Originally Posted by
acroy
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so anyway: say a speed-oriented commuter used 25c tires on his 700c wheels, the diameter of his wheel would be 26.5in. If the same guy used 1" tires on his mtb, the wheel diameter would be 24 inch. using equivalent rims etc, the smaller wheel would have around 20% less inertia. this is just a guesstimate. the road bike not only has the heavy parts of the wheel farther away from the axis, but since the rim & tire are also physically larger, everything else being equal, they weigh more too!
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The problem I have with the numbers you're using is that while 25c is a common size for a road tire, 1" is NOT a common size for a MTB tire. By choosing a tire that small you're eliminating the softer ride, the go anywhere capability and the flat resistance that many here claim as MTB advantages.
The rim on a typical road bike obviously has a larger diameter but they are usually thinner than MTB rims.
Typical MTB tires I would guess are MUCH heavier than a typical road bike tire and I might just throw my tubes on a scale because I bet the MTB tubes are heavier too.
I think you can make the argument that you could
get an MTB wheel/tire combo that has less inertia than a typical road bike wheel/tire combo but then again you can also get some very light road rims and tires or for that matter there are road bikes that use 650C rims.
In other words by choosing certain tire/rim combos you can stack the deck one way or another. Unless you're using typical sizes I'm not sure what value the argument has.