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Stage two, I've taken this from an American book about 20 years old but the information is sound (all be it in inches and MPH)
Tires Are “Gears” Too!
Having the latest hot-tip racing tire is nice — but not if its diameter fouls up your gear ratio. Can’t happen, huh? OK try this. You’ve been running the 5.50-9.20—14 tires on your 510 with a 4.11 rear gear. These have about 24.12- inch diameter at 24 PSI inflation. Get a gear/tire size computer wheel. This little circular slide rule calculator puts the whole thing in your pocket so you can impress your team mates and fellow racers with your knowledge of what’s happening. Set the tire diameter next to the gear ratio, then find 7,000 driveline RPM (same as engine RPM because this is measured in 4th gear, which is 1:1). Read the top speed (4th gear) of about 121 MPH.
Your ex-wife loses grasp of things one afternoon and write you an alimony check. You squander this new found fortune on a set of 10.45—14 tires with a shorter diameter (only 22.4 inches) than you had been running. Play the computer wheel game again and you’ll see that 7,000 RPM now gives 111 MPH. This might help you on a short track where you didn’t use much 4th gear — but if the next race features two long straightaways, you’ve got to figure out what to do. Change rear gears, twist the engine tighter, go back to a taller tire or just be content to finish somewhere in the pack with what you already have in the tire and gear departments? You might be better of leaving your new tires in their wrappers!
A 510 is the old Datsun 1600 '68-'73
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