At least with iPhone web apps, you can bring up the numeric keyboard by default - and with two fingers, it's much faster and more accurate for entering digits than the scroller (I use alarm clock quite often, and can't stand it - overshooting numbers all the time, for instance).

You're correct that the keyboard would obscure the rest of the interface, but you shouldn't often need to see the rest of the screen when entering mileage. If it's a frequent trip, for instance, you'd just use the frequent trip shortcut.