Datsun Roadster Forum-311s.org

I have searched the website - no joy - someone published a spreadsheet(on my old computer - somehow disappeared) - that allows one to put in tire diameter and calculate rpm/mph so that one knows what "speedo gear' to put in. I am running a 1970 SPL (5-speed box, 195/60/R14) - how do i find out what is correct speedo gear??

hmmm, I have the spreadsheet that has engine speed vs car speed for all Nissan trans gear ratios, allowing for adjusting engine sizes.
but ti doesn't have speedo gears in it, so not sure other than trying one out and calculating the adjustment (meaning ~20% off on a 19 tooth gear would mean stepping up to a 24 tooth)

Not exactly what you are looking for but it might help: -

http://www.miata.net/garage/tirecalc.html

You will need the stock tyre dimensions to compare with your new tyres.