How speed affects MPG

[SuperFudd]

Here is a link to an article about how much speed affects miles per gallon.

Slow Down-Save Fuel - Car Pro Commentary | Car Pro USA

One point it makes is if a car gets 40mpg at 50mph, it would likely get ~23mpg at 80mph.

 

[mitdr774]

The EPA test only hits a max of 60 and averages something like 48.5 MPH. My car has about a 8-10 MPG difference between 60 and 70.

 

[Scot]

The EPA test only hits a max of 60 and averages something like 48.5 MPH. My car has about a 8-10 MPG difference between 60 and 70.

The EPA now does a weighted average of five tests (instead of two). They've added a 20F cold start & drive test, the US06 (which has aggressive accels and tops out at 80 mph) and the SC03 - which tests at 95F, has a large sun load and the A/C is operating.

But, yeah, there's a HUGE difference in our cars between 60 and 70 mph. You're someone that would definitely know, with your current commute.

 

[cndctrdj]

Between 70-80 is not much of a difference. But at 55 this car shines. I can easily get over 55mpg at 55. At 75 I get around 42. 80 is about 39-40.
If I am low on fuel. I just slow down

 

[ahtoxa]

Ideal speed for most cars is about 45 mph. That's the sweet spot for fuel economy as it lets you be in the highest gear with least load. Above that and air resistance becomes and ever larger factor.

 

[mitdr774]

90 miles a day for work, winter tires, tune, and this awesome weather keeps me right around 34-36 mpg. If I go back to the stock tune it wil probably go to about 38-40. Can't wait for warmer weather to come back so we can get off of winter blend an I can go back to over 40 even with the tune.

 

[$ub$onic]

90 miles a day for work, winter tires, tune, and this awesome weather keeps me right around 34-36 mpg. If I go back to the stock tune it wil probably go to about 38-40. Can't wait for warmer weather to come back so we can get off of winter blend an I can go back to over 40 even with the tune.

I forgot your driving a crazy distance right now. To bad my c

 

[$ub$onic]

90 miles a day for work, winter tires, tune, and this awesome weather keeps me right around 34-36 mpg. If I go back to the stock tune it wil probably go to about 38-40. Can't wait for warmer weather to come back so we can get off of winter blend an I can go back to over 40 even with the tune.

I forgot your driving a crazy distance right now. To bad my car will no longer run on a stock tune

 

[mitdr774]

Dont you just need a different filter to run a stock tune? Not a fan of the distance for work, but the job is within the jurisdiction of the local and it is where my employer decided i needed to go.

 

[$ub$onic]

That is what we were thinking but not 100% sure.


You should be catching my mileage shortly then

 

[mitdr774]

Probably hit 38,600 by the end of the night. Work this weekend is close but should be at 39,000 by next friday.

 

[SonicBoy24]

GEARING and AERODYNAMICS

Nowhere in that article did I see either of those words. With the appropriate gearing, you could get a Sonic to get 50mpg at 100mph, and I'm sure you could get a silverado to get close to 40 at those speeds as well. Obviously these numbers are made up, but I would bet money that I am not far off, assuming someone has the money and time to put the appropriate gearing into the car/truck. Everyone talks about "slow down slow down," well how about gearing and aerodynamic improvements. Sure, the cruze and malibu have vents that close at highways speeds for just those reasons, but slowing down is the not ablution to everything.

 

[SuperFudd]

..."There wasn’t much correlation between decreasing fuel economy and the vehicles’ frontal area and aerodynamic drag. That’s one of the surprises the DOE team is still trying to figure out, because it seems to violate a law of physics."...

 

[mitdr774]

Sure you could gear it to get 50 MPG at 100 MPH (maybe) but I doubt it would be driveable in the real world. If you went the route of normal gearing for 1-5 and then a super tall 6th you would run into issues. Transmissions do not tend to do well with huge changes in gearing and it would only be usable at above normal highway speeds. You would only see the better economy if in the tallest gear and I doubt it would be that big of an improvement. Keep in mind that as speed increases so does resistance. As resistance increases you need more power to continue to accelerate. More power means more fuel. Can the engine produce enough power at a low enough RPM to achieve those speeds yet still return good economy? I doubt this very much.

 

[Swordsmith]

An alternative to different gearing might be simply larger diameter tires, I was reading an interesting article on that a couple weeks back, it seemed to make sense, larger diameter means fewer revolutions for the same distance, which means in effect that all your gears become taller. And our various tuners can adjust the ECU so the speedometer still works correctly, so the only real worry would be finding a diameter that still fits in the wells, right?

 

[mitdr774]

That works as well. If you can do the math you could always just adjust the speed based on correction for the speedometer. If you know that the tires will result in a 10% change in speed, just reduce the indicated speed by 10%.

 

[SonicBoy24]

Sure you could gear it to get 50 MPG at 100 MPH (maybe) but I doubt it would be driveable in the real world. If you went the route of normal gearing for 1-5 and then a super tall 6th you would run into issues. Transmissions do not tend to do well with huge changes in gearing and it would only be usable at above normal highway speeds. You would only see the better economy if in the tallest gear and I doubt it would be that big of an improvement. Keep in mind that as speed increases so does resistance. As resistance increases you need more power to continue to accelerate. More power means more fuel. Can the engine produce enough power at a low enough RPM to achieve those speeds yet still return good economy? I doubt this very much.

Who says we have to stick to 6 gears?
We already know Ford and GM are working on 9-10 speed transmissions for trucks, it's only a matter of time before cars have them too, although they'd probably have to be RWD.

I just threw 100mph for the number, but with the torque of all these turboed engines, who knows what they would be able to do in terms of moving the car at high speed and low rpm.

My point continues to be, lowering speed isn't good enough.

 

[$ub$onic]

Ford also working on a 300hp 2.7 v6 Eco boost and already have a 2.3 4 cylinder that is 300+ hp.

And have a rumor of developing a v6 diesel f150

 

[sx sonic]

9 and 10 speeds. **** that, too much shifting lol. Same thing can be done with half the gears.

As for speed and gearing, you can only go so far, and that's not a whole lot farther than where we're at. Wind resistance goes up by the square of speed, power requirements go up at the cube of speed. No matter how you slice it mpg is taking a substantial dive as speed increases.

 

[mitdr774]

Making 80hp at 1500 rpm or 3000 rpm (just using those numbers as an example) to go a set speed will still require about the same amount of fuel. If anything forcing the engine to produce the same power at the lower RPM might be less efficient and consume more fuel.

The problem with smaller displacement turbo engines in bigger cars is that they really only benefit as long as you can get it to a point where the turbo is not helping. As soon as you start using the power (higher speeds, getting up to speed, city driving, towing, full load, ect) economy drops off fast.