Loss of Mileage?

I have a 2002 Honda Accord, Manual transmission. When I got the car, I could do 400 miles to the tank without stretching it too much. I got 350 and change on my last tank, and that was with running completely out of gas and coasting into a station. I’ve noticed over the last few months that my tank distance has been decreasing, slowly.

So my question is what should I check and/or replace to try and get it back up? I don’t think it’s my driving habits or style, I’m still pretty much doing the same thing in the same way.

Was it always exactly the same fuel ?
Maybe you had one or two tanks of good alcohol free gasoline and now they’re serving up that ethenol laced stuff ?

I’d imagine the basics such as a decent tune-up and check the tires. That seems a bit dramatic for that difference but it sure isn’t going to hurt. Of course, as Ken said, E10 can also have make difference in some cases, though not so dramatic as this.

One other possibility is someone siphoned off some of your gas. I had that happen on my truck once; freaked me out then it happened again but they left the gas filler door open. I got a locking cap and it solved that.

I agree with Nilt. Has it had spark plugs and wires in the last 9 years? This could be the heart of your reduced mileage. How often do you check your tire pressure? And these are the basics. How about the fuel filter? When was that last replaced? Transmission fluid change? Coolant flush and fill with a new thermostat and radiator cap? Air filter change? The stock ones typically last 1 to 2 years.

I didn’t mention ‘tune-up’, because the ECM in today’s cars generally keep the engine running at it’s peak performance given the condition of it’s parts and the quality of the fuels pumped in. But, the parts need periodic maintenance to keep the engine running like a champ.

Find the owner’s manual, check the maintenance schedule, and bring everything up to date according to the mileage/months schedule.

Spend whatever it takes.

Then enjoy a return to better mileage.

Hondas are good cars, but you can’t just ignore them.

In addition to the necessity of keeping the car’s maintenance up to date, and keeping the tires inflated to the mfr’s recommended pressure (or slightly higher), let’s consider the likelihood that the OP has had to replace the tires at least once on this 9 year old car.

While Honda may have been very careful to equip the car with low-rolling-resistance tires, most consumers never consider this factor when buying new tires. If–as I suspect–the OP has purchased tires with a higher rolling resistance than the original tires, it is very possible that these new tires have lowered the gas mileage by as much as 2 mpg. Then, when you factor in the other possibilities of lax maintenance, low tire pressure, and ethanol-laced gas, it is easy to see someone’s effective driving range on a tank of gas going from 400+ miles to barely 350 miles.

How long have you had the car? How many miles when you “got it”? How many miles now? What is the maintenance history of the car? 1st item to check with a loss of mpg is the air filter. A dirty clogged air filter will drop your mpg significantly. It could be due for new plugs, the brakes could be dragging, could be lots of things. More info will help get more specific answers.

Besides dragging brakes, clogged air filter and low tire pressure, also think about the engine thermostat. Do you have a temp gauge, and if so, is the car running at normal operating temp or is it low? Is your check engine light on? Also, it is possible that the last couple of tanks of gas you did not fill up all the way? You might have a fault in the gas tank pressurization system that is shutting off the pump before the tank is full. A more reliable way to calculate mileage is to record your odometer reading every time you fill up the tank, and average the miles and gallons over several fills.

A difference between going 300 miles on a tank and going 400 miles on a tank could be as simple as a difference in driving habits and shifting patterns, but could also be effected by changes in the driving environment and weather.

Before you do anything, I encourage to use more precise methods to measure your fuel economy and compare those findings to the EPA’s estimates for your car. Then, if you still think you have a problem, take a look at maintenance items like air filters, tires, and spark plugs.

If the CEL is on have the codes checked and you’ll probably find the answer to your question. A bad or lazy 02 sensor will cause mileage to drop significantly as can a bad ECT sensor sending the wrong signal to the ECU causing the car to never go into closed loop. I’ve had 02 sensors go bad on my cars and see a 25-30% drop in fuel mileage.

What’s a “lazy” O2 sensor? What makes it lazy?

A “lazy” oxygen sensor is one that does not respond rapidly to changes in oxygen level, typically because its surface is coated in something, usually carbon or oil residue. A “bad” sensor simply sends the wrong signal or no signal at all. Often for the same reason.

Have you checked for dragging brakes?

Wow! Thank you everyone. To answer a few questions, I’ve had the car for about a year and a half. I replaced the tires myself about a year ago. The decrease in mileage only started about 4-5 months ago. It’s a bit hard to say exactly when because it’s been pretty gradual.
My engine temp is fine, according to my dash gauge. Check engine light isn’t on. I always fill the tank fully when I fill up. I doubt that the gas content is part of it, unless all the gas stations around me have changed the content simultaneously. I got 400 miles to a tank for over a year, and I don’t use one station exclusively. I’ll start paying more attention to which tank mileage is coming from which station, though. Just in case.
Car Mileage is roughly 121,000, I got it with about 80,000 on it and I have no idea what is new or old on it except the tires. I’m pretty good about checking my tire air pressure. I’m overdue for an oil change and I’ll replace the filter while I’m at it.
What should I be looking for when I check the spark plugs to see if they’re good? Or will it be really obvious?
So here’s the list of things I need to check/replace. Any additions?
Oil Filter
Transmission Fluid
Air Filter
Spark Plugs
Tire Pressure

While I agree with the above comments, I will make on more suggestion.

Don’t measure your fuel usage by the tank full. That is rather inaccurate for most people. Look at your receipt and record the miles and gallons. Do this for each time you buy fuel for the next few fill ups. That will give you a much more accurate reading.

Note: fall and winter are coming so you can expect a small drop in mileage due to driving changes and winter fuels that will be showing up at the pumps soon.

I would also suggest not bothering to even look at the mileage until you have all the maintenance items up to date.

Good Luck