Highway MPG

[Rayme]

Ok, the reason that got my started this is all those people saying Camaro's/corvette/ big displacement engine gets good fuel economy on highways. Also dealers like to advertise the highways MPG, for obvious reasons of course.

I say it's all baloney! JEEZ!!!!!!!!!! It's easy to get good MPG on the highway, it has almost nothing to do with the engine. It's all in the final drive ratio, good tires and speed. Saying "get a corvette it can get 32 MPG on the highway with 350 HP" is terribly terribly misleading on how the "overall" MPG will turn out. It doesn't take einstein to realize after a couple read around here that most cars can go over 30 MPG just depending on how you drive it.

Manufacturers will start upping gear ratio and slapping on some LRR tires just to meet the new CAFE regulations, flipping the bird to city ratings. to keep their previous SUV's selling and selling the highway MPG to potential buyers thinking their SUV is actually good on gas.

Let's talk about the MEAT!!!!! City MPG, where cold starts, stop and go, frequent gear change and long idling actually hurt MPG. This is where a car shine, this is is "real life, under-the-city-EPA-rating" MPG everyday people get. This is where technology will shine the most (valve timing, cylinder shut down etc..). Give me a constant 28-30 MPG in the city and I'll be amazed.

Just venting off...discuss..

EDIT: Here's some MEAT CITY MPG

Toyota Yaris : 29
Honda FIT: 28
Mini Cooper: 28
Hyundai Accent: 27
Nissan Versa 5 speed: 26
Honda Civic: 26
Chevrolet Cobalt 5 speed: 25 (gained 1 city MPG after tweeks)
Ford Focus: 24
Chevrolet Aveo 5 speed: 24
Dodge Caliber : 24
Mazda 3 : 24
Volkswagen Rabbit : 22
Suzuki SX4 : 22
Subaru Impreza 2.5: 20 (****, look at what I drive!!!)

A little less shiny than advertised highway eh?

[Hateful]

I'm not really all that impressed with the advertised highway mileage.Most do no better than my Cavalier.

[theholycow]

For some people, city mileage doesn't matter because most of their driving is highway.

As for the VW Rabbit, I think that city rating is way low, and the highway rating (28) is too high. The close-ratio gearing is great for city driving and you can keep it under 1200 rpm if you want, and get 30mpg city. A normal driver just shifting at 1500rpm would get all the acceleration he wants and still get 27mpg city.

Now, I have no clue how they expect anyone to get 28 highway out of it with the largish-displacement engine and silly low gearing. A 2.5l 5 cylinder at 3000 rpm can't be very efficient, but it sure is lively!

[dieselbenz]

Originally Posted by theholycow

Now, I have no clue how they expect anyone to get 28 highway out of it with silly low gearing. A 2.5l 5 cylinder at 3000 rpm can't be very efficient, but it sure is lively!

My GF's 12 year old 180k mile volvo 850 wagon with annoyingly short gears (3k @ 70mph in 5th) and a 170hp dohc 20v 2.5 liter 5 cyl engine (sound familiar?) does 30mpg @ 70mph, 32mpg @ 65mph and 36mpg @ 55mph. There is no direct correlation between displacement and highway fuel economy IMHO. I suspect the Golf can beat those numbers in the real world.
In city driving the volvo returns 20-24mpg depending on who is driving.

To the author of the thread: I'm willing to bet given an identical driving cycle the corvette will return better fuel economy than your wrx. Not trying to knock the wrx, its one of my favorite cars. But the GM LSx series of engines are extremely efficient at part load by comparison to any other engine of similar power or displacement. This is due to reduced friction from its 16 valve single cam design.

[theholycow]

Originally Posted by tjts1

My GF's 12 year old 180k mile volvo 850 wagon with annoyingly short gears (3k @ 70mph in 5th) and a 170hp dohc 20v 2.5 liter 5 cyl engine (sound familiar?) does 30mpg @ 70mph, 32mpg @ 65mph and 36mpg @ 55mph.

Extremely familiar. That describes my VW's engine and gearing perfectly, except you didn't mention torque and I don't know if the VW is dohc.

There is no direct correlation between displacement and highway fuel economy IMHO.

I'm not sure I'd agree 100%, but given appropriate gearing, I'd say the difference would be minor. I was actually thinking about something similar this morning. Imagine a 5 liter V8, and a 2.5L I4 made exactly the same - same bore, stroke, even the same head, just appropriately different cam and crank to keep the firing order even (unlike 1970s Buick V6 oddfire). At any given RPM, the V8 would have to make approximately double the power; and at any given amount of power, the V8 should run at half the RPM. So, given twice as tall gearing, the V8 should use approximately the same amount of fuel to cruise at the same speed.

I know there's MAJOR stuff I missed there, and I only thought about it for a minute or two while in the shower, and that idea has no practical effect on anything, but it was an interesting thought process.

To the author of the thread: I'm willing to bet given an identical driving cycle the corvette will return better fuel economy than your wrx. [...] the GM LSx series of engines are extremely efficient at part load

In addition to engine efficiency, the Corvette's drag coefficient is .286 vs. the WRX's is .33 to .35 (depending on trim level).

[fumesucker]

Originally Posted by theholycow

Imagine a 5 liter V8, and a 2.5L I4 made exactly the same - same bore, stroke, even the same head, just appropriately different cam and crank to keep the firing order even (unlike 1970s Buick V6 oddfire). At any given RPM, the V8 would have to make approximately double the power; and at any given amount of power, the V8 should run at half the RPM. So, given twice as tall gearing, the V8 should use approximately the same amount of fuel to cruise at the same speed.

Engine friction rises non linearly with engine rpm.. I suspect it's close to a square ratio like aero drag. This means the four cylinder turning twice as fast will have more than twice the friction.

When BMW set out to design a fuel efficient version of an existing model in the very early 80's, they used a six cylinder and actually increased the displacement from 2.5 to 2.7 l. Low spring pressures in the valve train, fewer bearings, low rpm torque and high final drive ratios.

In addition to engine efficiency, the Corvette's drag coefficient is .286 vs. the WRX's is .33 to .35 (depending on trim level).

For P&G a Corvette can be thought of as a kinetic hybrid with a large storage system and a very powerful engine.

[theholycow]

Originally Posted by fumesucker

Engine friction rises non linearly with engine rpm.. I suspect it's close to a square ratio like aero drag. This means the four cylinder turning twice as fast will have more than twice the friction.

Well, that pushed the result of my thought experiment far in the opposite direction of what I thought would be realistic, into the optimism zone.

I was thinking it might be useful to start a thread trying to collect a list of where fuel's energy potential goes (mainly, a list of points of inefficiency). It would start with obvious ones -- actual energy required to move the mass of the vehicle, heat produced, aerodynamic drag, engine friction, energy lost reversing pistons, tire rolling resistance, friction in wheel bearings, friction in the drivetrain, parasitic losses to accessories and alternator, etc...

[DRW]

I wouldn't mind seeing manufactuers up the gear ratio in 5th gear to increase FE. I think that's one of the advantages of the new auto trannies that come with more gears: they can add a tall top gear and still get good response since the trans can downshift when needed.

Manual trans drivers don't like to downshift on the highway to get power. They're too freakin lazy, so we all get trannies geared so low it's as if we're driving in 3rd gear all the time.

[Otistheminivan]

Well as someone that drives both of those big engine sports car. A Pontiac formula and Corvette. I race in a event that has us drive around the country. We coverd this yr 4400 miles in 8 days and 2004 with my Firebird covered 7800 miles in 9 days. The best milage I have to date on either car is just under 32mpg highway(75MPH Average). City mileage is 17-20mpg and the big one is race track milage is 5-6mpg.

So yeah they get crappy city mileage when driven normally. If you were so inclined could you get better mileage with hypermiling techniques yeah, but I choose not too. Let see Both of these car make just over 400RWHP. Both car capable of 170Mph Plus. I can cruise around on the highway without holding traffic up and get over 30mpg. What do you expect!!!!!

John

[96hb]

Well, the newer Vette's DO get pretty good mileage for what they are, and you look pretty damn cool driving one, too. I have been seriously considering buying myself one next year.