Most efficient gear for Interstate Cruising?

[bull27]

I read somewhere on this forum that 4300-4500RPM range is the most fuel efficient, I might be completely off on this? In a 5 speed Boxster that would be 4th gear on the highway (approx 70-75MPH). I dont know about you guys but in Massachusetts there is no "cruising" at 80MPH, that is a ticket for sure, especially on the Mass PIKE!!!!!!

[denverpete]

Originally Posted by bull27

I read somewhere on this forum that 4300-4500RPM range is the most fuel efficient, I might be completely off on this? In a 5 speed Boxster that would be 4th gear on the highway (approx 70-75MPH). I dont know about you guys but in Massachusetts there is no "cruising" at 80MPH, that is a ticket for sure, especially on the Mass PIKE!!!!!!

The tallest gear ratio available operated at the lowest RPM's, without lugging the engine, will provide the best fuel efficiency. Smooth driving is key. Limit acceleration and abrupt lane changes.

Also, remember that drag increases in proportion to the square of speed. This essentially means you experience more than twice as much drag at 80 mph than at 55 mph - even though you're only going 25 mph faster.

So, cruising at 80 mph instead of 55 mph can result in as much as a 25% decrease in fuel efficiency. Obviously, this is completely dependent upon the drag coefficient of the vehicle. It's entirely possible that a Boxster is more efficient at slightly higher speeds than less aerodynamic cars. But, in general, a vehicle begins to lose fuel efficiency above 55-60 mph.

It's difficult to say what kind of penalty you'd have with the top down. That would be dependant upon the car's drag coefficient up versus down. But, my experience is that the top down significantly alters the aerodynamics of the car - and not in a good way. So, I would also expect to see a corresponding decrease in fuel efficiency.

[RandallNeighbour]

I'm betting that driving with the spoiler up will also decrease fuel economy. This makes me think that if I'm going to settle into a long cruise control-controlled ride at 70 mph on the freeway after passing a truck at 80 mph, I should slow down below 50 mph just to lower the spoiler and then raise the speed back up to 70 and ride spoiler-free.

Problem is I don't think about that spoiler since I can't see it and rarely think about it.

[wild1poet2]

Trade it for a Prius.....

Don't worry about the gas prices. Just go....
If you are going to worry about gas, then worry about insurance and repairs...especially tires!

Just think how much better off you are then all the SUV owners....

[JP-s-in st. louis]

3rd gear so you can get to 102 in a blink.

[mtch]

Originally Posted by denverpete

It's difficult to say what kind of penalty you'd have with the top down. That would be dependant upon the car's drag coefficient up versus down. But, my experience is that the top down significantly alters the aerodynamics of the car - and not in a good way. So, I would also expect to see a corresponding decrease in fuel efficiency.

A note on drag coefficient differential between top up and down. I went cruising top up a couple months ago and pulled from 70-148 using 4th and 5th gears. The car pulled hard and felt like it had plenty left when I let off. A couple of weeks ago, I tried the same pull and could not get above about 128 top down. I pushed to redline and could not climb any more. Not only would it not climb as high, it took much longer to get there.

At one time, I knew the equations for relationships between hp, vehicle speed and drag coefficient. I don't know anymore, and I'm to lazy to look them up, but experiencing the difference was astonishing to me

[blue2000s]

Originally Posted by mtch

A note on drag coefficient differential between top up and down. I went cruising top up a couple months ago and pulled from 70-148 using 4th and 5th gears. The car pulled hard and felt like it had plenty left when I let off. A couple of weeks ago, I tried the same pull and could not get above about 128 top down. I pushed to redline and could not climb any more. Not only would it not climb as high, it took much longer to get there.

At one time, I knew the equations for relationships between hp, vehicle speed and drag coefficient. I don't know anymore, and I'm to lazy to look them up, but experiencing the difference was astonishing to me

It's not a really simple formula. It also has to include the rolling resistance of the tires, which varies from tire to tire and you'd have to know what the wind was doing.

[blue2000s]

Originally Posted by bull27

I read somewhere on this forum that 4300-4500RPM range is the most fuel efficient, I might be completely off on this? In a 5 speed Boxster that would be 4th gear on the highway (approx 70-75MPH).

That's where the engine makes the most torque, therefore it's where the engine's volumentric efficiency is highest. This means that it is most efficiently burning the fuel at this engine speed. Think of it as gallons of fuel per piston stroke.

When looking at the overall fuel efficiency of the vehicle, you need to look at the gallons of fuel per mile driven. As stated before, the lower the engine speed, the less fuel is being consumed per mile, even if it is slightly more fuel consumed per stroke, simply because there are alot fewer piston strokes per mile which more than makes up for the difference in volumetric efficency.

[denverpete]

Originally Posted by blue2000s

....When looking at the overall fuel efficiency of the vehicle, you need to look at the gallons of fuel per mile driven.....

Ummm, isn't that kinda the definition of fuel efficiency? Even if most people actually look at miles per gallon of fuel. Or as I like to call it, MPG. It's catchy. You can use it if you want.....

[blue2000s]

Originally Posted by denverpete

Ummm, isn't that kinda the definition of fuel efficiency? Even if most people actually look at miles per gallon of fuel. Or as I like to call it, MPG. It's catchy. You can use it if you want.....

Not necessarily. If you read through my entire post, I have highlighted two kinds of fuel efficiency. The first is the efficiency with which the engine uses the fuel to make energy (peak volumetric eff.), the second is the efficiency with which the car uses the fuel (max mileage).

[denverpete]

Originally Posted by blue2000s

Not necessarily. If you read through my entire post, I have highlighted two kinds of fuel efficiency. The first is the efficiency with which the engine uses the fuel to make energy (peak volumetric eff.), the second is the efficiency with which the car uses the fuel (max mileage).

Yup. Re-read post. Engine efficiency. Very hi-fallutin. Interesting but not much use to us folks lookin to maximize mileage. GPM. Got it. Yup. Very fancy inverting them there numbers. Purty neat what ya can do with that there math.

'Round these parts we like to stick with MPG n'all. Not much use 'round here for GPM 'cept if'n you're talkin bout how much water it takes to get a horse from the farm to the big city.

Gotta go and clear out some tumbleweeds. Them critters just be getting ever where!

Well, I do most of my country road crusing at about 65-70 and highway cruising at about 75-80 which means I'm taching around 2900-3000 rpm. I measured the past 3 tanks for mileage, and I average 30.2 mpg. On one tank full, where I know I was a bit more of a lead-foot than usual (ok, I was really putting my foot into it), I dropped it down to 28.5 mpg. Not bad at all, really.

[Jump]

Thanks for the replies and discussion. To back up PorscheDoc, things are inexpensive here. Also, the speed limit is 75 so the HP doesn't even bat an eye at 80 mph. There can also be some long, straight, boring roads when commuting from one community to another by Interstate, hence the original question on gas efficiency.

[Rail26]

That provided my humor for the day...Thanks! So let me get this straight...do the tumbleweeds and ex-wives go in the wood chipper in South Dakota?

Originally Posted by denverpete

'Round these parts we like to stick with MPG n'all. Not much use 'round here for GPM 'cept if'n you're talkin bout how much water it takes to get a horse from the farm to the big city.

Gotta go and clear out some tumbleweeds. Them critters just be getting ever where!

[Brucelee]

Pete is correct.

6th gear for 80 MPH.