KevinGary

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 5% 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 (26.4 mpg adjusting for Audi’s optimistic trip computer (3.33%))
60 MPH – 26.7 MPG (25.8 mpg adjusting for Audi’s optimistic trip computer (3.33%))
65 MPH – 25.8 MPG (24.9 mpg adjusting for Audi’s optimistic trip computer (3.33%)).

I was surprised to find that the car was only 6% 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).

I recently updated this experiment in November of 2010 using 35 mile intervals and confirmed the mpg numbers used above.

jetshine

Not surprising... you get lousy mileage at lower speeds because the engine is doing all the work to overcome the car's weight. But at steady speeds, mileage improves due to inertia and the engine is just working to overcome drag (and the occasional elevation change). But drag increases exponentially with velocity, so the engine has to work harder to overcome the growing drag!

jazznuts699

i think you should definitely try 70/75/80... i would here... but the elevation changes every half mile... so that would be hard...

i drive at 75-85 on hwy and i think i avg around 21-23mpg (based on miles driven and amount of fuel filled up)... so i think your numbers are pretty accurate...

BlueSander

lol

I drive pretty fast like an *** all the time but this is how i maintain great mileage on the car: take advantage of the weight when going down hill --> let the car run as far as I feel like with minimal pressure on gas as possible, whenever i'm about to go up hill, give the car a little more push early so that you don't have to push it hard midway to top of the hill. I experiement all kind of conditions, cruising at different speeds, trying to go steady w/o cruise at different speeds, do what i said above... for the whole summer. I'm getting so good at it now too.

am_ver

I averaged about 27mph doing between 80-90mph from SF to Vegas, fairly aggresive (non-cruise control) and with AC and one more passenger. But then I have the 3.2.

OP - you need to invest in a good radar detector to avoid those speeding tickets. Either *always* drive at or below the speed limit, or get a V1.

KevinGary

I have a V1. I got nailed by laser. I am thinking about having a Laser Interceptor installed.

jazznuts699

laser is tough... the other day i was merging onto the freeway and i could see the cop lasering the guy on the far lane... my detector didnt even chirp... unless laser is on top of you (or the guy in front of you gets tagged) it's usually too late...

hence the laser deflectors...

KevinGary

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 (26.4 mpg adjusting for Audi’s optimistic trip computer (3.33%))
60 MPH – 26.7 MPG (25.8 mpg adjusting for Audi’s optimistic trip computer (3.33%))
65 MPH – 25.8 MPG (24.9 mpg adjusting for Audi’s optimistic trip computer (3.33%))
70 MPH - 24.0 MPG (23.2 mpg adjusting for Audi's optomistic trip computer (3.33%))
75 MPH - 22.6 MPG (21.8 mpg adjusting for Audi's optomistic trip computer (3.33%))

I recently updated this experiment in November of 2010 using 35 mile intervals and confirmed the mpg numbers used above.

jefman

Wow - now I see why you are a lawyer. I average 22 MPG's.

I average 65 mph in the am and 70 in the pm, on my 80 mile round trip commute with a little city driving thrown in. Faster at night - must be psychological, I think they have a harder time seeing me and I want to get home faster I too have a V1 and it saves me all the time. Laser is tough but I mount mine high under the mirror and I've been lucky up to now. Learning to interpret your V1 alerts helps a lot too. Now that I know about being 3 mph shy - I'll kick it up a notch, although that may have helped me from being ticketed.

BTW, don't lie to your wife. One look in your eyes and they know right away you are lying- I never lie.....I.....

'08 4.2

jazznuts699

sweet thanks for the update... i may try cruise control for the next few days at 70... see how i do