AWD MPG penalty

[SeattleLegacy]

Tires flex. So driving a tire makes it flex more. Putting the power down through four tires is more efficient. But the mechanism required to do so is less efficient That's why you can't get your head around the statement.

 

Here's the simplest possible primer on automotive physics I could find that explains some of the drag factors, written in layman's terms and without any math concepts beyond multiplication....Give it a go.

Physics In Automobiles and Trucks

 

The twist flex (not flex caused by the weight of the vehicles) applied to tire is only for a brief moment when the vehicle begins to accelerate (from a standstill or rolling). Once the twist is at it greatest (the point where the torque/twist applied to the tire is equal to the forces it must overcome) it can be removed from the equation. This also applies during highway cruising as the tire is under a twist load that is proportional to the losses that it must overcome (drivetrain, drag, and tire rolling resistance) and is constant. Tire flex is only different between AWD and 2WD during acceleration, but the sum of the losses are the same. (Same tires, same acceleration) Therefore it can be concluded that the losses through the tire caused by flex (twist) under acceleration are very small and can be removed from the MPG calculation.

 

Any difference in losses caused by tire twist between AWD and 2WD under acceleration can be applied to reaction and not MPG. (Sure, it may take more gas to turn the wheel in 2WD, but its so small, who cares?) Also, the difference in drivetrain losses between AWD and 2WD are much greater than the tire flex during acceleration.

[psucaptainkickass]

Cars have 4 wheels, not 2. I think the Subaru commercials are working

[Garandman]

The twist flex (not flex caused by the weight of the vehicles) applied to tire is only for a brief moment when the vehicle begins to accelerate (from a standstill or rolling). Once the twist is at it greatest (the point where the torque/twist applied to the tire is equal to the forces it must overcome) it can be removed from the equation.

 

 

That must be why my tires stay so cool!

 

OK, we will now lead the horse to water.....

 

Movement Forward

 

In order for any vehicle to move forward, it must provide some Thrust. Thrust is necessary to first cause acceleration (increase in speed / velocity) according to Newton's laws of motion. Additional Thrust is required to overcome Drag forces, and that component is necessary even once the vehicle is traveling at a constant speed. Drag exists from two different sources. Aerodynamic Drag is due to having to push air out of the way of where the vehicle needs to go. Mechanical Drag is due to all the moving mechanisms in the vehicle that have frictional losses, most specifically the wheel bearings, but is actually nearly entirely due to the action of the tires on the road surface.

 

 

In nearly all actual situations, all the other causes of mechanical drag factors can be ignored, and just the Tire Resistance considered, regarding the Mechanical Drag.

It has been experimentally determined that the total Tire Resistance Drag is nearly proportional to the weight of the vehicle, and is slightly affected by vehicle speed. It is also affected by the air pressure in the tires and the temperature of the tires. In general, with modern synthetic rubber tires, inflated to the specified pressures, the total Tire Resistance Drag is generally around 1.2% to 1.4% of total vehicle weight at 30 mph, and around 1.6% to 2.0% of total vehicle weight at 70 mph, once the tires are driven at that speed for half an hour or so such that they get up to normal operating temperature (often around 240°F).

 

 

For example, a standard 3,000 pound car therefore has around 48 to 60 pounds of Tire Resistance Drag at 70 mph.

 

I'd also suggest looking up "Traction Circle" in case you ever have to turn or stop your hypothetical machine.

[SeattleLegacy]

 

That must be why my tires stay so cool!

 

OK, we will now lead the horse to water.....

 

I'd also suggest looking up "Traction Circle" in case you ever have to turn or stop your hypothetical machine.

 

Reread my post. I wasn't talking about tire flex caused by it rolling over the ground (rolling resistance). try to keep up with this horse.

 

tire flex due to rolling resistance will be equal for the same car, with the same vehicle load, with the same tires and pressures. REGARDLESS of drivetrain type.

 

I am talking about the twist of the caused by the torque applied to the tire by the wheel under acceleration. this twist only happens for a moment until the angular velocity of the tire and wheel match. once both the 2WD and AWD drive vehicles reach the same speed they will each have the same rolling resistance. rolling restance can now be removed when comparing the two.

 

the only thing different between the two vehicles is their drivetrain, AWD vs 2WD. AWD has more drivetrain losses when compared to a 2WD vehicle at the same speed. in order to maintain the same speed of the 2WD vehicle the AWD vehicle needs to use more energy/fuel to overcome its increased drivetrain losses. this is why the AWD version of a vehicle has worse gas mileage compared to 2WD version.

 

EDIT: sorry for the mis-spellings

[LongHiway]

How much MPG does an AWD vehicle loose when turning? Agreed, you'll have to generalize about what types of differentials are being used.

 

Second part, someone told me that boxer engines are inherently less efficient than traditional engines - gravity?

[SLegacy99]

I ran my '96 in FWD mode for two tanks and didnt save any gas.

 

32 mpg in '99 and proud of it.

[SLegacy99]

Second part, someone told me that boxer engines are inherently less efficient than traditional engines - gravity?

 

Never heard that before.

[SeattleLegacy]

I ran my '96 in FWD mode for two tanks and didnt save any gas.

 

32 mpg in '99 and proud of it.

 

FWD mode and true FWD are different. (I'm assuming that you have an A/T and put the fuse in that disengages the AWD.) Even in FWD mode you still have losses from the unused drivetrain. you may not be using it but it is still spining and causing friction.

[SLegacy99]

Right, but there are many people who claim to gave mpg gains from running in FWD and have even go so far as to install a switch. I say its BS.

[Garandman]

///

Second part, someone told me that boxer engines are inherently less efficient than traditional engines - gravity?

 

Entropy?

 

If you were Karl Benz, or have a BMW motorcycle; Honda Gold Wing; Beatle; Corvair; Citreon 2CV; Porsche; Tucker; Subaru; or a Lycoming-powered aircraft, a boxer (or flat) is a traditional engine.

[zildjiank]

let the weight of the car coast for you, and lighten up on the throttle, that'll help a lot.. listen and feel for the engine load.. there's a specific point to where the motor is keeping up with the speed that you are running, if you add too much gas, you are going to accelerate ever so slightly, if you are coasting or feel engine braking, you're slowing down and not covering as much distance as you can get.. smoother driving can get you as much as 5 mpg more.. but again, always carry light as possible, the weight will kill you.

 

 

oh, and try towing a car or a 3000+ lbs trailer sometime, watch that gas just get dumped right out as fast as you can pump it in.

 

+1 on this description of peddle imput...

I do the same thing everwhere I go now, and definitely can tell a big diff...

[SeattleLegacy]

Right, but there are many people who claim to gave mpg gains from running in FWD and have even go so far as to install a switch. I say its BS.

 

I agree. I've also heard driving in 2WD mode is murder on your drivetrain. I bought the car because of AWD... I think I'm gonna use it.

[LongHiway]

We have some (SW) engineers originally from the home of Porsche, and the exact comment was "boxer engines tend to be more thirsty". I did not challenge it at the time. For the reason following it seemed to make sense.

 

So, I like my Subaru this is not one of those Subaru's suck blah blah.

 

Without going through the exact specs, or in fact comparisons with "like" cars, we've all got a 2.5 L engine pulling / pushing (ok, it is AWD) roughly 3400 + pounds. I've driven (long term) 2 L 4 cylinders and 3 L 6 cylinders. This car seems to be more like the 3 L (maybe worse?) than at a mid-point between the two in terms of MPG. Agreed, very unscientific.

[SLegacy99]

I agree. I've also heard driving in 2WD mode is murder on your drivetrain. I bought the car because of AWD... I think I'm gonna use it.

 

And the car didnt handle as well in FWD mode and I peeled out when I really didnt want to.

[LIZARD]

I I'm getting old and need to learn how to better cope with sarcastic, negative internet sharpshooters who bring no positive or useful information to a thread. So help me out......

 

You appear to be doing fine.

 

no offense intended. Just making light.

[ahains]

We have some (SW) engineers originally from the home of Porsche, and the exact comment was "boxer engines tend to be more thirsty". I did not challenge it at the time. For the reason following it seemed to make sense.

 

So, I like my Subaru this is not one of those Subaru's suck blah blah.

 

Without going through the exact specs, or in fact comparisons with "like" cars, we've all got a 2.5 L engine pulling / pushing (ok, it is AWD) roughly 3400 + pounds. I've driven (long term) 2 L 4 cylinders and 3 L 6 cylinders. This car seems to be more like the 3 L (maybe worse?) than at a mid-point between the two in terms of MPG. Agreed, very unscientific.

My 91 turbo mr2 (rwd) averaged about 18-19 mpg.

My 91 eagle talon tsi (awd) averaged about 18 mpg, but had a straight-through exhaust (may have helped a tad).

 

Turbocharged sports cars just generally do not get great mileage

I'm thrilled that my WRX gets 20-22 and upper 20's if freeway only.

My wife seems to be getting 19-20mpg on her new OXT, and this seems pretty reasonable since it is bigger & heavier and has a larger engine than my wrx (my04).

 

Edit - I'm an idiot, I didn't notice I was in the normally aspirated forum.. disregard my idiot speak.

[SLegacy99]

Want more gas mileage? Get more gears.

[Nightstorm]

Just for some reference about AWD vs 2WD:

2006 Volvo S40 T5 6MT: 22/32

2006 Volvo S40 T5 AWD 6MT: 20/29

 

These cars are pretty much identical, although the AWD is 167 lbs heavier, all other dimensions (engine and physical) are the same.

[SeattleLegacy]

^^^ that's some pretty solid info. Thanks.

[LongHiway]

Interesting. If Volvos are representative, a 9 - 10% MPG hit for AWD.

 

Makes sense if I take my 27 - 28 MPG, divide by 90%, I'd be in the low 30's which my gut tells me is the ballpark for something between a 2 L and 3 L engine.

 

And probably the Myth is "busted" about any inherant loss of efficiency from a boxer engine - I could not visualize any efficiency difference from pistons going up and down versus side to side.

[Deer Killer]

If anyone is really interested, here is my approximate spindown loss in 3rd gear. stock wheels/tires if I remember correctly. Stock sedan exterior, no spoiler

 

Total loss drag+drivetrain at speed (by RPM in 3rd) is at the black line.

 

My estimate is half the loss >6500 is drag. So for example the chart says that at ~90mph in 3rd gear we need ~110HP to maintain speed. At 60mph in 3rd about 58hp. Yes it would be better in 5th, but what does 58hp cost in gas?

[erikvcl]

How much MPG does an AWD vehicle loose when turning? Agreed, you'll have to generalize about what types of differentials are being used.

 

Second part, someone told me that boxer engines are inherently less efficient than traditional engines - gravity?

 

Boxer engines are more efficient since the pistons don't have to fight gravity. Due to the lack of having to fight gravity and better balance, the engines are more reliable as well. Remember that Porsche uses boxer engines too for the same reasons.

 

Our Subarus don't get as good of gas mileage compared to Toyotas and Honda engines of comparable size since they're putting out more horsepower per liter of engine size (higher output) and have AWD.

 

I believe that the 2.4L 4-cyl engine in the Camry puts out 145 HP. The 2.5L engine in the 2.5i is 168/175 HP (depending on whether it's a 2005 or 2006) -- it's no wonder that the Toyota gets 5 MPG more.

[ahains]

Boxer engines are more efficient since the pistons don't have to fight gravity. Due to the lack of having to fight gravity and better balance, the engines are more reliable as well. Remember that Porsche uses boxer engines too for the same reasons.

 

Are you sure this is right?

When pistons are oriented vertically, they receive just as much of a benefit in the downward stroke from gravity as they pay out against gravity in the upward stroke.

 

I thought the main benefit of a boxer engine was that it allowed you to get the weight of the engine down much lower so it helped the CG of the car?

[LIZARD]

Are you sure this is right?

When pistons are oriented vertically, they receive just as much of a benefit in the downward stroke from gravity as they pay out against gravity in the upward stroke.

 

I thought the main benefit of a boxer engine was that it allowed you to get the weight of the engine down much lower so it helped the CG of the car?

 

Very true. Horizontally opposed = lower profile = lower CoG. My friend's Father owned a Subie dealership for many years (Somerset Subaru) His son asked him why his Impreza would pump out heat so fast in the winter, literally 1 min. after starting the car cold as opposed to other vehicles, he said it was the boxer's horizontally opposed pistons and the extra friction created by being alligned as such. Just something I remember froma few years ago.

[harrydog]

Very true. Horizontally opposed = lower profile = lower CoG. My friend's Father owned a Subie dealership for many years (Somerset Subaru) His son asked him why his Impreza would pump out heat so fast in the winter, literally 1 min. after starting the car cold as opposed to other vehicles, he said it was the boxer's horizontally opposed pistons and the extra friction created by being alligned as such. Just something I remember froma few years ago.

Extra friction? If that were true, I would think there would be more engine wear. The fact is, Subaru engines in general exhibit less wear than most automobile engines, based on oil analysis.