Either that or you're limited by revs - in other words, there's enough net torque that, in the highest gear, the engine is running at redline before wind resistance stops you accelerating.

You don't see that much these days, though, as having your highest gear that short is just leaving fuel economy on the table on the highway. If you can pick up speed going uphill without changing down at least one gear, you're wasting fuel to pumping losses on flats/downhill.