Just for fun, let’s look at my pacing segments on the mountain bike:
The power numbers are estimates from Strava because I don’t have a power meter on the MTB.
We can be fairly sure that the power numbers are grossly overestimated: I can’t produce 300+ watts for that length of time on my road bike. If they were correct I should be riding sub-25-minutes on the road bike.
I’m very happy with the time deltas as they suggest I paced it well, as well or better as I’ve ever paced a road-bike ride.
I was intuitively concerned that the gearing on the MTB would hold me back. In practice I did not spin out top gear at any point so it was not a problem; and subsequent calculation confirms that I should have expected it to be fine.
I have a 34 tooth chainring and 11-36 cassette on the MTB, but the tyres are larger travelling 2.35m per revolution. My maximum useful cadence is around 90rpm, so my top gear on the MTB allows me to propel myself at about 40 kph. I don’t go any faster than 40kph on a road ride on this course so this is no limit at all. This also suggests I do not need anything smaller than the 15 tooth sprocket on my road bike, so I am carrying around 3 (small) sprockets I don’t use.
The mechanical advantage I have on the MTB is further reduced because the cranks I wanted to use are only made in 170mm and 175mm lengths, not my preferred 172.5mm length, so I opted for the lesser-evil of slightly-too-short. It only makes 1.5% difference and I can’t say I noticed any big difference while riding.