After driving my car, a 2005 4.2, on the interstate for about an hour this morning, I decided to try an experiment regarding average gas mileage at different highway speeds. Using the trip computer’s speed, not the speedometer (which German law requires to be inaccurate by approximately 3% on the high side), I set the cruise control for 55 MPH, 60 MPH and 65 MPH and left it set at those numbers for a distance of 10 miles for each speed (because the Long Island expressway is relatively flat, with a total elevation difference of less than 100 feet, each 10 mile section was roughly comparable). The car’s trip computer reports the following average MPGs for each speed:

55 MPH – 27.3 MPG (25.3 mpg adjusting for Audi’s optimistic trip computer (7.5%))
60 MPH – 23.7 MPG (21.9 mpg adjusting for Audi’s optimistic trip computer (7.5%))
65 MPH – 22.8 MPG (21.1 mpg adjusting for Audi’s optimistic trip computer (7.5%)).

I was surprised to find that the car was 20% more efficient at 55 MPH than it was at 65 MPH. Unfortunately I had to return home before I could try the same experiment at 70, 75 and 80. Perhaps I can try it early in the morning next weekend before the traffic shows up. Regardless of the outcome I will continue to tell my wife that the car is designed for the Autobahn and does better on gas at higher speeds.

I think I have discovered a correlation between my recent speeding ticket for doing 88 MPH and why my trip computer typically shows that I average 16.5 MPG in combined driving (15.3 MPG adjusting for the trip computer).

 

Don't be surprised -- that's consistent with everyone's findings. I drove from Madison WI to the outskirts of Chicago last night (~100 miles) in rain, construction, and heavy traffic that restricted me to 50-60 mph. My trip computer averaged 30.2 mpg (it's optimistic by 2+ mpg, so true gas mileage was probably ~28). The rest of the way home, about 200 miles, I drove at my usual average speed of 75-80 and the average steadily dropped to around 27, reflecting a true average of ~24-25 mpg. I like the higher mpg, but don't have the patience to drive that slow. Especially when I have to cover 627 miles in a day.

 

Well I continued the MPG experiment this morning and took my car, a 2005 4.2, back on the interstate and using the trip computer’s speed, not the speedometer (which German law requires to be inaccurate by approximately 3% on the high side), I set the cruise control for 70 MPH and 75 MPH and left it set at those numbers for a distance of 10 miles for each speed (because the Long Island expressway is relatively flat, with a total elevation difference of less than 100 feet, each 10 mile section was roughly comparable). The car’s trip computer reports the following average MPGs for each speed (including the mphs from last weekend's experiment):

55 MPH – 27.3 MPG (25.3 mpg adjusting for Audi’s optimistic trip computer (7.5%))
60 MPH – 23.7 MPG (21.9 mpg adjusting for Audi’s optimistic trip computer (7.5%))
65 MPH – 22.8 MPG (21.1 mpg adjusting for Audi’s optimistic trip computer (7.5%))
70 MPH - 24.1 MPG (22.3 mpg adjusting for Audi's optomistic trip computer (7.5%))
75 MPH - 22.6 MPG (20.9 mpg adjusting for Audi's optomistic trip computer (7.5%))

I was so surprised by the bump in mpgs at 70 mph that I did the 10-mile experiment two more times on different 10-mile sections of road, getting a computer reading of 24.0 mpg and 24.5 mpg. As a result, there appears to be a sweet spot on the 4.2 for highway gas mileage at 70 mph, which is 73 mph on the typically German optomistic speedometer.

 

Wow, 55 on the L.I.E. They must have loved you!

You know I have often wondered if most traffic jams are really caused by speeders slowing down to give someone doing the speed limit the Bird.

 

Its not surprising... HP required goes up by the square of the speed. Since it takes x amount of fuel and air to make x HP, increase HP required and increase fuel required. Simple physics.

 

It was a Sunday morning around 7 am, and the traffic was light. Had I tried it later in the day somebody would have run me over in their SUV.

 

It is nice to see that the transmission is set up to save fuel at 55 mph. If you can keep it a 55 mph try the L.I.E. Monday morning at say 7:30-8:30 if it's anything like 684 then 16mpg seems about right.

I have actually seen Idiots creating the traffic jams at 6:00with little or no traffic they slam on the brakes and slow down for no reason and back up traffic then take off again. WTF??? I guess that's why they wouldn't give me a full carry in NY! I would have given him a reason to slow down.

LEAD POISONING!

 

The 70 mph results seem right. Ive driven from san francisco to portland and back several times and average about 24 mpg driving between 70 and 75 mph. This is calculated from miles traveled and how much gas i take at the pump. I can live with a 4000+ lb car with 300 hp gettin that kind of milage.