Well, I can feel differences of just a few gear inches on my daily commute route, because I know the route so well, the slope of the hills etc. Do small changes in gearing prevent me from climbing hills or dramatically change my average speed or ability to slow down? Definately not. For me something in the low 70s is great for leisurely riding or in a dense urban environment. I like 75-76 inches for fast road rides (with hills). I like 80+ on the track. If you just put me on some random bike on a road I've never been on before without any kind of cyclocomuter, I probably couldn't guess very accurately how many gear inches I was riding.