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Sudden and mysterious improvment in gas mileage

I have a 98 cavalier at 194K, manual and have been tracking my MPG for about a year now. For the last 4 fuel ups the MPG has been increasing from 27 and change to 33 and change. I did make the one change in my habit of gassing up at a 1/4 tank rather than letting it get into the red and today’s gas up my MPG was at 49.84. I checked it at least 20 times and had my son look at it to make sure I copied the odometer right. There’s been little to no maintenance over the winter (have only checked the tire pressure). Other than a change in the brand of gas and warmer weather I can’t figure out why the sudden change.
Any incite would help.

Shrug. You have provided no data.

I am certain your car is behaving just as it always has. For now all I can do is chalk this one up to some mathematical error, possibly miscopying the figure on the pump. If that is the case we will never know.

Continue your record keeping. I have no doubts your car’s fuel economy will henceforth return to its “normal” condition.

If you try to calculate mpgs on each tank of gas you will end up with very large amounts of error, especially on a tiny little tank like the cavalier’s. This is mostly owing to the fact that you don’t actually have any way to know that you are getting the gas tank to the same level each time. You are at the mercy of the automatic shut offs on the pumps along with other random things.

Track your mileage over about 5 fill ups and then do the math. The more miles/gallons in the calculations the less the result is affected by the fill to fill variations. (In fact all of that error should cancel itself out).

That said, if they pay attention many people (including me) have probably noticed a bit of an up tick in fuel economy recently owing to warmer weather, including a move of refineries from winter back to summer gasoline mixes.

Differences between tanks are not uncommon. However if you record each tank and average the results, you get a very accurate number. I recommend as cigroller has, get some more data. Post the actual numbers for us.

While I doubt if it is the reason for the change you are seeing, it is generally better to fill up sooner than later. Running low on fuel can (but seldom does) lead to lower fuel mileage and the difference is usually small.

And while you’re doing what Cigroller said, keep in mind that gas gauges are notoriously inaccurate. So when you think you’re filling up at 1/4 tank, you could actually have 2 or 3 gallons difference between levels at fillup, which would significantly skew your mpg calculations.

shouldn’t. I would assume he’s looking at the pump for the number of gallons, not estimating from the gauge.

Where do you live? How much has the weather changed?

I was getting a little over 300 miles to a tank. I took a 255 mile trip the other day on about a half tank of gas. Im not sure whats going on

As suggested, the only way to be even close to accurate is to use the full amount detailed on the receipt (like 9.283G), and the mileage over several tanks (from the odometer, not the tripmeter, which you’ll most likely be resetting after each tank). Even then, depending on how particular you are, there may be minute changes because of those 10th’s of a mile that aren’t considered.

IMO, every tank you go through will vary. From time sitting at lights, to the time you let it idle for 10 mintues when you ran back into the house to get that soda you forgot for your drive to the mall, to the time you sat talking with a friend with the A/C on before you went into said mall, or whatever. I know it does for me, but it’s the overall that’s important.

Now, if you suddenly get 100 out of a full tank when you normally 350, that’s serious enough to really look into.

Well it is the spring…and where you buy your gas from probably now is rid of the reformulated winter gas.

I get about +/- 1 mpg variation tank to tank driving in a fairly routine pattern. About 4-5%.
Maybe 28-30 MPG in the summer, 24-26 winter.

I use the same technique each time when filling up:
Switch the nozzle to slow when I know the tank is close to full, then stop after the first click-off: no topping off.

MPG = (miles since last fill) / (gallons this fill)

I use the trip odometer and the pump display. The car’s fuel gauge doesn’t factor in.

MPG per season is going to vary depending on make model ect.

circuitsmith…you still have a variable there depending on Highway/City driving

Nothing better to do this morning other than throw in my 2 cents

someone’s pranking you and putting gas in the tank when you aren’t looking.

Tell him to move to my neighborhood, and put fuel in my truck for me. :slight_smile:

badbearing…I have a pretty consistent driving pattern. About 75% city. I excluded road trips.

I get 33-38 mpg on the highway, depending…

By the way, OP, it’s “insight”, not "incite’.

Just wondering, anyone else ever drive the car, or just you?

Many many years ago I worked at a trucking company where someone was bragging about the gas mileage on his 58 Ford fliptop convertible claiming unbelievable numbers. The first week we added a gallon of gas to his car, increasing a gallon a week for 5 weeks. He became ecstatic! We then siphoned out gas in the same pattern, he became very quiet and never mentioned his gas mileage again.

It’s safe to assume you’re not get 49.84 mpg, so either you’re writing something down wrong or making a math error. Get the world’s cheapest digital camera and take a photo of the odometer and the gas pump each time you fill up.

Disregard all your past information as possibly tainted and start calculating over the next 3 to 5 tanks. You should be calculating over a rolling period anyway.

I haven’t noticed any significant change in my fuel mileage since I bought the car last year. I believe I’ve averaged 14mpg for most of my fill ups. Though, 93 octane might not see the ethanol content raised for winter fuel like 87 does; I’m not real sure on that.