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Nissan 300ZX Gas Mileage Issue

Hi. I have a 1993 300zx. Recently its gas mileage has been not great, as it’s lucky to get 200 miles per tank. A couple years ago it was getting 350 or so, but has been slowly declining and this year it’s just about tanked out. I took it into a Midas in March to get a major tuneup and mentioned the mileage issue and they did a lot of work but I haven’t noticed a great improvement. I took it back today to see what else could be done but they said they did everything they could; replaced the spark plugs, fuel filter, air filter, added cleaner to fuel tank. They said it could take 6 months for the stuff to take full effect and to see the improvement. Is this true? The car generates a lot of business whenever it’s brought in but they flatout said there’s not much they can do. Is this 6 month thing legit?

First off…Find a better mechanic. I wouldn’t take my car to Midas for air…

6 months to take effect…they haven’t a clue…

How is the car running besides the gas mileage???

The #1 gas mileage contributor is driving habits. This is NOT the type of car you drive gingerly to and from the grocery store. Nicer weather means more fun driving…

If your driving habits haven’t changed since before the gas mileage drop…then it does seem to point to something mechanical…but Midas is NOT the place to take it. Find a good independent in your area…or even the dealer…but Midas??? NEVER!!

200 miles per gallon? 350 miles per gallon? I’m confused. Do yo mean per tank? What’s your mileage in miles per gallon?

I assume you mean 200/350 miles per tank of gas. Whenever the gas gauge reads low, you fill up and reset the trip odometer, right? And average distances have declined, right?

This is a worthless method of determining fuel economy. It is possible your mpg is the same as it has always been but your fuel gauge is giving faulty readings. The only acceptable method of determining gas mileage is to keep a record of gallons purchased and miles driven.

It is too late to know what kind of true mpg you were getting previously, but you can determine the accurate mpg you are getting now. Such records may show your actual fuel economy to be normal, and may also indicate a real problem should one exist. Also, it will determine if a particular repair has had any success.

Wow…I missed the 200mpg…

Man if you’re getting 200mpg…Don’t let ANYONE touch that vehicle…It’s blessed.

My fault - it was getting 350 miles per TANK and now it’s getting 200. I’ll go back and fix that ha-ha.

I thought it may be the fuel gauge, but when I fill up the tank when it’s about 3/4’s empty it still fills up the same amount it always has. And the thing is — I mostly drive the car around locally, so it’s not going on the major trips it once was when it would easily get over 300 per tank. Maybe the reduced freeway driving is a factor. Would that affect it? Keeping track of the miles is a good idea and I’m gonna do that because even small trips can add up quick. Could be a combination of these things. It’s just when they told me 6 months to take full effect, that seemed kind of out there but I wasn’t sure.

Thanks everyone for your responses!

“I mostly drive the car around locally, so it’s not going on the major trips it once was when it would easily get over 300 per tank. Maybe the reduced freeway driving is a factor.”

That is THEE most likely answer…I’ll pretty much guarantee that’s the reason. With the exception of Hybrids - Driving on the highway at a reasonable speed will yield much better gas mileage then driving around town.

That does make sense. My freeway driving has really gone down with that car, and mileage in the city is less than highway…didn’t even dawn on me until I wrote all this out ha-ha.

Very helpful, thank you!

Replace the thermostat, and every 5 years or so (when the coolant is changed) from now on.

Your fuel tank is about 18 gallons. At 200 miles per tank that’s about 11 MPG. At 350 mile per tank that’s close to about 19 and a half MPG. The car is rated for 16 MPG city and 22 on the highway. However you mentiont that fill up when the tank is 3/4 empty, this means that you should only be putting about 13.5 gallons in. Now if you’re going 200 miles on 13.5 gallons of fuel, you’re getting close to 15 MPG. Which is very close to the city MPG rating for this car. If that’s the case then there’s probably nothing wrong with the car.

As others have pointed your methodology for determining the fuel economy seems to be a bit flawed.

Miles per tank is generally a not a vary accurate method of determining fuel usage.

Allow me to suggest a better way of determining gas mileage. Fill the tank until the handle self-stops, then set the trip odometer to zero. Then every time you fill, record the miles driven and the total amount of gas it took to fill until the handle auto-stopped. Divide the gallons used into the fuel pumped.

Do this a half-dozen times and you’ll have a good measuerment of your gas milage. Continue doing it and you can keep track of gas mileage changes.

Thanks everyone for your responses! I’m taking this all into consideration. Very helpful.