typical driver, I would not be worried about it. Say, you get worse than 25mpg average, then I'd be worried.

 

You actually get better MPG by accelerating brisky at 80% throttle. That’s to spend as little time in lower gears as possible. Because you’ll do more revs-per-mile and burn more petrol in lower gears.

In my manuals, I’ll even skip gears to get into top-gear ASAP. I’ll shift from 1st to 3rd then 5th. Big-bore GM V8s even have nifty trick. There’s a little angled finger that blocks shift-gate, so when you shift out of 1st, ECU flips this finger out and it diverts shifter movement from going straight back to 2nd and angles it across into 4th gear instead!

 

That is indeed something I've always wondered about, since the engine stays at the higher revs when having to go from one gear to another so slowly. I ultimately decided, however, to continue accelerating slowly simply because I've heard that engine load is more likely to cause engine wear than revs, and I'm willing to sacrifice several mpgs if it means 400k miles. That's my goal for this car.

 

Different engine parts wear out differently.

high-revs/low-throttle - wears out rings most

low-revs/high-throttle - wears out crank & rod bearings

idle/low-revs/low-throttle - wears out valvetrain (cams, valve-buckets, valves, valve-seats)

Best to use full-synthetic oil with high ZDDP levels. Makes significant reduction in wear.