I just finished my first tank on my new '06 civic - I got 49.1mpg over 550 miles - I was very impressed.

However, yesterday before work I filled up and did the exact same commute and I got 59.1 mpg!!! Can anyone tell me why the difference?

I had my cruise control set to the same as I had on the first tank - 63mph - and I drove in the same traffic which I fight every day - I commute 50 miles one way to work. It was a little warmer out - probably 8 degrees warmer - would this make a big difference?

Why would I get 10 more miles to the gallon? Is the car breaking in? I have over 550 miles on it now.

Your feedback would be appreciated...thanks!

It's really tough to evaluate changes like this from one day to the next. I find that my mileage varies by probably 8 mpg (42-50) from one day's commute to the next. It depends on how you catch traffic lights, battery charge, AC vs. no AC, etc. etc. I guess what I'm saying is to increase your sample size a bit more and see if you can figure it out. My biggest swings come when the battery back is either fully charged (7-8 bars) or almost ready to go into forced regen (4 bars).

Eric

Wind can play a big part in gas mileage, especially on the highway.

Because you had just filled up. It's more extreme high and low milage at the beginning of the taken. And maybe it's slighly downhill on the way to work; if so it will even out on the way home.

Wait and see what the overall tank FE comes out to. Should be about the same, unless you did a major change like pump up tires or something.

Have fun!
--A

Actually the reason is because the first tank was your first tank! The car needed to break in. Even the tires alone can reduce mileage by alot when they're new, as they have more rolling resistance initially.

I understand your overall average will change more rapidly at the beginning of a tank, as there's not as many miles logged yet, but he was talking about swing between trip mileage, presumably being reset each time.

My guess would be a swing in the direction of the wind is the most likely candidate. Going from a headwind to a tailwind can make a huge difference at highway speeds, as drag increases quadratically with airspeed.

As for temperature, while 8 degrees is not a big swing, temperature can make a difference. However, that shuold be less distinct on a long 50 mile trip as a shorter one-way commute. The main advantage of warmer temperatures is that the whole drivetrain warms up faster. A secondary advantage comes from the fact that the air is less dense, reducing drag.

I just filled up after my first tank, and averaged about 51mpg, based on 515 miles and 10.2 gallons put into the tank. According the the trip computer, I have been averaging about 60mpg (over my last 300+ miles). I suspect that my tank was not really full when I got it. I suspect unless you topped yours off after you purchased your car, it was not really full either. Unless the tank is filled to the exact same level as you started with, you cannot get an accurate measurement.

It does take several tanks of gas averaged out to get as close as possible to the true mileage, as you likely do not fill the tank to the same level each time. As long as my driving habits do not change, I expect my 2nd tank to reflect what my trip computer says, which, like you, is around 60. Bravo!

Just out of curiosity, what did your trip computer say, 49 or 59?

I don't drive a Civic, but I'll tell you the single biggest factor that influences my short-trip fuel economy: initial engine temperature. Making a short trip on a warm engine can make a difference in fuel economy approaching 25% depending on just how short that trip is. Take this example with a grain of salt:

The trip to the local grocery store is 4 or 5 miles, down hill the entire time. I just have to get over one "hump" and then gravity does 90% of the work until I get into about a half mile of frequent traffic lights. On a cold engine I might manage 38-42mpg. (Did I mention that this is down hill the whole way?) But yesterday I did the same trip and broke 53mpg because I did it right after work, with the car fully warmed up. (And before anybody cries foul, the best return trip I've seen was 28 or 29mpg.)

So what I'm saying is this: if you drove to the gas station, filled up, and then made your trip to work, you may have reaped the benefit of not having to factor in that cold-start penalty that comes from running rich and idling high until the coolant temp gets into the "normal" zone.

As stated earlier, I have found that wind is the #1 variant in mpg. Having no wind, or a tail wind makes a huge difference.

I'm on my 2nd tank in my 06 HCH and finding it much better than the first. When I got mine from the dealer, the MPG indicator was at 9.6 MPG and the car had 11 miles on it. I can't imagine what type of tests, etc were happening on the car before I got it, but the FE was horrible. I only got up to 45.3 on my first tank with 500 miles on it when the fuel light started blinking. I'm up to 48.5 on my 2nd tank with over 430 miles on the tank. I have 6 gas bars left too. If it wasn't for the immense amount of stop and go traffic I've had on this tank (and the fact that my wife drove it a bit ), I would be much higher. I'm anticipating somewhere around 50 or so when I'm done. Every trip to and from work it keeps going up and up.