Most efficient gear for Interstate Cruising? - 986 Forum - The Community for Porsche Boxster & Cayman Owners

Jump · 06-14-2006, 02:30 AM

Does anyone have any thoughts as to what the most fuel efficient gear would be on a Boxster S for Interstate cruising at around 80 mph? Obviously, the lowest RPM's would be in 6th but I don't think that means it is the most fuel efficient at this relatively slow speed? With gas prices the way they are, no sense in wasting it when performance isn't in the equation.

Mike · 06-14-2006, 03:07 AM

Cruising at 80 MPH in a BoxsterS requires less than 50 HP. On this basis, 6th gear should provide optimum fuel economy, especially if you use cruise control and keep the top up and windows closed for the maximum aero advantage.

HOWEVER, the small penalty in gas mileage for running with the top down is well worth the price.

MEB

06-14-2006, 03:48 AM

Quote:

Originally Posted by Jump

Does anyone have any thoughts as to what the most fuel efficient gear would be on a Boxster S for Interstate cruising at around 80 mph? Obviously, the lowest RPM's would be in 6th but I don't think that means it is the most fuel efficient at this relatively slow speed? With gas prices the way they are, no sense in wasting it when performance isn't in the equation.

Maybe your concern should be the price of speeding tickets vs. the MPG's. What is the legal speed limit in North Dekota? If it is 70 or 75...I guess 80's not that bad.

bull27 · 06-14-2006, 04:28 AM

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 · 06-14-2006, 06:49 AM

Quote:

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 · 06-14-2006, 07:23 AM

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 · 06-14-2006, 10:49 AM

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 · 06-14-2006, 11:58 AM

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

blue2000s · 06-14-2006, 02:10 PM

Quote:

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 · 06-14-2006, 02:29 PM

Quote:

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 · 06-14-2006, 03:31 PM

Quote:

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).

PorscheDoc · 06-14-2006, 04:02 PM

Quote:

Originally Posted by bmussatti

Maybe your concern should be the price of speeding tickets vs. the MPG's. What is the legal speed limit in North Dekota? If it is 70 or 75...I guess 80's not that bad.

Lol, I am from North Dakota. The last ticket I got for 19mph over cost me 20 bucks.

And the speed limit is 75 on I94 and I29.

06-14-2006, 04:07 PM

Quote:

Originally Posted by PorscheDoc

Lol, I am from North Dakota. The last ticket I got for 19mph over cost me 20 bucks.

And the speed limit is 75 on I94 and I29.

I remember about 8 years ago, the speed limit in Montana was "Reasonable & Prudent". I think this has since changed to a posted limit. I think the nightime limit was 75.

rctboxsters · 06-14-2006, 04:13 PM

Quote:

Originally Posted by PorscheDoc

Lol, I am from North Dakota. The last ticket I got for 19mph over cost me 20 bucks.

And the speed limit is 75 on I94 and I29.

Are you for real? 19 MPH over the speed limit and only cost 20 bucks. Man, even parking ticket are much more than that.

06-14-2006, 04:16 PM

Quote:

Originally Posted by rctboxsters

Are you for real? 19 MPH over the speed limit and only cost 20 bucks. Man, even parking ticket are much more than that.

I think PorscheDr. is dating himself!

PorscheDoc · 06-14-2006, 04:16 PM

Quote:

Originally Posted by rctboxsters

Are you for real? 19 MPH over the speed limit and only cost 20 bucks. Man, even parking ticket are much more than that.

Living in the Dakota's is a little cheaper (ok, a lot) than the rest of the country. Life is a little less complicated, lol. You just have to put up with 6 months of winter. Can't wait to get back.

PorscheDoc · 06-14-2006, 04:18 PM

Quote:

Originally Posted by bmussatti

I remember about 8 years ago, the speed limit in Montana was "Reasonable & Prudent". I think this has since changed to a posted limit. I think the nightime limit was 75.

They changed it back to 75, because before they got rid of the speed limit, everyone drove 75. After they got rid of the speed limit, everyone drove 75, with the exception of a few jackasses that would buzz through at 140. So they just decided to drop it back to 75 again.

06-14-2006, 04:24 PM

Quote:

Originally Posted by PorscheDoc

Living in the Dakota's is a little cheaper (ok, a lot) than the rest of the country. Life is a little less complicated, lol. You just have to put up with 6 months of winter. Can't wait to get back.

Yea...but don't they chop-up people there with giant tree shredders? Or is that just in the movies?

PorscheDoc · 06-14-2006, 04:26 PM

Quote:

Originally Posted by bmussatti

Yea...but don't they chop-up people there with giant tree shredders? Or is that just in the movies?

Haha, you know only the 1st 5 minutes of the movie "Fargo" took place in North Dakota, the rest of it was in the damn Minneapolis area

denverpete · 06-14-2006, 06:01 PM

Quote:

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!