No-one can say really, it's down to the tracks. I was using a 42 rear on some tracks.
As long as you top out somewhere near the end of the longest straight in 6th, you're spot on.
If this leaves you with crap drive from a couple of corners, shorten the gearing, as the time lost cornering will be greater than that lost on the straight...
Unless you wanna drop the gearbox and swap ratios at the track!
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