In another thread elsewhere, there was mention of the Octavia 5- and 6-speed gearboxes. The consensus was that top on both gearboxes was roughly the same.

If this is correct, and I've no reason to suspect its not, the obvious question is why bother at all with having different gearboxes? And why have a greater selection of ratios in the stronger gearbox?

One final question: at an indicated 60mph in my car in 6th, engine is doing about 1800rpm. In the wife's Toyota, its nigh on 3000 rpm for the same speed. That only has a 5 speed manual box.

Does the fact that my car is diesel and hers petrol have anything to do with it?

Does the fact that my car is diesel and hers petrol have anything to do with it?

In a word YES.

The gearing on a petrol engine is completely different to a diesel. Just look at the redline on the rev counters of each car.

... the obvious question is why bother at all with having different gearboxes? And why have a greater selection of ratios in the stronger gearbox?

For the same reason you can't go very fast in 1st and can't pull away in 5th.

Since the engine produces most power within a fairly narrow rev range, having more ratios spaced closer together means that you're able to keep the engine within this range more of the time, so use the power more effectively and drive faster more economically The power band on a diesel engine is narrower than on a petrol, as well as them not revving so high so more gears are potentially even more useful in that respect.

As an experiment I might suggest driving using only 1st, 3rd and 5th and seeing how much more difficult it is (or at least understand why my old 3 speed auto Corolla was rubbish at going up 40mph hills...)

Electric motors are the only ones that don't really need gearboxes, since they produce their maximum torque at 0RPM, but can still achieve much higher RPMs than combustion engines - see trains for an obvious example.