I'm going to try something novel, and just talk about bikes:
I've been riding a touring bike with 622 x 37 tires for about 2 1/2 years, and I've been very happy with these tires. They deal with broken pavement well, they're comfy, and they pretty much never get flats. This Christmas, though, my girlfriend, noticing the wear of my tires, gave me new ones, which I greatly appreciated. The only thing that gave me pause was that the new, very expensive tires she gave me (Schwalbe Marathon) are 622 x 28. Being curious about the benefits of skinnier tires, I put them on my bike, and went for a ride. At first, I was unimpressed; my speed was about the same with the new tires. Then, as I continued to ride, I gathered these impressions:
1. On hills, there is no difference; weight is much more of a factor than anything else going uphill, and downhill, well, then it's just a matter of gravity...
2. On flat ground, though, there seemed to be a noticeable difference; I seemed to be able to go about 2.5 mph faster on the new tires with the same amount of effort, all other things being equal.
My question is this: Am I fooling myself? Does tire size even make any difference at all when you're riding a heavy steel touring bike on a beer run? I'm having a little trouble believing that such a small difference in surface area can mean much in terms of performance.