Why are cars geared so low?

[darxus]

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?

Comments

[tech-charles.livejournal.com]

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.