My BMW Z4 can get over 40mpg.

The other day I was going to work and I remembered I didn’t have much gas in my car. The little low-gas warning light was already on when I started the car.

It’s got one of those little computer things that tells you how much gas mileage you’re getting and how many miles it thinks you have left in the tank.

It said I had 57 miles left, the drive to work is 35 miles, I figured I could make it. New car there should be no sludge at the bottom of the gas tank….

So I figured I should drive nicely. Now the little computer thing has to go by how much gas is in the tank and your driving pattern to determine how many miles you have left. I don’t know how far back in my driving history it goes to gauge but it’s more than today’s trip.

When I first got the car, I drove it nicely (since it was still in its  break-in period) and I got it to say I could get 400 miles to the tank when I just filled it up. I figured that was pretty good. But obviously that was pretty non-fun driving, especially for z4.

So I drove to work, got gas, reset all my computer settings, and went to the office.
Starting to drive home I noticed that it said I had 472 miles left in my tank. Not bad I thought, it must have remembered my driving style on my drive to work. Let’s see if I could get it to 500.

Well, as the miles clicked by the number went up, and I eventually got it as high as 512, and that was after having driven 20 miles or so, so I should be able to get at least 530+ miles out of  a newly full tank.
The mpg computer said I was getting 41.5 mpg.

A BMW z4.

Now of course this isn’t entirely fair. I had the windows open, the A/C off, it was all highway, manual transmission, and I think the trip from work to home is, on average, downhill (though there are plenty of up hills on the way.)

So my question is, if BMW can make a sports car that can put out 250 hp, that can still (if you try very very hard) get 41 mpg, why does EVERY OTHER CAR suck by comparison?

And it seems to me with a smaller engine and a lot of tweaking to force you to drive like I was driving, LOTS of cars should be able to get in the 30 mpg range, not just the mini.

So what gives?  How hard is it to make a car 10-20mpg more efficient than the going rate now?

You can forget all the hybrid stuff, and spend your money better on tweaking a known good platform.

I just don’t get it.

2 Responses to “My BMW Z4 can get over 40mpg.”

  1. J.T. McQuitty says:

    I don’t understand the thermodynamics of the hybrid. Don’t you have to burn something somewhere to get energy for the electric motor? Whichever motor is running, doesn’t it have to accelerate the mass of the second motor in addition to that of the car?

    The extra mass means frame has to be heavier – more mass – and you need more braking power, meaning heavier braking apparatus, still more mass.

    If you hold overall mass of the car constant by lightening everything else, doesn’t that mean a decrement in collision safety?

    You have to burn something somewhere to get electricity to accelerate all that mass, so where is the energy saving? Does generating a little electricity from the brakes to charge the battery really capture enough to make up for hauling the extra mass around?

    I’m just asking; I really don’t understand.

    I do understand that the thermodynamics of ethanol production is a joke.

  2. admin says:

    I’ve wondered about how the hybrids manage and the conclusion I’ve come to is: just barely. I think what it comes down to is that gas engines are just really really inefficient. So anything you can do to not run the engine is a big savings. I remember my high school physics teacher telling us that cars get on average 8% of the energy out of gas. Very telling.
    So anyway, consider: the car is heavier, more frame more hardware and batteries, but if you can not run the engine some of the time, you can save a noticeable amount of gas. Any time you run off the battery is free (gas mileage wise) and charging the battery is probably a negligible drain if you’re already running the engine to move the car.
    But consider: with all that, those cars still only get 30-40 mpg.
    So all that engineering doesn’t buy you much except a government subsidy.
    The other thing is that you probably CAN drive a hybrid to get worse mileage than a conventional car, but a) the kind of people who drive hybrids don’t drive like I do, and b) the car’s performance is limited for just that reason.

    I agree, the thermodynamics seem questionable but they obviously made it work, and again the benefit is pretty small, so it probably comes down to the big savings you get from not running the engine as often as possible.

    I read a thing the other day that in europe a great way to save on mileage is to stop the engine at traffic lights. Apparently this is coming to the US soon. Called stop-start systems.

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