The 40-MPG Challenge

[yottabit]

I think the perception of "better fuel economy at higher speed" stems from the fact that most associate low speed driving with more stop and go. If you are able to sustain a lower speed then you will almost always get better efficiency. A Brake Specific Fuel Consumption chart will show you sweet spots for your engine if there are some available to help determine the right gear to be in at a certain speed.

You're right that say being in 5th going 30 would not be the best for fuel economy. But Being in 3rd or 4th going 30 (depending on your gear ratio) and cruising at a steady speed could definitely yield better mileage. You're right that some cars don't have the right gearing for efficiency at a certain speed. But I assure you there is an ideal speed for efficiency in each gear. So if in 8th it is 85, in 5th it could be 40, etc

I used to drive the same route 400 mi a lot in several different cars.

In a Chevy Beretta (GM V6) I would get about 31 MPG if I went ~65 MPH
Picking the speed up to 75-80 I would get about 26 MPG

In my old Saturn (1.9L) I would get about 33-34 MPG if I went ~75-80
Traveling at 60 MPH I got 41 MPG

 

[yottabit]

BAD F150 Gas Mileage

I drive my boss' truck a lot to pick up equipment

He has a similar F150 (5.4, king cab 4WD)

When I get in his car the average MPG is usually ~16

Driving hundreds of miles at ~70 MPH and then picking up a piece of equipment that weighs 500+ lbs and driving back with it protruding out the cab at the speed limit, I got 19+ MPG highway. I used Tow/Haul mode up hills.

On short highway trips I can get 21-22 MPG with it

I'm confident if I took it on a long cross country trip going 55-60 I could get about 24 MPG

I don't understand how people drive so bad

 

[LTC8K6]

I would think that because of aero drag and the way it increases with speed, there's very little chance that you'd get better economy at 75 than at 60 in high gears.

Plus you have the rolling resistance in there somewhere, which isn't huge but it does increase with speed, I think.

The engine is going to have to work much harder to maintain the higher speed, as far as I know.

 

[nicolaskl]

I would think that because of aero drag and the way it increases with speed, there's very little chance that you'd get better economy at 75 than at 60 in high gears.

Agreed. It's far more likely that the people that think they've experienced this had other less obvious factors in play. More or less traffic, more or less hills, more or less slipstreaming, different temp/humidity, different energy content of the gas, etc.

The power required to overcome drag increases as the cube of the velocity, there's basically no way your drivetrain efficiency is increasing at a rate that is MORE than that between 60 and 75.

 

[50 mpg by 2012]

Makes me really want a diesel. Too bad the difference in cost savings over an already decent MPG car is small, even on an annual basis.

I averaged one of my last tanks of gas with mostly rural highway but with some city and it was 37.6 MPG. No special care taken to achieve higher. I think I could easily get 40 on rural highways.

I agree with you that small incremental improvements in fuel economy do not, in my opinion, justify spending money for a new vehicle purchase.

However, I just increased the base cost of my 45 mpg highway (at 60~70 mph with AC on) vehicle by ~30% today. I bought it a $300 set of new tires. That seems reasonable for a 95 Civic CX 3dr HB with 150k on it and still running great ... only paid $900 for it a few years ago. I guess my total cost of ownship is very low since no comprehensive insurance is justified and personal property tax is <$10/year. Plus, <2.5 gallons/100 miles does not hurt.

This vehicle is one of the reasons I am looking for a >50 mpgUS combined diesel minivan.

 

[50 mpg by 2012]

Honda, long known for their brilliant motors, needs to step it up. Look at the Mazda3 2.0i SkyActiv and even the Ford and GM options for their compacts like the Focus and Cruze. It's been proven that you can actually have a nice balance of usable hp/tq along with good fuel economy in an economy car.

Watch for the US version of the Mercedes 2.143 Liter twin turbo diesel AT 2500 Sprinter in 2013. It is predicted to provide average 30 mpg(US) user experience in mixed driving route applications with max 55 mph speed limiter. This vehicle has a base curb weight of 5,700 pounds and GVWR of 8,550 pounds.

Just think, take a 2012 Chevrolet Sonic 1.8 L, 4 cyl, Automatic (S6), Regular Gasoline with a 28 mpg(US) combined and MPG Estimates from Drivers Like You ... crush the sides in 7 inches ... put the Sonic into the 2500 Sprinter ... and possibly get better average mpg than the Sonic alone.

Size and weight are NOT the problem dictating small economy cars ... it is US engines, in my opinion.

 

[SparkyJJO]

Regarding gear ratios I replaced the rear axle on my old 98 Camaro V6 with a lower ratio axle. Stock was 3.08 open diff, the replacement was 3.42 with a posi unit. Best tank I ever got out of that car before the swap was 29 highway. After the swap I got 32. Yes, I corrected for the speedometer being off (which I got fixed via tune later). Had I not corrected for the speedo being off it came out to 35

This goes back to the torque curve that someone else mentioned. The engine was simply more efficient at 2100 RPM vs 1900 RPM. I thought my mpg would drop a little which I was OK with, but the 3mpg boost was certainly welcomed

Honda, long known for their brilliant motors, needs to step it up. Look at the Mazda3 2.0i SkyActiv and even the Ford and GM options for their compacts like the Focus and Cruze. It's been proven that you can actually have a nice balance of usable hp/tq along with good fuel economy in an economy car.

I don't understand, what is (or was) so brilliant about Honda's engines?

 

[Throckmorton]

Regarding gear ratios I replaced the rear axle on my old 98 Camaro V6 with a lower ratio axle. Stock was 3.08 open diff, the replacement was 3.42 with a posi unit. Best tank I ever got out of that car before the swap was 29 highway. After the swap I got 32. Yes, I corrected for the speedometer being off (which I got fixed via tune later). Had I not corrected for the speedo being off it came out to 35

This goes back to the torque curve that someone else mentioned. The engine was simply more efficient at 2100 RPM vs 1900 RPM. I thought my mpg would drop a little which I was OK with, but the 3mpg boost was certainly welcomed



I don't understand, what is (or was) so brilliant about Honda's engines?

32 highway mpg in a Camaro? I know those cars are very aerodynamic but I'm surprised

 

[Throckmorton]

Agreed. It's far more likely that the people that think they've experienced this had other less obvious factors in play. More or less traffic, more or less hills, more or less slipstreaming, different temp/humidity, different energy content of the gas, etc.

The power required to overcome drag increases as the cube of the velocity, there's basically no way your drivetrain efficiency is increasing at a rate that is MORE than that between 60 and 75.

I think the amount of energy usage increases at the square of velocity. Then that's offset partially by the trip taking less time.

So theoretically if you drive twice as fast, you burn 4x as much fuel per unit of time, but get there in 1/2 the time, which means you use 2x the fuel.

 

[Zenmervolt]

I don't understand, what is (or was) so brilliant about Honda's engines?

Honda's founder was basically just a crazy engineer and while he was running the company he spent a lot of time encouraging his own engineers to do new and interesting things. Honda's experiment with air-cooling (which never made it stateside) was unique in that it used an inner and outer block with "coolant passages" for air to pass through between the two layers. This allowed Honda to use shorter, fatter cooling fins which, in turn, nearly eliminated the excess noise associated with air-cooled engines (apparently most of the noise is actually harmonics from the cooling fins vibrating).

When new emissions standards came out in the US and everyone else switched to catalytic converters, Honda met the same emissions standards without requiring catalytic converters (and therefore allowing owners to use cheaper leaded fuel) with their CVCC system.

And while it has become a ricer joke now, Honda's VTEC was the first commercially viable system for altering camshaft profiles and variants of this system are used almost universally now. Though there had been experimentation with systems to alter valve lift (Fiat experimented with a system that altered the fulcrum for rocker arms), VTEC was the first ever to allow the use of completely separate cam profiles (altering not only lift and timing, but also duration and speed of opening).

For a very long time, Honda essentially regarded itself as an engine company that made cars and motorcycles because the engines had to be put into something.

ZV

 

[Zenmervolt]

I think the amount of energy usage increases at the square of velocity. Then that's offset partially by the trip taking less time.

So theoretically if you drive twice as fast, you burn 4x as much fuel per unit of time, but get there in 1/2 the time, which means you use 2x the fuel.

Nope. Drag increases with the square of speed, so total power required increases with the cube of speed.

If you double speed, drag increases by a factor of 4. So you end up with initial power times 2 (to double the speed) times 4 (to overcome 4x the drag) for a total of initial power times 8.

In your scenario, you burn 8 times as much fuel per unit of time, but get there twice as fast so you burn 4 times as much fuel over the entire trip.

ZV

 

[yottabit]

Regarding gear ratios I replaced the rear axle on my old 98 Camaro V6 with a lower ratio axle. Stock was 3.08 open diff, the replacement was 3.42 with a posi unit. Best tank I ever got out of that car before the swap was 29 highway. After the swap I got 32. Yes, I corrected for the speedometer being off (which I got fixed via tune later). Had I not corrected for the speedo being off it came out to 35

This goes back to the torque curve that someone else mentioned. The engine was simply more efficient at 2100 RPM vs 1900 RPM. I thought my mpg would drop a little which I was OK with, but the 3mpg boost was certainly welcomed

Just to be clear it's not necessarily the torque curve that determines the ideal RPM for fuel efficiency, it's a measure of the Brake Specific Fuel Consumption (power generated for a unit of fuel)

You can see some BSFC charts here:
http://ecomodder.com/wiki/index.php/Brake_Specific_Fuel_Consumption_(BSFC)_Maps

It's not usually widely published data on engines

You're right that some cars are undergeared though, just as some are overgeared

On my old Saturn the sweet spot for the BSFC was near 2750 RPM, a lot of people swapped in a steeper 5th gear going for more fuel economy and I told them they were wrong. That would be good if you were going to go fast, but at 55 the stock 5th gear was best (with a steeper overdrive it would put RPM too LOW)

 

[LTC8K6]

Watch for the US version of the Mercedes 2.143 Liter twin turbo diesel AT 2500 Sprinter in 2013. It is predicted to provide average 30 mpg(US) user experience in mixed driving route applications with max 55 mph speed limiter. This vehicle has a base curb weight of 5,700 pounds and GVWR of 8,550 pounds.

Just think, take a 2012 Chevrolet Sonic 1.8 L, 4 cyl, Automatic (S6), Regular Gasoline with a 28 mpg(US) combined and MPG Estimates from Drivers Like You ... crush the sides in 7 inches ... put the Sonic into the 2500 Sprinter ... and possibly get better average mpg than the Sonic alone.

Size and weight are NOT the problem dictating small economy cars ... it is US engines, in my opinion.

Why did you pick the Sonic with the worst economy instead of the faster and higher economy version?

http://www.fueleconomy.gov/feg/Find.do?action=sbs&id=32052

 

[jagec]

Nope. Drag increases with the square of speed, so total power required increases with the cube of speed.

If you double speed, drag increases by a factor of 4. So you end up with initial power times 2 (to double the speed) times 4 (to overcome 4x the drag) for a total of initial power times 8.

In your scenario, you burn 8 times as much fuel per unit of time, but get there twice as fast so you burn 4 times as much fuel over the entire trip.

ZV

You're thinking of the "quarter mile equation", which gives the power required to ACCELERATE to a certain speed. At a steady speed, power scales with the square of speed, not the cube.

 

[nicolaskl]

You're thinking of the "quarter mile equation", which gives the power required to ACCELERATE to a certain speed. At a steady speed, power scales with the square of speed, not the cube.

The power it takes to accelerate to speed x is the same as the power it takes to maintain speed x.

Imagine a 100 horsepower car that's top speed (drag limited) is 120 mph. It takes all 100 horsepower to get to 120, and it takes all 100 horsepower to maintain 120. You don't get to 120 mph and then are suddenly able to let off the gas and still maintain 120.

Drag varies as the square of velocity, the power to overcome that drag is therefore the cube of velocity:

drag = 1/2(density of fluid)(velocity^2)(drag coefficient)(frontal area)

Power needed to overcome drag = drag*velocity, or all that stuff listed above multiplied by velocity again, = velocity^3*(all the other stuff).

You can see the proof pretty roughly (but still easily adequate to prove it's the cube, not the square) in cars like the Veyron. It takes about a thousand horsepower to hit 250 mph, while I remember during my misspent youth doing around 110 in my my moms 102 horse Corolla. If it was anywhere near the square, rather than the cube, cars like the GT500 would have top speeds in the 250 mph neighborhood

 

[Primergy]

+1 on the Mercedes Sprinter. The motor is just one thing that shines in those.
Drive one and you will realize that a van does not have to feel like driving a bowl of Jello! Aka Ford & Chevy Vans.

As for my own economy:
32mpg from a 26yr old Volvo 240 Wagon at 70mph hwy. Accelerate like a grandpa, shift early (M/T). Gear ratios help - 2450rpm at 70mph (max torque @ 2750rpm).

 

[ultimatebob]

I'm bummed that they chose the Chevy Sonic instead of the Cruze Eco. The manual version of that car is supposed to do 42 MPG.

My automatic is rated 37 MPG highway, and can do slightly better than that with just highway driving.

 

[50 mpg by 2012]

Why did you pick the Sonic with the worst economy instead of the faster and higher economy version?

http://www.fueleconomy.gov/feg/Find.do?action=sbs&id=32052

I chose the base Sonic because it was the first one I came to when I looked up USER MPG Estimates.

So to make you feel better ...

http://www.fueleconomy.gov/mpg/MPG.do?action=browseList2&make=Chevrolet&model=Sonic
shows 30.8 mpg for the 4 cyl, 1.8 L, Manual 5-spd, Regular Gasoline

And http://www.fueleconomy.gov/mpg/MPG.do?action=browseList2&make=Chevrolet&model=Sonic 5
shows 36.9 mpg for the 4 cyl, 1.4 L, Manual 6-spd, Regular Gasoline, Turbo

In my opinion all this demonstrates is a 20% improvement in USER MPG Estimated for the Sonic 5 AT which is roughly 1/4 of the size (and capability) of the Sprinter 2500.

As for not picking the Cruze ... it would not fit into the back of the Sprinter with minimum crush.

However, for the Cruze fans ... EDIT (corrected link)

http://www.fueleconomy.gov/mpg/MPG.do?action=browseList2&make=Chevrolet&model=Cruze Eco



shows best case 41.7 mpg for the 4 cyl, 1.4 L, Manual 6-spd, Regular Gasoline, Turbo ....


which is still substantually less vehicle than the 2500 High Roof 170" WB Sprinter

• Interior Height: 78.22 in.
• Cargo Vol: 494.0 cu. ft.
• Max Payload: 2,988 lbs

already demonstrating 30 mpg(US) average in mixed driving outside the US using the 2.143L TT diesel with AT.

Just real comparisons ... like it or not.

 

[skyking]

50, can you hook me up with a link about this?

It is predicted to provide average 30 mpg(US) user experience in mixed driving route applications with max 55 mph speed limiter.

Thank you.

 

[Zenmervolt]

Watch for the US version of the Mercedes 2.143 Liter twin turbo diesel AT 2500 Sprinter in 2013. It is predicted to provide average 30 mpg(US) user experience in mixed driving route applications with max 55 mph speed limiter.

I've bolded the relevant part for you. It's much easier to get high economy numbers when the vehicle is electronically limited to 55 mph max.

ZV

 

[skyking]

yeah, I have that part.

I was looking for a source.

 

[thedarkwolf]

Probably can't do much more than 55mph anyway but doesn't really need to for what it is intended.

 

[skyking]

Probably can't do much more than 55mph anyway but doesn't really need to for what it is intended.

That rig probably tops out near 90 MPH, depending on gearing.

 

[ultimatebob]

+1 on the Mercedes Sprinter. The motor is just one thing that shines in those.
Drive one and you will realize that a van does not have to feel like driving a bowl of Jello! Aka Ford & Chevy Vans.

As for my own economy:
32mpg from a 26yr old Volvo 240 Wagon at 70mph hwy. Accelerate like a grandpa, shift early (M/T). Gear ratios help - 2450rpm at 70mph (max torque @ 2750rpm).

That thing looks hideous, though. Most Eco hipsters want to drive something that looks trendy.

Hell.. I spent the $800 on the Cruze Eco package mostly because I liked the 17" rims and the look of the aero package. The extra 3 MPG is just a bonus.

 

[50 mpg by 2012]

50, can you hook me up with a link about this?

It is predicted to provide average 30 mpg(US) user experience in mixed driving route applications with max 55 mph speed limiter.

Thank you.

Sorry I can not provide a link ... my sources are industry insiders.

However, you may be able to confirm it yourself ... if you contact a major Sprinter dealer's fleet specialist and ask specifically about the 2.143L twin turbo diesel AT Sprinter 2500 average fuel economy in mixed driving expected in the US for 2013. The number were presented at a (sales?) conference within the last 10 months.