At $4.40/gallon, OUCH!
June 4, 2008 8:44 AM   Subscribe

Why is my cars gas mileage so low?

2 months ago I bought a '97 Honda Civic EX with 150K miles on it. Since that time I have performed some repiars including: New wheels and tires, replace all the belts, replace water pump, replace engine seals near the belts.

I have been watching my fuel usage, and have been seeing a measly 26mpg highway and 22mpg city. The manufacturer claims the car should get 38mpg highway and 32mpg city.

There are two factors I know probably reduce the mileage. One is the current tires are lower profile than the stock tires, and so probably knock about 2mpg off the mileage. Also, the car is old and has a lot of miles off it, so that should knock off a few mpg also. But other Civics the same age and miles driven get much better mileage than my car, so there must be something else.

The two ideas I have currently are get an oil change (its almost due anyway) and replace the air filter. But I am curious to other suggestions you MeFites have that could improve the mileage.

As for things like driving slower and combining trips, such suggestions are good tips but I already follow all such suggestions to the T. I drive an average of 150 miles per week (not including drives for vacations), and rarely go above 70mph. I also coast as much as possible, accelerate slowly, and use the brakes only when there isn't room to coast.

In any case, after this long post, I am eager to hear what suggestions you guys have for changes to make to the car or engine to improve the mileage. I am willing to spend as much as a couple hundred dollars to improve the mileage, as I estimate the lower mileage is costing me around $36/month in wasted gas.
posted by LoopyG to Travel & Transportation (34 answers total)
Did you have a mechanic look at it closely before you purchased it?

At that mileage, depending on how the last owner treated it, all sorts of things could be happening. Does it lose fluids, like oil or coolant? Say, for instance, the head gasket has started to leak - that loss of compression will cause serious reductions in fuel efficiency.
posted by M.C. Lo-Carb! at 8:54 AM on June 4, 2008

How did you determine you were getting 26 mpg highway and 22 mpg city? Also, is it manual or automatic?
posted by Dec One at 8:55 AM on June 4, 2008

I had a bad alternator (it was running over-voltage) and replaced it on my 2000 Accord v6. I immediately got 30% better mileage.
posted by devbrain at 8:59 AM on June 4, 2008

The EPA-provided MPG estimates are always way off and aren't based on real-world use.

I'd bet you're doing about as well as you can do without going to extremes.
posted by unixrat at 9:06 AM on June 4, 2008 [1 favorite]

Response by poster: Hi Guys. To answer the questions. I purchased it from a mechanic, and he is the same one who performed the repairs mentioned above (except for the tires). I determined the highway mileage when I drove 700 miles last weekend on flast, straight roads going the speed limit. So that 26mpg is pretty accurate. The 22mpg for city driving is less accurate as I have accumulated less data.

I check the tire pressure once a week, and so it is nearly always at the reccommended level.

So far the suggestions are: alternator, losing fluids (how would I check this), or a leaky head gasket. Any others?
posted by LoopyG at 9:06 AM on June 4, 2008

Response by poster: Unixrat: I agree they are off. But I have talked to 5 colleages who drive Honda Civics that have late 90s models and over 100K miles, and they get between 30-35 highway. My girlfriend also drives a 99 civic that has 146K miles and she gets 35mpg (measured when we drove to the Grand Canyon). So there is some data showing that my car could get ~33mpg (maybe 30-31 because of the tires being lower profile), as opposed to the 26mpg I am getting now.
posted by LoopyG at 9:08 AM on June 4, 2008

In addition to the oil change and air filter, you might want to replace the in-line fuel filter and replace the spark plugs. If those are still the original plugs, they are way overdue for replacement.

You don't say if this is a manual or automatic. If a manual, you might have a clutch going south. If an automatic, you may need to change the fluid.
posted by Thorzdad at 9:10 AM on June 4, 2008

I would check: spark plugs, spark plug wires, 02 sensor, and have a compression check done on the cylinders.
posted by jaimev at 9:11 AM on June 4, 2008 [2 favorites]

If your tires are a different diameter than original, then your odometer reading will be off. This will affect your calculated mpg.

I am not familiar with your car's engine, but I might check that the exhaust manifolds are properly tightened down and free of leaks.

Leaky manifolds mean outside air leaking into into the exhaust. On many cars, this means that the oxygen sensor which monitors the exhaust falsely sees a lean burn condition. The engine computer attempts to correct for this by making the engine run rich.

This can lead to a 30% decrease in mpg.
posted by zippy at 9:16 AM on June 4, 2008

That is pretty bad for a Civic. You might want to see if there are any codes in the ECU. Some car accessory stores will read the codes for you for free. I would be looking for codes that don't throw faults like excessive spark retard, cam position sensor, idle air control.

Dragging brakes can cost mileage, but probably not that much by themselves. If the car's an automatic, it's less obvious that the brakes are dragging than it is in a manual shift car. If it's automatic, you can put the car on a slight hill and verify that the car will coast down the hill when the transmission is in N.
posted by jet_silver at 9:17 AM on June 4, 2008

A roof rack and driving with the windows open will both cut gas mileage by 1 or 2 MPG. Are either of those a factor?

Also, carrying a lot of extra weight -- junk, hefty 6-footers, etc. will cut MPG (by 1 percent for every 50 pounds!).
posted by beagle at 9:20 AM on June 4, 2008

A vacuum leak can clobber your MPG.
posted by neuron at 9:22 AM on June 4, 2008

Response by poster: Nice suggestions, and yes the car is an automatic. So the list I am compiling contains:
-Oil change
-Air Filter
-In-line fuel filter
-Spark plugs
-Spark plug wires
-O2 sensors
-Compression check on cylinders
-Leaky manifold

And jet_silver suggested: Putting car in N when going down a hill (can test tomorrow morning on drive to work) to see if brakes are dragging, and reading the ECU to find faults that might indicate reduced mileage.

Another question, I have an idea how much Oil change and air filter change costs (was going to do those two next week anyways), but how much would it cost for the fuel filter, spark plug with wires, and O2 sensors to be replaced?
posted by LoopyG at 9:23 AM on June 4, 2008

If your tires are a different diameter than original, then your odometer reading will be off. This will affect your calculated mpg.

But the new tires are smaller than the original tires. The wheels have to spin more to go the same distance. So wouldn't the odometer be overestimating the miles driven, and therefore result in a higher calculated mpg?
posted by Dec One at 9:36 AM on June 4, 2008

Did it come with the low profile tires? Usually the guys who buy those kind of wheels and tires also tinker around with the engine to make it go faster. There may be some modifications to the engine that would explain your low gas mileage.
posted by 517 at 9:47 AM on June 4, 2008

OP said new wheels and tires, not just tires. Are the wheels the same diameter as the originals? If it has bigger wheels, the overall height/diameter could be larger, thereby underreporting your miles.
posted by daveleck at 9:51 AM on June 4, 2008

Response by poster: I replaced the tires after my rear-right had a blow-out. The previous owner tinted the windows and installed a large sound system (I have since removed the sound system and the tinting from the front windows). However, I do not think they made any modification to the engine.

Dec One, you are right, and I hadn't thought about that. I had a chance to compare my odometer to the actual miles driven when I did that 700 mile drive 2 weeks ago, but did not do so. But it could be I am actually getting like 24mpg in reality.
posted by LoopyG at 9:52 AM on June 4, 2008

This might be a bit far-fetched depending on where you live, but if all of those things check out fine and you're still getting low gas mileage, could it be that someone's regularly stealing your gas?

My boyfriend's fastidiously maintained 1990 Civic consistently gets about 32mpg city/39 highway, but we got 24mpg on a weekend trip shortly after he got the car. He remembers looking at the fuel gauge when we parked, and it was noticeably lower when he returned to the car. He suspected a leak, but since then, it's been in the 32-39 range. Neither of us were able to come up with any other ideas.
posted by kiripin at 9:56 AM on June 4, 2008

Response by poster: Hi Kirpin, I seriously doubt that is the case. Especially because during that 700 mile drive I was with the car at all times, and would have noticed someone stealing my gas while I drove 70mph down the freeway =). But thank you for the consideration, definitely something to look out for in my neighborhood.
posted by LoopyG at 10:01 AM on June 4, 2008

but how much would it cost for the fuel filter, spark plug with wires, and O2 sensors to be replaced?

O2, assuming you have two, is going to be 300+ dollars. Fuel filter might be half that. Spark plug and wires prob 60-75 or so. All of this is with labor.

I wouldnt replace them. Id ask the mechanic to check them out.

Frankly, on a car with 150k, even a honda, dropping $1,000 for the chance you might get +2 MPG isnt the smartest thing to do. Hopefully your mechanic can isolate the issue but if he is just going to keep replacing costly items with no guarantee of MPG increase then obviously this is not the way to go.
posted by damn dirty ape at 10:19 AM on June 4, 2008

However, I do not think they made any modification to the engine.

If this is an ex-ricer car then there's a slim but real chance that previous owner modified the ECU via a mod-chip or other hard to detect modification.
posted by damn dirty ape at 10:20 AM on June 4, 2008

Response by poster: I agree damn dirty ape. I estimate I will drive this car a maximum of 50,000 miles. I did the math and the loss of fuel (assuming I could obtain 31mpg) is about $1600 or so assuming the average price of gas over the next 4 years is $4.50/gallon. I do not plan on spending more than $400 in order to try and improve the gas mileage to the 30mpg target I have in my mind. However, fuel filter, spark plug, and wires might be a good choice, as I can probably get those done for $200, maybe a bit less if I have it done when I take it in for other things such as replacing air filter and the oil.
posted by LoopyG at 10:26 AM on June 4, 2008

2nding having a mechanic LOOK at these things rather than just replacing them. Also, take it to a different mechanic than the one you bought it from. The guy you bought the car from would have an interest in hiding problems if were any.
posted by cnc at 11:20 AM on June 4, 2008

Seconding, thirding, whatever-ing spark plug wires...I changed the wires on my previous car, an Accord (1993 with 108,000 miles) and noticed a surprising improvement in performance. You may also want to check to see if there are any obstructions in front of the air cleaner....this would reduce the available amount of air-to-fuel, which would result in running rich, which can severely affect mileage. Maybe replace the air filter while you're at it, too.
If you're really a stickler about mileage, you may want to have the wheel bearings replaced or re-packed with lubricant. This will result in less rolling resistance which could make an infinitesimal difference in mileage, but probably isn't worth the expense if you're trying to keep total expenditures under $400.00.
posted by motown missile at 12:46 PM on June 4, 2008

damn dirty ape writes "O2, assuming you have two, is going to be 300+ dollars."

Are Honda O2 sensors really that much money? It only takes a few minutes to change them if they aren't buried and Dodge sensors, even the 3 wire ones, are less than $50.
posted by Mitheral at 1:28 PM on June 4, 2008

Where do you live?

IF you live in nys that is a good mpg . NY uses ethanol in the gas which reduces your mpg a lot.

I get about the same in my 2004 corolla here in ny.
posted by majortom1981 at 1:55 PM on June 4, 2008

Also driving is a huge difference. Try not going over 3,000 rpms . That will make it go up also.
posted by majortom1981 at 1:56 PM on June 4, 2008

Response by poster: I live in Southern CA. As for RPMs, I usually hold it steady, and not far about 3000. As mentioned, I accelerate and decelerate slowly in order to conserve fuel.
posted by LoopyG at 3:32 PM on June 4, 2008

Also, smaller tires will make the engine turn more RPMs per mile traveled. So even if your calculations were dead-on, you'd still be running lower than advertised.
posted by gjc at 4:46 PM on June 4, 2008

I ... rarely go above 70mph.

70 mph is too fast if you're trying to maximize mpg, due to aerodynamics. 55-60 mph is optimal for most cars, according to the EPA, and my recent experience supports that. My mileage has climbed noticeably ever since I try to keep it under 60 mph (in the right lane, in case you're wondering). There was a reason why the national speed limit was lowered to 55 mph back in the 1970s.

The EPA-provided MPG estimates are always way off

They were, or so said popular opinion, but now shows adjusted figures ... which are actually too pessimistic, in my experience. The old numbers were pretty accurate for me.
posted by pmurray63 at 9:42 PM on June 4, 2008

If you want to get the best mileage the car can get, buy you a vacuum gauge and tee it into the vacuum line. keep the gauge where you can watch it while you drive. With the gauges that have a red, yell, green range try to keep the gauge in the green area all you can. IT WILL change the way you drive.
D. Hair
posted by thehairs at 8:57 AM on June 5, 2008

Are these the stock wheels/rims on the car? If the previous owner put it larger wheels the odometer may be off.
posted by damn dirty ape at 9:00 AM on June 5, 2008

Yeah, you never know what weirdness a previous owner might install. I had a car fail the smog test, and the mechanic finally found a BB in a cable, which he said someone had done to influence air flow. Popped it out and passed smog.
posted by Scram at 9:30 AM on June 5, 2008

Response by poster: These are not stock wheels/tires. They were replaced with new ones about a month ago. They are slightly smaller profile then the stock tires.

I just checked, and they claim 24city - 32 highway. So by their standards I am only about 6mpg below what I should be getting (assuming odometer reports accurately, which I haven't been able to test yet). Though 6 is still a fair amount (~21%).

I put my car in neutral on a large hill today on the way to work, it coasted just fine.
posted by LoopyG at 12:32 PM on June 5, 2008

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