Road: off road mileage equivalence

Does anyone have ideas or views on this?
what I mean is what the equivalent road mileage would be for, say, 50km on fire roads?
One way I looked at it is time/ ave speed, so if 2 hours off road I can cover 36km, on roads I bike at 24km/h (sloooowww) so should cover 48km?
This is primarily as I am mainly MTB but converting to roads & looking to do a Century Sportive thing.
what I mean is what the equivalent road mileage would be for, say, 50km on fire roads?
One way I looked at it is time/ ave speed, so if 2 hours off road I can cover 36km, on roads I bike at 24km/h (sloooowww) so should cover 48km?
This is primarily as I am mainly MTB but converting to roads & looking to do a Century Sportive thing.
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So 13 mph av off-road => 19 mph av on road
BUT of course it depends a great deal on the off-road conditions. Anything technical or muddy is going to slow you down a lot. On fast, flat fire roads you will probably average not too much slower than road speeds.
ALSO depends on how good your bike handling is off-road. I have generally been likened by club mates to look like "a pig on roller skates" when on my MTB
20 really tough miles off road is about the same effort as 60 road miles.
IMO
I grew up in the house just on the top left point of the 'M' in your map!
Cheers, Andy
http://www.stirlingtri.co.uk