Did I Get E85 by Mistake? Nearly 20% Drop in MPGs

I just completed two long trips less than two weeks apart. Each time I drove from Chicago to St. Paul, taking pretty much similar routes and driving at the same time of day in the same kind of weather. I drove similar (or the same) roads and at the same speeds. The difference? The first trip I had an MPG of 61.8 going north. The second trip my MPG was 51.1. OK -- it isn't exactly a 20% drop, but it is pretty significant.

I purchased the gas at the same station -- a BP close by our house. I suppose I should not have bought the gas the second time when I saw a delivery was under way, but I needed to get gas and get on the road (and it was under $3.80/gal!).

I kept hoping the mileage would increase, but it wouldn't. After about 360 miles, I gave up and bought gas, even though the guess gauge still showed 4 pips remaining. (And -- the gas was only $3.20!)

After leaving the gas station, I noticed no real difference for the first 5 to 10 minutes, then the mileage started to increase dramatically, running up close to 60. The next day I returned to Chicago, and the mileage stayed up in the upper 50s (even though I was driving much faster). Then, once I was down to about 4 pips remaining, the MPGs just plummeted! It would not get up above 50! I was extremely upset and tried to baby things, but it stayed very low. I gave it about 20 minutes and could stand it no longer, so I pulled over for gas. Once again, about 5 minutes or so into the continuing drive, the MPGs just shot up again.

It is possible that the earlier tank was E85 that mistakenly entered the regular tank at the first BP? Or could it be that something else was in the gas (old gas/varnish) that got picked up by my car because I was filling my tank at the same time a delivery was underway at the gas station?

What would explain the fact that the mileage would drop dramatically, again, once I hit the "4 pips remaining" zone?

 

Highly unlikely, John. The bad gas I bought was in Illinois. The later tanks from MN (Red Wing) and WI (Madison) both produced MUCH higher mileage.

 

No. There was no noticeable wind during either trip.

 

I would agree if this was a trip at 70-75 mph. I've done this trip several times at better than 60 MPG, though, by driving on county and state highways at 50 to 60 mph. It takes longer than driving the interstates, but it is much more enjoyable!

 

You are correct. Still, I check tire pressure regularly, especially before a long trip. I didn't add or lose air, and the MPGs varied only with the change in gasoline. Also, I was not using A/C or heat (perfect temps to drive in lately!).

 

I don't quite understand your suspicion. The method for measuring mileage is simply reading the MFD. This is the way I've always done it, and I don't see how there could be a problem in doing so. I'm not talking about small changes over a period of minutes or a few miles. I'm referring to very significant differences that continue for hours at a time across hundreds of miles.

There is no "hint" about the mileage dropping after getting down to 4 pips. The mileage DID drop, and it dropped dramatically.

It started when I filled up my tank that was on fumes. I drove about 360 miles right after that, and the mileage was horrible. As you can see from the earlier posts, I was driving a route I had just followed two weeks earlier. I think the mileage comparison is clear enough -- if I had a drop of 5 MPGs, then it might not be a statistically significant change. The drop I had, though, was quite significant. I filled the tank and started driving right away. It was only a few minutes later that I saw the mileage go up, again rather dramatically, and I was just continuing on the same drive. The very next day I continued driving, and the mileage remained high, until I was back down to the "4 pip" level. I had been driving hundreds of miles by then, and the circumstances were just like that of the previous day. That is where the mileage dropped -- again, it was a dramatic drop, and it seemed to me the car was just not as responsive.

I know that gasoline mixes well, but that is why I was thinking I may have received quite a bit of E85. Gasoline and alcohol do mix, but so do alcohol and water. Really, the big concern is that I received a jolt of water and other junk that got kicked up during the filling of the station's tanks. It could be that the new gas I received simply was quite a bit cooler and thus dropped right to the bottom of my tank. If it was a better quality, then it rather quickly would have started to feed the engine. Why it would continue to remain separated for more than one-half of a day is the real mystery.