out-frugaling an accountant is a risky business, but you could put new tyres on front and back for wet rides and put the old ones back on for dry rides.For true frugality - dispose of back, move the front to the rear and get a new front.
My Marathon+ Brompton tyres are indestructible. One puncture in four years (and none on Susie's in a year) and no sign of wear on tyres that are eighteen months old.