Do you mean the big chain ring? It's better to use the chainring and rear cog tooth counts, since bikes can vary on their drivetrain arrangement. The ring and cog, along with the tire size, determines the speed for a given cadence.

"Top gear" would usually mean the highest combination, typically a 50-11 or 50-12 these days. That's for downhills or for bike racing finish line sprints near or above 40 mph.

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You have 28, 38, and 48 chainrings, and a wide range 11-32 cassette. And 700c x 28mm tires.

I have a bike with 30, 39, and 52 chainrings. And either a 12-25 or 12-29 cassette. On flat roads, I'm usually in the 39 chainring, good for cruising speeds up to about 22 mph, which is higher than my usual speed.

My other bike is the typical road bike, with 34 and 50 chainrings, and 11-28 cassette. On flat roads, I'm usually in the 50 chainring, but the small ring can go up to about 20 mph, so I'll often stay in that small ring if it's rolling hills.

If I'm trying to hang on to a fast-for-me group ride (perhaps 20-23 mph on the flats), I'll be in the 50 chainring and a middle cog, and spinning faster at 95 to 105 rpm.

A lot more power to go a little faster
Wind resistance goes up dramatically as your speed increases. For example, it takes more than twice the watts to go 20 mph compared to 15 mph. See my comment in another thread.

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High cadence, moderate effort
This is better for longer rides for most riders. So, many riders aim for a cadence anywhere from about 85 rpm to 100 rpm. Their flat road speed depends on their power at their cadence.

High cadence, high effort
This is the definition of a sprint, a short, hard effort to the finish line. The racers don't use a huge front chain ring -- they can put out lots of power at high cadences, so most use a quite "normal" big chainring.
For example, Mark Cavendish at the 2009 Tour de France. Youtube link. High cadence, and a max effort!


Low cadence, easy effort
Just cruising along.

Low cadence, hard effort
Most road bikes don't have a wide enough range of gearing to climb steep hills at a high cadence. So the riders are pushing harder at a lower cadence. They can get even more power for a short time by standing up, too.

Your short burst of speed is similar. It's probably not sustainable for very long, but riders can put out quite a bit of short-term watts, perhaps in the range of 5 times their longer term power.

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Gearing and "spinning" cadences in the 90 rpm range
Here's a chart for a typical road bike, 34 & 50 chainrings, and 11-28 cassette, at typical cruising cadences.
You can see that the 50 chainring, in black, is effective for about 14 mph on up. The 34 chainring, in red, is good up to about 20 mph.

From 15 to 20 mph: either chainring is fine. The choice depends on what's ahead -- going faster or slower.