Whoops, yes you are right.
The expansion of the wheel adds 7 mm per turn of the wheel, but to turn the wheel once at 20/70 gearing requires 3 and a bit turns of the crankshaft so each turn of the crank moves the kart 7/(20/70)= 2mm
The change of gearing from 20/70 to 2/72 means that each turn of the crankshaft moves the kart 6.7mm less (0.0079*853=6.7mm)
The change in pressure from 10 to 30 psi is the same as 2/3 of a tooth on the gears.
And as pointed out , the circumference change from 10 to 20 psi is larger than 20 to 30psi and the OP was asking from 18 to 28psi.
So the answer is very little effect!
(I always need someone to check my maths, thanks Hi 99)
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