AndyM / done a little research
Gearbox internal ratio stays the same but mmpr (millimetre per revolution) is the bit that matters when going to a larger tyre. Without changing the pinion/spur from 20/81 your rollout was to big a change for your 37's. On top of that you also had long, damp grass that creates drag on the car. Not sure how tall TR's are so guessed @ 86mm to show how they affect rollout.
Schumacher/Ballistic tyres
20/81 = 23.87mmpr
YokomoTR32
20/86 = 25.66mmpr
so you would need to fit 18/78 = 23.98mmpr to get near to 23.87 if you need to run TR32's
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