Darxus

Why are cars geared so low?

Why are cars geared so low?

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2009-09-29
For a given speed, the higher the gear you're in, the less gas you burn.

I can drive 30mph in my Accord in fifth gear comfortably on a level road.

I'm sure the relationship isn't entirely linear, but that should mean I'm burning roughly twice as much gas as necessary at 65mph only because of how low the top gear is.

Why?
  • I have often wondered the same thing. My guess is that there is a space consideration for the transmission, but that's only a guess. Also, there is the idea that they want to make it a bit more difficult to speed. If you had a logical sixth and seventh gears, you would likely be more prone to speeding.
  • If you put your foot to the floor when doing 30mph in 5th, what happens? Not much. You have no torque at low RPMs, and that makes acceleration or going up hills difficult.

    Also, if your car is geared to run at the lowest possible RPM in top gear at 65 MPH, then it would lug the engine every time you had to step on the brakes, and you'd have to downshift to have any power. This would make even the lightest highway traffic very annoying to drive in, and you'd probably end up not using your top gear much at all.

    As a side note, plenty of cars have 6 speed transmissions. Mine does. More expensive luxury cars have 7.
    • I spend lots of time most days on cruise control at 65mph. I believe I would use that top gear quite a lot.

      Number of gears does not necessarily imply anything about their range - "Mine goes up to 11."
  • Because the amount of power (i.e. gas burned per unit time) required to overcome internal engine friction (ie, whetever you would need to idle your engine at a given RPM) is miniscule compared to the power required to overcome air drag at higher speeds. The power required to overcome the air drag is roughly proportional to the *cube* of your airspeed. IE, your air drag losses at 60mph are 8 times what they are at 30mph.

    Your engine also has an RPM range at which it is most efficient, and one at which it can develop the most torque. Neither of these are likely to be in the RPM range you want to use here (probably about 1500 i'd guess), unless you're planning to retrofit your car with a big-ass marine-or-locomotive-style diesel engine.
  • Perhaps it's because cars are much heavier than motorcycles, so they need more torque at lower speeds.
  • because it doesn't actually work like that.

    If the power generated with each power stroke were the same at all RPMs for a given fuel and air supply, it would be true, but it's not.

    Engines have a sweet spot where they generate the most power per *fuel* consumed, and that's not always at low speed.

    When you spin an engine faster, the harmonics of the intake track change and can become more or less efficient. You can tinker with these curves with variable valve timing, turbochargers, variable intake volumes, etc, but you can't fix the problem entirely.
    • An internal combustion engine generates peak torque, peak power and peak efficiency at different RPM. The RPM for peak efficency also varies with load.

      Choosing the highest gear ratio is an optimisation of efficiency at cruising speed (or a combination of cruising speeds), and available torque at those RPM.

      Bury the throttle in a high gear at low engine RPM and nothing much happens other than wasting fuel, and poorly chosen gear ratios lead to more frequent gear changes, which can be inconvenient, annoying or even dangerous depending on the situation.

      I'm just guessing here, as I don't have the specs for your drive-train to hand, nor access to an Accord, especially a US model but I suspect 30mph in 5th is round about 1300RPM, not much above an idle, but engine doesn't hit a respectable amount of availble torque and power until at least 2000 rpm, 45mph, peak efficiency at about 3000rpm and around 65mph, but peak power is somewhere over 5000, which then drops off sharply towards a red line of 6000.

      Note that air resistance is roughly proportional to velocity squared, and thus power to overcome drag is proportional to velocity cubed, hence an awkward cruise at low rpm is possible.

      Continuously Variable Transmissions allow for much wider ranges of gear ratios, but they are notoriously unpopular as the transition between peak efficiency and peak acceleration can be jarring, and the constant engine note mind-numbing.
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