I ride in some open areas near the coast. Wind is more of an issue than hills. 48x12 is not something I'd want to get caught with fighting 35 knots going home. I think a strong young guy could handle that, but I'm not so strong anymore and have always been a spinner. Big, long gears kill me. 48x18 and no more.I have used such a ratio since age 15 and have ridden 50 miles at a time including many steep hills and descents on such a bike.
Mike, I like to think in gear inches also, but you're off a bit. That number you come up with after doing your formula represents the old fashion Pennyfarthing big wheel diameter. Example: 50x15 gearing = 90 inch gear if using a 27 inch tyre. That 90 inches is what a Pennyfarthing's wheel diameter would have to be in order to equal a modern 50x15 with 27 inch tyres.I'm used to gear inches. If you divide the chainring number by the cog number and then multiply by the diameter of the rear wheel you get how many inches you will go in one revolution of the peddle. I like running a small gear, something like 65 inches. But I'm a fat old guy.
Mike Frye-the bike guy