What are some reasons a car wastes so much gas?

old car, engine size, depending on if its carburated or fuel injected can be a differace as well as carbed vehicles tend to waste more gas than fuel injected..dont worry i drive an 88 jeep and its a v6 as well GAS HOG..could just be the car..

Incorrect mixture, timeing, drity airfilter, incorrect valve clearances, worn engine.

Well the car is old I assume high mileage? Could be poor compression in the cylinders you may just need a good tune-up as well. I would look into spark plugs, wires, distributor cap & rotor, fuel filter check the vacuum lines replace any of them that are cracked or broken. This may help also go to a 10w30 Weight oil with a lucas oil stabilizer this works to help with oil consumption as well as helps the compression.

Also ensure your tires are properly inflated to the correct pressure. I do it all the time – Yes, they do always need adjusting (everyone’s tires are like that, it is just a very overlooked issue)…and yes, it does save gas, trust me.

Other than that, what everyone said above me is right on key… besides the fact that the vehicle you own isn’t exactly a gas-saver in itself.

*Tip: Fill up with a full tank of “Shell” “V-Power” gasoline and drive with that fuel until you are almost empty. It is excellent at cleaning the fuel system and restoring power and gas mileage.

@ 15 min away, it’s unconscionable to drive there.

Ride a bicycle.

And inflate your tires evenly from 10+- lbs to at least 30, including the spare, and get it aligned, and tuned and all scheduled maint done only @ GM.

Faulty Fuel Pump

The above suggestions about tune up and maintenance and tire pressure are all good ideas. If they don’t lead you to a solution check to see if your transmission shifting into overdrive. If you are riding around in “3rd” and the trans isn’t shifting into overdrive the engine will turn much faster than it has to at speeds above about 35 or so. The more rpms you are turning at the more fuel you are going to consume.

Although it is a V6, that doesn’t necessary guarantee better gas mileage. Sometimes, an engine is just too small for the size and weight of the car and so, it works harder and the transmission shifts down more often, especially going up hills, so the car just uses more gas.
My friends car is a 85 Buick V6, but it has a turbo and on the road, it gets around 30 mpg. The extra boost from the turbo pushes the car easily and saves gas. The V6 alone is not large enough to move the Buick Riveria without using a lot of gas.
But Detroit makes everyone think that a small engine saves gas. It does, if it is used on a small car. Try putting a lawn mower engine on a car and see how hard it would have to work, and the gears required to get the car moving would have to be really low range gears, so the gas used would be immense.

Okay…all that said, I am not saying that your engine is too small for your car, just that having a V6 doesn’t always mean gas saving.

If your car is fuel injected, have the injectors checked, to see if they are leaking fuel, while parked. Sometimes they leak, and over a period of time, gas is wasted. Also, check the sensors, such as the Manifold Air Pressure sensor or Main Air Flow sensor, whichever one is on it (both would not be used, only one or the other) These measure air flow into the manifold and adjust gas accordingly to keep the ratio at 15 to 1, or close to that. If these are bad, you could be running rich, and wasting gas.
check the Oxygen sensor. If this is bad, you could be running rich or lean, and in your case, probably rich.
Check the coolant sensor. This senses the temperature of the engine and when cold, it adds lots of gas to keep the car running when cold, since cold gas doesn’t burn to well. Once the engine warms up, the coolant sensor tells the computer to go to normal, and run leaner. So if this is bad, and stays on cold reading, you would be running rich again.
so you see, these play a major part in keeping your car running lean.

If you have a carburetor instead, your float level might be too high and causing it to use more then it should. Jets will leak by and leak into the manifold.
You could have the outer air jets mis adjusted so that it runs rich instead of on the lean side. Or the main jets inside might be set up to run at a lower altitude, to run richer using the denser air, and so at your altitude and thinner air, they run to too rich. I don’t know what altitude you are at. But cars at sea level run different then those at higher elevations. Car computers try to keep the ratio the same no matter where you are, but sometimes they don’t.

Have a tune up done. Being out of time could affect gas mileage. Spark plugs not firing right, could waste gas.

First things first, you need to give us a baseline. Absolutely pointless to try and pinpoint “the problem”, when there is no idea of exactly what is going on, or how bad it is.

Is the speedometer accurate? Take a GPS along, compare with speedometer reading. Same? Very close? Good. If the speedometer is not accurate, compose another question to fix it.

Fill up tank (don’t top off, let it fill up and automatically shut off). Reset trip odometer to 0. Drive until you are down to 1/4 tank. Fill up at same station, preferably at same station. Let fill automatically and shut off. Divide trip odometer mileage by fuel required to fill it back up. Now you’ve got your MPG.

Tell us what your MPG is.

Does your check engine light work? Does the heater in the car work good? Have you inspected underhood to make sure everything is plugged in? Have you inspected all the vacuum hoses (and there are a LOT)? If you aren’t a pro at it, get someone who has been working on cars for awhile, and doesn’t drive a piece of junk themselves, to look over the engine bay, and see if any previous owners have messed with any sensors, wires, or hoses. If the computer controlled carb on these things is disconnected, fails, or otherwise doesn’t work correctly, it defaults to “full rich” and will of course waste a lot of fuel.