As you note, this graph is just an example - and I’m sure the curve could vary significantly between various types of cars.

For a 4-cylinder, I would imagine the mpg would peak out around 50. But for a 6 or 8 cylinder, might it not peak out around 65 or even higher? It would be interesting to see a graph for various cars…

Also, it depends on the aerodynamics of your car, and whether you are going uphill or downhill… going downhill, the curve would shift to the right… and going uphill, to the left.

I heard someone say that on the highway, your fuel consumption is most closely related to how much pressure you are applying to the throttle … so if you are coasting on the highway, and are in your top gear - then your optimum speed is probably where you are barely pressing the gas and still staying in top gear - for an automatic, anyway.

Basically it is important to understand what is working here. You have a body hurdling through a medium (air), which provides resistance. You also have tires rolling against asphalt, which provides friction (this is beneficial as its the only reason we can steer). You require that the engine spins at some RPM, using an essentially fixed amount of gas in each cycle, which turns a shaft in the gear box, which is further reduced by a differential and ultimately ending up connected to the drive wheels.

Now, air resistance is proportional to velocity. So the faster you drive, the more air resistance your engine has to overcome. Further, friction is essentially fixed (since it is a coefficient * normal force, which when you are on flat ground is simply the weight of the car plus cargo and occupants).

Alright, so you have something like an efficiency. With each gear shift you increase the MPG, if you are at the same speed (where air resistance is fixed). We all intuitively know that 5th gear provides higher MPG than 2nd or 3rd. It’s blatantly obvious that your RPMs drop and the speed stays the same. Now, the ideal speed to travel is the one where the top gear is just barely usable. For me, I can use 5th gear at about 35mph. But if I need to rapidly accelerate, forget about it. I have a 170hp V6, but in 5th at 35mph it’s doing about 1600rpm — just not enough torque there (peak torque is 4250, 165ft-lb). But for straight cruising, 35mph is where 5th gear is usable and of all the speeds 5th gear covers, 35mph has the lowest air resistance. Notice here that it doesn’t matter the aerodynamics of the car. Any car will perform similarly — air resistance is proportional to velocity (there are indeed other factors such as cross sectional area, but these are FIXED for any given car).

Now, the part about this efficiency business. Since air resistance is proportonal to velocity, it is important to realize that some of the power generated by the engine simply goes into overcoming air resistance and friction of the tires. This is what the engine is doing when you cruise — it provides no net force, just enough to overcome the air resistance and friction at your speed and gear.

Now, check this out, you can work backwards. Suppose you know which car we’re talking about, and that you know a) the tire size (so you can get the rolling radius), b) the differential’s gear ratio, c) the gearbox’s top gear ratio. You can compute backwards and show what the RPM should be to drive the car at that speed and gear. Guess what? It’s lower than you’ll see — air resistance and friction, again!

So there’s something amiss here. Suppose you can drive at 35mph at 1600rpm, and when you work backwards you should have 1500rpm. You’ve lost 100rpm due to the air and friction. Then suppose you are at 70mph, theoretically you should be at 3000rpm. Now, from this information about driving at 35mph you would expect to lose 200rpm, so you figure the car will be at 3200rpm. It’s not. It would probably be 3400rpm (air resistance being proportional to velocity, we would expect the gap to double, but we doubled the initial RPM so the gap really quadrupels).

So, you see, at 35mph you lose 100rpm, at 70mph you lose 400rpm. But each of these cycles uses fuel. You are losing more fuel by going faster, in the same gear.

This is what the graph is showing — at low speeds the decrease in gear ratio is enough to provide a MPG bump, all the way up to 30mpg. In top gear, however, air resistance saps power from the engine causing an increase in lost RPMs and decreasing MPG.

Now, as to this 4 cyl vs 6 or 8 cyl business: the cylinders don’t matter. The only thing that changes going from a 4cyl to 6cyl is that you can use lower gear ratios (since it has more power), to increase MPG. But the increased number of cylinders will take away that advantage, which is why 4cyls get better gas mileage. You’ll notice that the same car equipped with a 4cyl will use higher RPMs in the same gear as the 6cyl version — this is because the weaker engine needs to run faster to produce the same power as the 6cyl. Although they will actually gear it to give the 6cyl a speed advantage, the 6cyl engine will still run slower than the 4cyl at the same speed and gear (the gear ratios and differentials are different).

So, what will happen at high speeds with a 4cyl vs a 6cyl. Put simply, the 6cyl can go faster. Since its geared lower, it has a higher top speed. However, the 4cyl may run into an issue where increasing the engine speed doesn’t increase the car’s speed — this is because the engine is fighting as hard as it can to overcome air resistance and simply can’t. A 6cyl would be able to do that, so it would be less likely to encounter such a problem.

Here’s a way you can notice it. When you’re driving on the highway, check out which way the wind is blowing. When you’re going say, 70mph, see if the engine is at higher RPM when the wind is coming towards you rather than pushing your from behind. Then compare it to a calm day. Here’s another cool effect: about 100 feet behind an 18 wheel truck is a little gap of air that doesn’t move. Here the air resistance is greatly reduced. If you drive there, you should notice the car’s RPM drop (if you don’t let off the gas, you simply speed up when you previously were cruising). Now that is max MPG! (Note: Driving this close to a big rig has dangers associated with it, I’m not encouraging you to do this, merely pointing out an effect).

Wow thats long!! Thanks for the post Jeff lots of good information there. Provides some excellent background details.