shopdog

A few weeks ago I took a 1680 mile trip in the Corvette. Since most of the trip was over relatively flat and straight roads, I decided to find my car's MPG sweet spot. All cars have a speed where they get the best MPG. Obviously, aero drag increases with speed, but so does engine volumetric efficiency up to the torque peak RPM. So there will be a speed where economy is best. Faster, and aero drag requires more power and more fuel consumption per mile. Slower, and engine volumetric efficiency falls enough to hurt MPG.

So, I used cruise and varied my speed in 1 MPH steps from 60 MPH to 75 MPH, pausing at each step long enough for things to stabilize and noting the instant MPG on the DIC. My car is a 2005 coupe with the A4 and optional 3.15 rear ratio.

Now to summarize the results. Best MPG (31 MPG) occured at 68 MPH. MPG dropped to 29 MPG at 75 MPH, and on the low side, MPG dropped to 29 MPG at 60 MPH. So there you have it, for my car, on basically level ground, the speed for best MPG is 68 MPH. Faster or slower, the number declines.

What surprised me most about this is the relative symmetry of the result. With many cars, MPG declines slower with falling speeds than was the case for this car. MPG typically "falls off the cliff" as speeds increase above the sweet spot. That isn't as much the case with the C6 coupe either. I think that's because our drag coefficient is much better than for most cars.

Now 68 MPH is right at 2,000 RPM in my car. It would be interesting to see what happens with A6 or MN6 cars, which typically turn lower RPMs at 68 MPH. I suspect the sweet spot for those cars would be somewhat faster because their engines are working further down the efficiency curve at 68 MPH in top gear, but I have no data to confirm that suspicion.

FlyArmy

I'm always impressed with your posts as I am now. I've been considering doing that same test but have been too lazy to actually do it...so I'm glad you did it. I just wish you had an MN6 z51 haha. I have an MN6 z-51, and from the limited amount of testing i have done I've noticed the sweet spot somewhere between 60 and 80 (i think between 65 and 73 but don't quote me, yet). Right about at 80 mph I am at 2000 rpm just fyi. I will check it next time I have the roads/time to do it and find the spot and update this thread...

LS1LT1

I have seen similar results in my A6, I have yet to attempt a solid, scientific approach like you've done but I think I too have experienced as good (better?) fuel economy at around 70mph than I did at 60.
Thanks for the info.

VetVetter

I have an 05 C6 6-speed manual. I did the same thing on a 2k mile trip.

I found 71 mph as the sweet spot.

shopdog

What was your MPG? And what speeds above and below were 2 MPG worse?

fyreline

Well, I had the same question so I had to go out and try it. My '07 3LT M6 convertible's "sweet spot" is also 71 mph. I'm showing 31mpg at 71, 28 at 72, and 29 at 70. Got the exact same numbers on a second test (level ground, 72 degrees both times). I'm sure this isn't a precise, scientific test but it's good enough for me. That's unbelievable mileage for a car of this caliber. Bravo GM.

quickride

SHOPDOG: Your postings are always informative, thanks for the good information.

Our cars are exactly the same right down to the gear ratio. Your car wouldn't happen to be VY would it? I didn't get as precise as you but I did run a few tests a while back. I used the same patch of freeway to conduct my test. This patch of road is not totally flat but there are no real grades, only gentle ups and downs. What I did was set my cruise control to various settings, drive the exact same path, and notated the MPG numbers. I would also reset the Average MPG on the DIC before each run.

I found my 'sweet spot' to be 69 mph with a MPG reading of 29. At 70, the MPG was 28. At 72, MPG was 27. These are Instant MPG readings from the DIC. Since I also reset and used the Average MPG information, it should be noted that the Average MPG numbers on my test runs were maybe 1 MPG lower on all tests. Also the car's geographic location (how many feet above sea level) will affect these numbers.

What I didn't quite understand until reading your post is that my MPG dropped to 27 MPG when I set the speed to 65 mph. I laughed a little when I thought to myself that I would have to exceed the legal speed limit by just a little in order to get the best gas mileage.

Finally, the highest instant MPG reading I've ever seen was 32 MPG when I was on a long flat stretch of highway at 70 mph. It was about 7:00 am and the air temperature was about 50 degrees.

6spdC6

All cars have a sweet spot. My 98 was around 73 (if I remember right.)
The 05 was at 71.(both Verts) Too early to tell on the 07 Z06.
I have noticed for years that contrary to safety naizis and Gov that a lot of cars (not only Vettes) can get good MPG figures at speeds up to 80MPH. The difference is minor and in a lot of cases the milage is better up to a point going above the speed limit.