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Let's say a person has a mountain bike. How much faster would this person go (on long roads, not commuting) when using slick tires (the kind commonly used on road bikes) vs. using knobby, mountain bike tires? Assume that the power output is the same in either case.
Note that this is related to this question: How do on-road mountain bike speeds translate to road bike speeds?
Assuming everything else is the same, the wattage lost to best-rated road tyre test is 7.0W
And the Fatbike is 36.9 W
None of these tyres is great at anything but having a low rolling resistance - this is gained by sacrificing tread thickness and puncture protection.
There are plenty more reviews and tests at the same site. Whether its real-world doesn't matter too much - each one is tested the same way for consistency.
Then the second part is "how fast do these wattages allow?" Using http://www.bikecalculator.com/ and presuming a 75 kg rider, 10 kg bike riding at 150 Watts, will roll at 28.26 km/h (approx)
If this speed was done on the fatbike tyres, then moving to the touring bike would save 39.6 Watts, essentially increasing the rider's input power by that much, and returning a speed of 30.96 km/h
Likewise, going to the fanciest road tyre would result in 32.18 km/h
Since a bicycle has two wheels, losses are doubled. Some of these tyres will cost hundreds of dollars too, and may be tubular.
A common GP5000 from Continental is measured at 10W, resulting in 31.82 km/h, only 350 metres/hour slower than the costly Vittoria.
It very much depends on the individual tyres in question. Not all MTB tyres are slow, and not all road tyres are fast. In an extreme example, it is even be possible to swap from a MTB tyre and get slower.
However if swapping between tyres of an equivalent quality and market segment (for example a high quality fast rolling MTB tyre to a high quality fast rolling road tyre) you would typically see a gain of ~3km/h. Moving to a wide slick touring tyre will see very small gains.
Having experimented with this a bit myself, I came to the conclusion that it's just not worth ruining the versatility of a MTB for small speed gains.
