Low Gas Mileage on 99 Honda Accord LX

I’ve got a '99 Honda Accord LX (4 cyl) with 125K miles on it and I’ve taken it in for every single recommended maintenance (just did my 120K service that replaced spark plugs, dist caps, etc.) I change my oil every 3K and drive mainly freeway (80 miles a day). I recently noticed, before I got my 120K service, that I only got about 22 mpg, whereas I was able to get about 25 mpg before. Even after my 120K service, I still get 22 mpg. Any idea why?

You’re only talking about 10-15% change, which is pretty small, and can be caused by a large number of things, like driving habits, low tire pressure, type of gasoline. How long as it been since you got 25 mpg? what else changed since then?

I agree that the difference is too small to suggest there is anything wrong with your car. It drives just like it always did. As Bill has suggested, there are many non-mechanical variables that affect your fuel economy. And if you figure mileage by miles-per-tank, as too many people do, there is room for even more variation.

The perceived change in fuel economy could be a number of things. Is the weather colder than the last time you measured it? Some fuel pumps shut of sooner than others based on the rate of fuel they dispense. The faster it pumps fuel, the more splash-back you get, leading to premature shut-off. So if you really want to measure fuel economy accurately, you need to track fuel usage over several tanks of gas, using the same fuel pump for your first and last fueling to ensure that you start and finish near the same level of fuel. How often do you check your tire pressure? If you check it once a month, you might get slightly better fuel economy right after you fill the tires with air and slightly worse fuel economy right before you fill them. There are really so many variables that you should not worry about this small of a difference. If, however, your fuel economy drops below 20 MPG, let us know. In the meantime, it sounds like you are doing everything right.

I can relate to your obsessive fuel monitoring. Since my motorcycle doesn’t have a fuel gauge, I use the trip odometer and fill up every 100 miles. This makes it easy to look at fuel economy since I usually put in about two gallons. The math is easy. If I put in more than two gallons, I got less than 50 MPG. If I put in less than two gallons, I got more than 50 MPG. However, I realized that I am not always filling the tank to the same level. I try to fill the tank slowly so that it doesn’t overflow or splash. I realized that the fuel pump will stop at different levels based on how fast the fuel is coming out. If you apply this to your car, do you ever notice that different station’s fuel pumps dispense fuel at different rates? Have you ever noticed that a station that is low on fuel will dispense it very slowly? What about the sensitivity of the shut-off feature? This mechanism isn’t very precise, even if you top off your tank (which is bad for the car). So try not to read too much into small fluctuations in perceived fuel use.

3 mpg is not a large difference worth fretting about. I would simply check your tire pressure and make sure proper.