Nobull

I have a 2014 c7 z51 auto with npp package. Can anyone out there explain why I get better gas mileage at higher speeds, i.e., 27/28 mpg at a constant cruise control 65mph on a flat freeway surface vs 32/33 mpg at 75mph on the same surface. Several of my club members told me that they noticed this in their c6 corvettes. This makes no sense to me and does not relate to the physics I have been taught. The drag resistance has to increase at speed. I am a relatively new member to the forum, and this may have been addressed previously, but I am puzzled. Any answers out there?

Patman

I haven't noticed that at all with my C6. I have gotten as high as 36.1 MPG on a perfectly flat highway stretch with the cruise set to 60 mph (this is with a 6 speed manual and non ethanol gas), but have never done anywhere near that figure when I've had the cruise set to 70 or 75 or 80. The higher I set the cruise, the lower the MPG.

With a C7 though, I could understand how some might see better MPG at higher speeds, as it might stay in V4 mode continuously at the higher speed, versus kicking in and out of V8 mode if the speed was too low in the highest gear.

CaryBob

The automatic will go into eco mode at about that (75) constant speed won't it?

Rad22

I must admit that I have gotten exactly the opposite results in my Z-51 M7.

On a relatively high speed run on an interstate with a 75 MPH speed limit and the cruise control set at approximately 80 MPH for over 100 miles, I only got 27 to 28 mpg.

Recently, I took another trip of just over 100 miles on a limited access four lane with a 55 MPH speed limit and the cruise control set at 60 to 65 MPH. I got 30 to 32 mpg.

Both trips were in the relatively flat outback of eastern North Carolina. Both trips were made in the eco-mode as well. Temperatures were similar on both days and it was fair and sunny each time with no other definite variables that I can think of.

lastcowboy

what rpm's are you running at ? lower rpms will = less gas

FYREANT

Same here or close to.

This all has to do with two factors; the resistance of the road and wind (drag coefficient) and the rpm to drive ratio. The lower you are in the RPM band in the high gears, the harder it is for the engine to turn over and therefor it uses more fuel trying to do so. Then you factor drag into this. Basically, each gear has a sweet spot for RPM and drag which will change based on how windy it is. You only get the best mileage when you are in the sweet spot..

W88fixer

Sure will on my C7 base model. The only time I use ECO is on flat or downhill stretches of interstate cruising, otherwise SPORT mode. Don

romaja

Their are many factors. The grade of the road and wind are 2 factors that are seldom discussed.

Roads often appear perfectly flat yet might have a slight grade which can help or hinder mileage. ( This is also true when recording 0-60 times)

Wind is another factor. Seldom is it dead calm.

The best way to eliminate these 2 factors is to do a run in one direction, then turn around and immediately do a run on the same stretch of road in the opposite direction.

Best,

JerryU

:agree: It's a blend of fuel use efficiency versus engine rpm, as you say sweet spot, and the aerodynamic drag which increases with the square of the speed. The low coefficient of drag does reduce this force with the C7 but it still is a big factor. Found these two graphs, for folks that like graphs, that helps define why there is an optimum speed for the Vette. If on a flat road and in 7th gear, ~ 60 to 65 sounds like the engine is at its optimum fuel efficient rpm versus the extra drag from air resistance. When I have looked at instant mpg believe I saw about the speed being best.




Fuel Consumption vs Engine RPM on Bottom Graph. Top Graph Shows a Typical Optimum mpg vs Speed (Note, not graphs for C7, just examples)

thrilled

On one form A guy said he got 38 MPG.he would have it on ECO and in 7th gear.

64drvr

Think of it in aviation terms. Max range speed. Ie fuel consumption vs. vehicle velocity.

Example, you may be thinking the slower you go, the better. If you are idling in 1st gear going 3mph, you are likely at your max endurance speed(lowest aerodynamic drag, and lowest engine rpm). You may last 15 hours on 10 gallons, but youll only go 45 miles on 10gallons= 4.5mpg

However,
Try going 70 as you did and achieve 33mpg.

There are lots of variables like profile drag, induced drag, parasite drag, engine powerband, winds, DA, etc.
the bottom line is how many drops of fuel you consume per mile driven(or whatever metrics you like).

The simplest way i feel in a car like ours to quickly evaluate your max range speed is to use the instantaneous MPGs on the DIC. In constant road conditions, hold about mph. Note inst. mpg, slowly increase your speed at 5mph intervals and record your results. You will quickly bracket your best speeds. My wifes c5 doest best around 62. My c5 is best around 53. Our new z06 does best around 60.
Treat the dic as a trend indicator, then later hand calculate to validate dtc %error.

Bill17601

Mine gets better milage in the hills then I do on the flat. I think the downhills are super efficient...

SFZX

Did a three thousand mile round trip to Colorado and back to visit friends.
M7 even running mostly 6th gear, absolutely no eco-mode, with rpm avg in the 1800-2000 range and running shifts up pretty close to 4500. Using V-power Shell avg mileage was 29.8. Blew my mind never had this with any car couldn't believe how well it did, totally fun drive! I love this car!!!
Could not see much difference in eco mode after about five minutes driving so shut the blasted thing off and have never used it again.

Pocket Aces

That's physically not possible if you are in the same gear. I think you must be imagining things.

NavyBlue2

I got 30+ mpg going 100 mph in 9th gear on 2 cylinders,
going downhill both ways...

JoesC5

Under certain conditions, cruising at low RPM's in a very high gear can lug the engine, hurting gas mileage.

tambeng

Is there such a graph like the above for C7?

Thank you!

JerryU

It is dependent on so many variables, like hills or lack thereof, wind speed, etc etc would not be a generic one. These were just put in as examples. I have done my own tests on a fixed highway between my house and town (16 miles.) It is pretty flat. I get my best mileage at ~65 mph in 7th gear Sport. Was not much better in Econo (4 cylinder mode) so I just keep the car in Sport all the time. My Z51 has Biltein shocks so no effect on ride..

thrilled

I cruise at 60mph in 6th gear.I think it's around 1500 RPM's.
I don't believe in hurting the engine.If I want to accelerate I drop it down
A gear or 2.

Larry/car

Aerodynamics, it is just not air flowing over a surface, other factors are at play. Impact air can flow differently over the car's frontal area as speed changes, high/low pressure areas form. Another factor, the C7 has engine cooling airflow over the top of the hood effecting drag. Explanation could take pages of dialog. GM engineers probably have volumes of data.