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answer = buy marathon pluses
FTFY. I love the sound of crunchy bits of glass under my Marathon Plus tyres first thing in the morning. Ride quality's abysmal, but life is too short for fixing punctures on a Brompton.
And I know @edscoble will claim that fitting Marathon Plus tyres will make your Brompton explode in a fiery ball of death (until they changed the sizing) but mine hasn't.
answer = buy marathons
the amount of time wasted changing a Brompton rear tyre once will negate all the savings made in rolling resistance between that and a standard tyre...
4 times, and you may as well just walk...