Death to "gear inches"
Always been bugged by the presumption of "gear inches". Mostly because tire selection, profile, and wear plays into the actual number of inches a ratio will produce. Ratios are ratios. Gear inches is a Major Taylor vintage holdover.
Last few years I've been using a spreadsheet printout to determine ratios and also to figure out at a glance if I needed to add a link or remove a link to make a gear combo work. For pursuits I fine tune things a lot more than for mass start events, and this was a quick way of getting where I wanted.
I also set up a side chart to figure out the approximate range you'd need in "typical" road gears to produce a similar ratio during training. Of course I'm shooting myself in the head a bit because again, tire circumference plays into this, but it's at least ballpark.
Here's a zip of the excel file.
The way most people use gear inches, it's just gear ratio multiplied by 27. It's convenient because under the gear inch system, one chainring tooth is roughly two gear inches and one cog tooth is roughly 6 gear inches. This gives someone an easy way of finding approximate gear equivalencies without too much math. So, basically, anyone using "traditional" gear inches (meaning a 50/15 is a 90" gear), is essentially using straight gear ratios.
Good spreadsheet. Rank ordering gears in some way is essential. I've never found the traditional tabular gear charts to be useful. Mine is posted around here somewhere, but I'm told it's too nerd to be seen with in people's gear bag.
Ratios are ratios, but if you want to know how hard it's going to be to spin up, or how far you're going to go with a pedal stroke, you need the tire size. Hence gear inches. To give you an extreme example- a 50x15 is a ratio of 3.33 for both a 650c tire and 700c, but they're drastically different in gear inches (and development), and will be very different to spin up or maintain speed on.
Most people (including me) use them as Brian describes. If you want to get down into very detailed fine tuning and very accurate comparison you need to add actual tire size to your sheet, or use one of the smartphone apps that lets you select your tire size. And roll your wheels out at the pressure you're using them to make sure you're using the right size. And then advocate a new term like "actual gear inches" so the rest of us know you aren't using ratio*27. Kind of like MB vs. MiB for specifying memory and hard drives.
I like the idea of using ratios. I once tweeted to Shane Perkins asking him what kinds of gears do they ride in the JKA Keirin league in Japan and he replied in a ratio.
I think the idea of converting to gear ratios is easier than when I convered from MPH to KPH. THAT took a few weeks to get used to. But, after that initial hurdle, I loved it...til I had to talk speed with someone other than my coach, hahaha.
The weight of you on the bike also affects tire diameter/tire rollout.
Time for my periodic "that's how they do it in Japan" post: That's how they do it in Japan, ratios only.