Things you should know about gas mileage.

Fuel efficiency has been a hot topic lately. All of this hippy “green” talk and gas approaching $4 a gallon has people in an uproar. Everyone is looking to get the most out of every drop of fuel. But a lot of the fuel consumption stuff can be fairly confusing to the average person. EPA….MPG….WTF?

If you’ve read any of my previous articles you’re probably aware that I view most people as legally retarded. Yes, that’s sort of like being legally blind. I’m not sure if the government has standards yet for such a thing. If they don’t, my thought is that they’ve put off this legislation because the majority of the public wouldn’t make the cut. But rather than bitch about it (like I normally would) today I’m going to teach. Hold on to your asses everyone.

No gas guzzler tax means you're not trying hard enough...

 What are the factors?

Let’s get a few things ironed out before we get into it. A car’s efficiency is determined by two things. The first is how much fuel the vehicle’s engine requires to achieve a certain power level. The second factor is how efficient the vehicular structure is as a rolling device. I’m sure your first instinct is to argue this point but I assure you everything falls into one of those categories.

The engine in a car is the first place people place blame with regards to fuel consumption. Though it’s flawed logic, I do understand the thought process. The engine is the entity consuming the fuel so it’s natural to use it as a scapegoat. People typically assume that larger engines are less fuel efficient than smaller ones. As a general rule this holds true. The first reason behind this related to the volume of fuel required to fill a certain cylinder volume and still maintain a proper air to fuel ratio. The second reason is directly correlated to the increase of the rotational mass of a larger engine. Larger, and more numerous rotating parts all increase the rotating mass of the engine and require more fuel to turn.

She get's about 30....GPM

We have seen a few cool technologies come out lately that yield more power and better fuel consumption. Things like direct injection and variable valve timing have allowed us to have our cake and still consume it in a manner that’s efficient and socially acceptable to the aforementioned hippies with their rants about baby seals and the 99%.

But if I’m honest, the easiest way to alter the economy of a vehicle has nothing to do with the engine and everything to do with the car itself. I’m going to ask you to think about the car without its engine as an entity. The shape, wheels, axles….everything. So now think about this entity and how good it is at rolling. Let’s hit each topic one by one so we can keep them separate.

Drivetrain Loss

So your engine makes 300hp right? Any idea how much of that makes it to the ground? If you’re hitting 260whp (wheel horsepower) then you’re doing pretty good at around 12% drivetrain loss. That means 12% of your engines output is being used just to turn stuff like axles, gears, the driveshaft, and the wheels. The heavier these parts are the worse this number will be. Since these are rotating objects the larger their diameter the worse more power required to turn them. So if you’re sitting on 24’s I hope you’re sitting on an oil strike because you’re going to use a lot of gas to turn those ugly pieces of crap. I’m also going to throw tires into this category since it’s most relevant. Large tires with aggressive tread patterns don’t roll very well. This is the double whammy; larger diameter and increased weight. So what do you do if you’re looking to save money on gas? Get smaller wheels and a car that only turns two of them.

Makes 200hp...takes 180hp to turn the wheels

Gearing

The gear ratios for the transmission are CRITICAL with regards to fuel consumption. Try to imagine that an engine in a car going 60mph spins at 4,000RPM when the car is in 6th gear. Assuming the engine has some balls, we could gear it down so that the final gear ratio yields an RPM of only 2,000 whilst still propelling the car to the 60mph mark. So now we’re turning the engine half as fast which means we’re firing each piston half as often which leads to a large gas savings. This of course, assumes that the engine is sufficiently powerful to move the car at the lower RPM. You can typically gear down most cars to yield a boost in fuel economy. However, there’s a line that you can’t (or rather shouldn’t) cross. It’s the one where the engine struggles to meet the power requirements and you must give it more throttle (and more fuel) to meet demands.

The Shape

Aerodynamics are a HUGE portion of fuel economy and they are the part that’s overlooked the most. When you start to exceed around 50mph the  shape of a car really starts to come into play. When the national speed limit was 55mph it wasn’t as big a deal. But let’s be honest, in this day and age, if you’re in the left hand lane and you aren’t doing 70 you’d better be in the process of moving the hell over. My truck is about as aerodynamic as a house. This is why I get about 18mpg in the city and 18mpg on the highway. On the other hand, the Toyota Prius has a 0.25 coefficient of drag. It’s literally the most aerodynamic car on the road. Not surprisingly, it’s a fuel efficiency icon. The fact that the Prius can slip through the air untouched means that highway speeds don’t require hardly any work from its engine.

Why Not?

I’m no rocket scientist and I’m not likely the only person who’s figured out all of this. So why don’t manufacturers do these things? One word….Marketing.

I’m going to throw Marketing under the bus and I’m more than allowed to do it since that’s what my degree is in. I realized mid college career when I was pursuing an engineering degree. I realized that engineers are capable of building whatever they want but are restricted to building only what they’re TOLD. Can we build a 300mph consumer car? Sure, easy. Can we sell it to a great enough number of people to turn a profit? Probably not. Therefore it doesn’t get built. It’s that simple.

So what’s the marketing reason why most cars aren’t sufficiently gearded down from the factory? I’ll tell you why….toeing that line of sufficient power suuuuuuucks. The car isn’t fun to drive and it feels like a pig. It would be the same feeling as starting your car in 2nd gear as opposed to 1st. It bogs the engine and is actually counterproductive with respect to your fuel economy goals. My old Lexus IS300 really needed a 6th gear to fully gear down the vehicle. With one more gear (or a longer 5th) that car could have easily achieved 28mpg regularly. However, with my tachometer spinning damn near 4,000RPM on the highway that car usually averaged about 22mpg. And every mile felt responsive and quick due to the gearing being as high as it was. So in short, manufacturers are aware of the benefits of low gearing but choose not to embrace it as a solution because gearing a car high puts a positive checkmark in the “feels sporty” column.

So what of aerodynamics? Well good aeros take time in the wind tunnel and lots of money. Not only that, but the most appealing vehicle (looks wise) isn’t always the most aero dynamic. On top of that, there are some easy solutions we’re simply not ready for. Ever wonder why Smart Cars, Honda Fits, and other “mini”economy cars really don’t as good of gas mileage as you’d think they would? The reason is that their coefficient of drag is still quite high due to the fact that they need to sit two people wide and must be a certain height to accommodate headroom. But what if we got creative? Ever see a tandem bicycle? The second person sits directly behind the driver and the vehicle could be half as broad. This would lower the coefficient of drag by a HUGE margin and lead to outstanding mileage. But marketing is here to kick us in the face again. Two deep vs. two wide is a great idea but consumers aren’t ready for something like that. It’s crazy and outlandish which means sales won’t be good.

So in the end we (the consumer) prove that we truly do control what car companies build. Whether we realize it or not they’re taking cues from our buying habits and we’re shooting ourselves in the foot. So based on what we’ve learned today what’s the most fuel efficient mode of transportation?

Hit the apex hit the apex don't hit a tree...

4 Responses to Things you should know about gas mileage.

  1. Texasimportstoday Car Meets says:

    Nice write up, thanks for the info!

  2. Greg LaPointe says:

    Left out the myth about high octane. It actually does nothing to increase power at all. That is done through more aggressive timing which high octane allows you to do.

  3. Well….kinda

    Most cars today with variable timing trickery are always set to have a specific tune. When you put lower octane fuel in the tank the engine will knock. When that happens the knock sensor tells the car to PULL timing to keep that from happening.

    And yes, I think it goes without saying that not all cars benefit from higher octane fuel. It’s mostly those with the aggressive timing and high compression. But then again, there’s a lot of cars with higher compression these days…

  4. Greg LaPointe says:

    I like how the new genesis coupe makes the same power on regular as it does on premium…and I also wouldnt be surprised if there is one in my stables in a few months ;)

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