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How to improve highway MPG

I drive a 93 Civic, 2-dr, hatchback w/very low mileage (<69K miles). Normally my highway MPG is 400+ miles per tank of gas.Recently this has dropped to <350 miles per tank.I can think of 5 easy, cheap fixes but is there anything else (either cheap or expensive) I could do to improve my highway mileage? My fixes are proper tire inflation, new air filter, use 100% gas, change spark plugs and use a gas additive.(Air filter changed, Jan 08; spark plugs never replaced, to my knowledge). Thanks.

If your spark plugs are really 17/18 years old, then they are way past due for a changing. However, you may find them to be very difficult to remove. The plug wires and distributor cap/rotor (if it has one) should be changed too. Any other normal maintenance that has been put off should be done too. Don’t most Honda’s require valve lash adjustment every so often?

I do a parking lot check when someone complains of poor fuel economy. The check is simple: Find a nice flat parking lot and shut off the engine. Put the vehicle in neutral and see if you can roll it back and forth easily. There should be no resistance. If there is check for a stuck parking brake cable, brake drum or caliper. Winter gas is notorious for poor fuel economy and the oil companies will continue to sell it through the spring until supplies are exhausted. Did you change the oil recently. Sometimes you get a heavier weight than you asked for. That will affect fuel economy. I see that you use a gas additive. I have never found an additive yet that will boost fuel economy. None.

The distributor housing assembly was replaced in June 08. Would the spark plugs have been replaced at the same time? Somehow I had a bad feeling that changing the plugs wouldn’t be easy. If they were ever specifically replaced prior to June 08, I don’t have the paperwork. I really know very little about cars (that much is obvious!) so I don’t know what a valve lash adjustment is. I guess I’ll need to google it. Thanks.

Your measurement is woefully inaccurate if using the fuel gauge. A 50 mile “error” is very easy by using fuel gauge as a measure.

With that amount of swing I would not bother paying any money to fix a car this old.

Actually I don’t use a fuel system additive. When I had a fuel system tune-up done in Aug 08, the mechanic suggested it but I never tried it. Would changing the fuel filter help? Is it difficult or expensive? As far as I know, the fuel filter has never been changed. However, would the fuel filter have been changed when they did the fuel system tune up? Even though I’m a 115 lb, 55+ year old female, I have moved the car backwards,on my own, kind of like you suggested but I wouldn’t swear the area was flat. It was in my garage not a parking lot.

“The distributor housing assembly was replaced in June 08. Would the spark plugs have been replaced at the same time?”

Don’t you save your maintenance and repair invoices?
If not, how do you know when the car is due for a certain maintenance procedure?

Many people know very little about cars, but they compensate for this by being very careful with the maintenance and the operation of their cars. If you are really this careless with documentation you will skip some vital maintenance procedures (such as those 17 year old plugs), and wind up duplicating other services prior to when they would be needed. Either way, you are not helping yourself to operate the car economically or efficiently.

If we asked you when the timing belt was replaced, would you know the answer to that question? If not, you are looking at potential repair bills that will make poor fuel economy look like true “chump change”. If that timing belt is older than 6 or 7 years, you have to be prepared for repair bills that will exceed the book value of the car.

You really need to get organized.

This is just stupid.

This vehicle has been abused for 17-18 years…and all of a sudden you wonder why you’re NOT getting good gas mileage anymore???

Yup
From all available evidence, the OP has some seriously misplaced priorities and some very unusual expectations. This is sort of the automotive equivalent of a lifetime cigarette smoker being surprised that he cannot compete in a marathon.

I am prepared to change my opinion if the OP can give us some evidence indicating that he/she has actually maintained the car properly.

First, you need to measure your fuel economy more precisely. Miles per tank is about as imprecise as you can get. Reset the trip odometer (or write down your odometer reading), fill the tank, drive, then fill tank again. Then divide the miles driven by the amount of fuel added. If you want it to be even more accurate, use the same fuel pump both times, and don’t top off the tank.

Second, what kind of additives are you thinking about using? Most of the fuel additives, like octane booster, are a waste of your money.

Try actually measuring your fuel economy to see how many miles per gallon you are getting. I am willing to be they are in the normal range for your vehicle.

Don’t ignore all of the comments above.

IF you ensure that maintenance is up to date and verify your mileage more accurately, and there is an mpg problem - you should have your thermostat and coolant temp sensor checked. Is the car, by chance, running cooler than normal (if you even know where the gauge normally is).

This is just stupid.

I wouldn’t blame the OP for going away forever.
This was an overly harsh thing to say, given the limited information so far.
You don’t KNOW that it’s been abused.

If the OP ever comes back . . . could you give us a little bit of info on the maintenance schedule done on this car say in the last 5 or 6 years? Oil & filter changes . . . fuel filter changes . . . brakes . . . plugs, wires, cap & rotor . . . timing belt . . . if you know? How long have you had this car? Hard to trouble shoot it with limited info, but I have a '95 Civic and may be able to offer some suggestions if you can give me a little more info. Rocketman