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Losing Mileage

Since about 6 months ago I’ve noticed a significant drop in my mpg, as in from 20 down to 15. I drive a 1998 Nissan Quest with 98k miles, and keep it in very good condition.* The head mechanic at the shop says it’s due to the 10% addition of ethanol that’s being added to the gas. All of the stations here in Austin are adding it.

*New tires, change oil & filters at 3,000 miles, transmission fluid is clear and pink, I try to keep the tachometer as close to 2000 as possible. I only use the air conditioning when really hot and avoid afternoon driving (too hot). I don’t use additives. New plugs about 2,000 miles ago.

E10 won’t make that big of a difference.

Are the new tires the same as the old ones? If not, I suspect the new ones have higher rolling resistance. Speaking of tires, are you keeping them properly inflated?

They’ve been adding the ethanol for a couple of years now, everywhere. So if this just started, I don’t think that is the reason.

New tires could have something to do with it, especially if they aren’t inflated to the proper spec. Even tire brand and model have some influence but low pressure could drop your mileage.

As noted, you’ve most likely been going on 10% ethanol for a lot longer than 6 months and it isn’t the station that mixes it. It shows up that way on the truck.

Tire pressure is a big one.

Your mechanic should check the coolant temp sensor, and should put the van on a scanner & check out the fuel trims to see what they are up to. (You will, of course, have to pay for the time).

Try some sea foam, as directed. Are the air and fuel filter changed per the manual?

Are the replacement tires the same size as the original? Check the size on the sidewalls of the tires and then check the pillar on the driver’s side door pillar for the correct size. If the tires are too big, this will slow down the odometer. It then won’t register as many miles as you have traveled. This will make it seem as though your gas mileage has gone down.

“ … a significant drop in my mpg, as in from 20 down to 15 … with 98k miles …”

Most likely you need a new O2 sensor.

If the tires are more aggressively treaded than the ones they’re replacing this will also drop gas mileage.

The reason the guesses are all over the map is because there just isn;t enough information here to make a preliminary diagnosis…or even a good guess. There are just too many possibilities. Many would trip a check engine light, but many would not.

You’re to be commended for trying to give us as much detail as you can, but there are just too many things that affect mileage.

I’m afraid the answer is that there is something wrong with your engine, and you need to take it to a different mechanic, one who had a good head for diagnostics (which this one does not). If you really just switched to E10, you would only see a 3% drop in mileage. If your tires were so underinflated that you lost 25% of your fuel mileage, they would be visibly flat and you would probably overheat and blow them up on a modest highway trip. And if you just changed tires and got the wrong size that your odometer was reading 25% low, your speedometer would also read 25% low and I think you would notice that.

So we’re left with an engine problem. Here is a basic list of things to watch for while you are looking for a new mechanic:

check engine light on?
smoke coming out the tailpipe, on start-up or while warmed up?
air filter and fuel filter changed on schedule?
recent change in exhaust noise?
if you are handy, pull the plugs and make sure they are not fouled
how’s the transmission fluid?
is the engine temperature normal? Do you get heat from the heater if you request it (you probably never run the heater but check to see if you get heat)

Change the thermostat if it’s over 5 years old.