All good answers. Fortunately for me my father was a mechanic before cars became robots so I’m not all in the dark. I bought the car at 30K in 1999 so I’m familiar with it. I’m also sure that the gas gauge is off which is why I started filling up at 1/4. All of my numbers come from the receipts. I’m in Iowa were we use ethanol here. The most plausible explanation here is the switch from cold weather formula to warm.

But I’m going to provide some figures for those that are curious:

2/26/11 302.9 miles since previous fill up 11.38 gallons = 26.6 mpg

3/6/11 298.8 miles since 11.25 gallons = 25.76 mpg

3/16/11 316 miles since 13.45 gallons = 26.84 mpg

3/26/11 386.4 miles since 13.82 gallons = 27.95mpg

4/3/11 355.9 miles since 12.80 gallons = 27.8 mpg

4/12/11 332.7 miles since 13.31 gallons = 24.99 mpg ( odometer # 192525.6)

4/21/11 320.9 miles since 11.81 gallons = 27.17 mpg (odometer #192846.5)

4/30/11 366.5 miles since 10.82 gallons = 33.85 mpg (odometer #193123.0)

5/9/11 326 miles since 6.54 gallons = 49.84 mpg (odometer #193449.1)

The higher MPGs came from the same station which is not my normal station.

Everything except your last figure is normal deviation. I think your last figure is probably skewed due to not getting a full tank. Some pumps will shut off prematurely like that. I don’t every keep track of mileage this way. I just monitor the trip information program that gives me average and instantaneous mileage until I reset it. I can get wide fluctuations even on long trips due to terrain, whether going from low altitude to higher, wind, weight in the car, and so on. One stoplight can throw my average off quite a bit. So don’t be too anal about it.

I’m not sure where some of your numbers are coming from.

For example for 4/21/11 and 4/30/11 you have odometer readings, but 193123-192846.5 = 276.5, you appear to have 366.5

I also can’t get your mpg numbers for 3/6/11 and 3/16/11 to work.

Also I would say you’re doing your math wrong. You have “302.9 miles since previous fill up 11.38 gallons = 26.6mpg”. If you assume you totally fill the tank each fill up it should be “302.9 miles before a fill up of 11.25 gallons = 26.9mpg”. It doesn’t matter too much if you always empty your tank to nearly the same level however, but having your miles traveled and fill up amounts bounce around as much as they do indicates you aren’t doing that.

Given the error in your miles traveled on 4/30/11 I personally suspect you made a mistake when you recorded either your odometer and/or how much you filled the tank on that date.

Glad to see other people pointing in the same direction as me. Your last mileage figure is simply wrong. Start over.

3/6/11 298.8 miles since 11.25 gallons = 25.76 mpg <- wrong, should be 26.56

3/16/11 316 miles since 13.45 gallons = 26.84 mpg <-wrong, should be 23.49

It could also be your driving habits. You get better mileage if you do more freeway driving that is not stop-and-go. So if you’re driving more freeway miles and less street/stop/go miles, you’ll see an increase in MPG. Be thankful. Gas is climbing up in price.

Unlikely that driving habits resulted in a doubling of gas mileage.

Since a Cavalier, despite being an economical car, will not get almost 50 MPG, I’m assuming you either had quite a tailwind on a long trip, or the gas pump shut off prematurely, causing you to think it took less fuel than normal to fill the tank. If you get really cruddy mileage when you calculate it next time, you’ll know that’s what happened.

Every once in a while, I get a decent tank (from Costco, normally cheaper around me) that will produce 400 miles to the tank, instead of the normal 340/350. I just put it down to luck, and they got that particular load from someone with less ethanol mix than the last time. It happens, so for that tank, I’m just grateful. Not much I can do, but I’m happy about it, anyway. I would suspect something like this is what happened to you.

Chase

One way to make calculating and recording easier and less error prone is to drop the extra significant digits.

Since you’re not driving under test conditions random variation makes .1 mile, .01 gallon and .1 mpg meaningless.

Thus:

3/6/11 299 miles, 11.3 gallons = 26 mpg

I think the problem with the first two dates is that I simply typed them into here wrong. Sorry about that, but I don’t think 1 or 2 MPG is a big swing. I have a calculator so I don’t mind using decimals. The formula for figuring gas mileage is the same whether you get a lot of gas or only two gallons. On the last tank I had a gas card with $25 dollars and the pump shut off at that point. My commute is pretty much the same all the time. I had a 2nd set of eyes double check my odometer, however I am thinking that I recorded the 4/30 odometer reading wrong.

Well, then, looks like you have your answer. The numbers were reasonably close to each other, given the variability in pump shut-off, until the last two entries. The 4/30 entry sounds like you discovered an error, and the 5/9 entry was fouled thanks to an un-filled tank. Your next fill-up should have a big correction.

I think you have a bad thermostat, that it is stuck in the open position all or almost all of the time. As a result, your engine runs too cool in cooler weather. So in cool weather it stays in lower gears when it should shift into a higher gear in order to the raise the temperature in your small engine, which needs to run hot.

If you have a tachometer, see what the RPM is going 50 mph when it’s 70 degrees and what it is when it’s 50 degrees. I’d bet at 50 degrees it’s something like 3,000 RPM and at 65 plus degrees it’s under 3,000 RPM. That’d be where your gas went.

I didn’t come up with this solution, The top twenty guys on this site told me this about my Corolla and they were right. My concern was the high RPM’s and inability to shift into overdrive. But my gas mileage definitely suffered: it went from 33 mpg to under 25.

The place you are buying gasoline pump may not be registering the amount delivered correctly. You could be getting more gas than the pump says. Also, your car odometer could have gone out of calibration for some reason. I expect the answer though is more simple. You made a mistake in the odometer, or maybe someone else put some gas in and you didn’t know about it. It’s a good problem to have!