how long did u drive at 100 mph to figure out your mpg. i hope ur right

 

i cant get past 23mpg

 

I drive 38 miles each way to work on an empty highway. I've done test over an entire tank using the cruise at 65, 75, and 80. I've noticed that at 75 I get 29.5MPG, at 80 I get 29MPG, and at 65 I get 28MPG.

The only thing that I can come up with to explain it is that although the change in drag increase the square of the change in speed, our engines are designed to run at higher RPM's. (Just look at any HP curve). So holding higher speeds on the highway, with a very low (easy to turn) 6th gear, places the engine up at an RPM where it is producing more power and can punch through the extra drag. Couple this the fact that you are covering more ground at higher speeds and the result is better gas milage.

Now, from what I've seen, as you begin to go over 80, the gas milage should being to drop off quickly.

Anyone with a VAFC could easily test this by taking their car on a flat stretch of road, set the cruise at various speeds, and record the throttle % at each speed. The lower the throttle % the less fuel you are using. Then take this value and divide it by the speed. The lower the result at a given speed, the better MPG you are getting.

I'd be interested in seeing if results vairy based on model year and engine type (F20C vs F22C).

If anyone decide to test this, post your results.

-Chris

 

I know nothing about drag co-efficients, etc. However, at 80-85 my mpg is 24-26 as opposed to the normal 21 in town. I really don't care as long as the top is down and I've got music on.
cruise with attitude

 

Let's clarify something. The ENGINE may be more efficient at a particular speed, this does not make the CAR more efficient at higher speeds. If you truely believe that the ENGINE is more efficient at a particular higher RPM you can SHIFT to keep the engine at that RPM.

-b

PS My bet still stands. 55vs85, but I'll make it 55vs100 if dolebludger insists.

 

It certainly can. The efficiency of the car is the energy use divided by the distance travelled. See my post above for how the efficiency of the engine gets factored into that.

 

mikegarrison,

Engine efficiency is a function of RPM, not vehicle speed. The gearshift can be used to select the RPM that is most efficient. Granted you would be better off with a continuously variable transmission, but a 6 speed will put you close enough.

If indeed the rich area of the fuel map has such a great effect (which doesn't matter to me either way), it is still more efficient to go slower and downshift to keep the engine in the sweet spot than it is to go faster.

Lets assume that the fuel map is significant. The resulting conclusion should be that the car still isn't more efficient at higher speeds, it just takes a counter-intuitive technique (downshifting) to make it efficient at lower speeds.

I read your earlier post. You've got efficiency shown as a constant. I think we both agree it is not. Your also missing a term in the equation to reflect the amount of energy consumed by the engine just to keep running.

I think the equation should be more of the form:

Work/distance=eff(rpm)(a+b*v+c*v^2+e/v)

where a, b and c are the terms of the original equation, and e is a term that reflects the baseline energy required to keep the engine running.

If we assume rpm is one of 6 ratios with speed, we can solve for 6 different curves and pick the one that is the most efficient.

-b

 

You are misdefining efficiency. Engine efficiency is energy out divided by energy in, which (yes) is a function of RPM and doesn't really care what gear you are in.

But MPG (vehicle efficiency) is a function of speed. It very much does care what gear you are in.

Just think about it -- at 7K rpm in fifth gear you have about the same engine efficiency as at 7K rpm in sixth gear. But in sixth gear you are moving faster, and thus covering more distance, and thus making a better mpg.

This fundamental error is the source of your great confusion.

By the way, the energy needed to keep the engine running is the whole reason why the engine efficiency is not 1.000! Just mentioning "engine efficiency" means you are accounting for it.

 

MPG is indeed a function of speed. Agreed. Drag, rolling resistance, etc all require more power from the engine the faster you go.

What you're describing is not a steady state condition. Assuming the throttle plate is open the same amount and therefore both engines are producing the same power, then the car in 5th gear will accelerate. If the throttle plate is open less, then the car in 5th is drinking less fuel. It's also fighting less drag and rolling resistance because it's going slower, so if it's running at the same engine efficiency as the car in 6th, then it will have better fuel economy.

This fundamental error is the source of your great confusion.

-b

 

This assumes your engine is making the same amount of power. In sixth gear @ 7K engine speed, the car is moving considerably faster than in fifth gear @ 7K engine speed. Using the AP1 gearing, that would be an increase in speed of about 19.7% and an increase in required power of about 72%.