> Well, I've seen quite a few cars, and almost all of them had 4 gears.

Funny, I usually take about 10 rental cars per year and I don't recall any 
of them ever having 4 gears, even when I picked the absolute cheapest one 
they offered.  Can you give any examples?

> And yet, the gears are spaced so that changing gear is only useful at 
> speeds below 50 MPH. Once you're doing more than 50, you're in top gear 
> and there's no advantage.

Some people like to accelerate above 50 MPH you know :-)  I suspect at 50 
mph in your car you still have the choice of 3rd, 4th, 5th and 6th gears, 
how is that not useful?  Even at 80 mph I suspect that 4th gear would still 
be available to give you the maximum acceleration (should you ever need it). 
That's the idea of gears.

> It's just that when the engine revs get high, you feel like you should be 
> changing up a gear. (But there aren't any.)

Driving around at low revs might be fine at low speeds, but at higher speeds 
you *need* the higher rpms to generate the power to overcome the air drag. 
6th gear is designed specifically so that the engine is developing its 
maximum power at the top speed of your car, otherwise you wouldn't be able 
to reach that speed!

> I guess it might allow you to cruse along the motorways at high speed 
> without having to have the engine running at 8,000 RPM,

If you are revving near the redline in top gear then it probably means you 
*need* that much power from your engine to maintain that speed.  If you 
change up a gear you're no longer going to have the torque needed to 
maintain that speed.

> Interesting. I always through disk brakes are cheaper, so they use those 
> where they can't afford to fit propper drum brakes.

Hehe no, otherwise F1 cars would use drum brakes and cheap cars would use 
disc brakes.  It's the other way around.

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