Gas Mileage Mystery

`OK Guys, I give up. Why do I get better gas mileage going North than going South? I drive a 2009 Audi A4 2.0T Quattro automatic to work on Fort Leavenworth. The trip is 18.3 miles about 9 miles is 65 mph highway and the rest is town traffic. On the way to work the trip computer shows the average mpg always as 27 point something and the return trip as 24 point something. Morning and afternoon traffic seem to be about the same volume. The differential is there summer or winter. Is there something in the earth’s magnetic field to cause this or am I just seeing the first ravages of senility?

I don’t know the terrain there. Is there an elevation difference between point A and point B so that - on average - there is more climbing one way rather than the other?

Another possibility - the averages are running averages so they’re always a little behind where you are. If you start out with in town traffic, then get to the highway your running average will be lower than if you start out highway and end up in town.

Those computers can also be a little wacky. Have you ever checked yours out the old fashioned way? Fill - drive - fill - divide miles by gallons to refill.

What is the elevation at the origin point and the elevation at the destination point?

Good idea to check - start point is 900 msl, end point is 860 msl. 40 ft in over 18 miles may have an effect but it won’t be the 15% or so difference we’ve experienced.

So are the highway miles at the beginning of the run, at the end, in the middle or mixed in? If I had to guess based on your numbers I’d put the highway miles closer to home and the around town miles closer to work. If I’m wrong then what I’m thinking is wrong.

Does your car average the miles for your current trip? If the in town driving is done first It may weigh the average mileage down.

That computer in your car does not really measure miles per gallon. It sort of does. It looks at several streams of data and then guesses. Usually, if it has been calibrated for your kind of driving, it is close.

Going north or south could mean going up hill or down a little. That may be the difference. Something like morning and afternoon will usually mean different temperatures and that can make a difference.

I thought about the morning and afternoon thing, but it is backwards - though I also admit the assumption that the way to work is in the morning. If it was a cooler vs. warmer temperature thing the worse mileage would be on the way (runs cooler/richer longer) and better mileage on the way back (doesn’t run cooler/richer as long).