Very Low MPG in Traffic

[Keggers]

I use my '98 Carrera as my daily driver into work and back. While I get what I would consider reasonable mpg on the open road (about 28mpg over an average motorway trip (this is 23.3mpg in US gallons)), and around 18-20mpg in free-flowing traffic around town (about 15-16.5mpg in US gallons), on my commute into work through stop-start traffic, I'm getting about 12mpg (about 10mpg in US gallons). This is driving as smoothly as I can, so I don't think its me being overly aggressive.

This seems ridiculously low to me, but maybe this isn't that abnormal for these cars in these sort of traffic conditions? Its a 6 mile trip each way with about the first 3 miles reasonably free flowing then the second half in true bumper to bumper traffic (I averaged 13mph over the course of this week). The whole trip in takes me about 35-40 minutes.

It really seems to be the sitting still in traffic that is killing it, as I can watch the OBC mpg display literally tick down 0.1mpg every few seconds while I'm sitting there. Idles a bit below 800rpm and is reasonably steady.

I've done a few manual checks on the mpg being shown on the OBC by filling and then refilling and calculating manually and it seems pretty spot on, so I think the numbers are pretty accurate.

So I was curious as to whether people think this is acceptable, or could I have a problem with the car? Are there any logical places to look for vacuum leaks etc that might be causing a problem at idle? Anything else worth checking?

Any guidance appreciated.

Cheers

Greg

EDITED TO SAY THAT IT IS A TIPTRONIC - FORGOT TO MENTION THIS.

Edited January 25, 2008 by Keggers

[White987S]

Looks normal to me.

[jsoto]

That is barely enough miles to get your car warmed up. Short trips like yours will dramatically influence mpg. Have you ever taken longer trips to see if it is just your short commute each day.

I travel on a freeway everyday and get between 25-28mpg, but it is all highway travel. In the city, it reduces down below 20. I don't think any car would have good gas mileage on a 6 mile commute. I just think your circumstances dictate the mileage.

[hooster]

That is barely enough miles to get your car warmed up. Short trips like yours will dramatically influence mpg. Have you ever taken longer trips to see if it is just your short commute each day.

I travel on a freeway everyday and get between 25-28mpg, but it is all highway travel. In the city, it reduces down below 20. I don't think any car would have good gas mileage on a 6 mile commute. I just think your circumstances dictate the mileage.

Just think of it this way...how many miles per gallon do you get when you are not moving, but your car is running? None. On your trip (since there is so much stop and very little go) you should think of your fuel consumption in minutes per gallons, rather than volume per distance traveled. It is not a great number, but expected if you sit so much. Thankfully my 7 mile drive to work averages 35mph, including stoplights.

[Keggers]

Yeah Minutes Per Gallon sure is another interesting way of looking at it! Best not to think about the fuel being sucked up while I'm sitting there getting frustrated with the traffic!

Sounds like everyone is right and this is par for the course on such a mpg unfriendly commute. Had been getting around 20mpg in my last car (1989 3L 944S2) on the same route so had been surprised how badly the 996 compared, but then again its certainly worth it when I get the chance to use the car properly on the weekends!

[der meister]

Hi there,

Sounds like the traffic where you're at is worse than LA.

I see that at least you've got a Tiptronic to deal with that.

Otherwise, you ought to get a little beater (old car) for that commute.

Edited January 25, 2008 by der meister