Speedo, Gears and Wheels

reliablejunkie · Post by **reliablejunkie** » Tue May 03, 2016 12:46 pm

Hi everyone,

I am actively keeping an eye out for some new wheels for the 280zx and I have been putting thought toward the circumference of the wheels and how this, the diff and speedo are all in relation to each other. I know we can change the small gear in the gearbox for the speedo but I want to rule that option out in this case.
I currently have a 3.9 LSD R200 and the wheels are a 16 inch with a tyre profile of 55 and the speedo reads correctly. If we were to put in a higher ratio diff (eg 4.1) the wheel circumference would need to be larger for a correct speedo right? I am doing a small physics project at uni and I am going to come up with a mathematical formula that puts the diff size, wheel circumference and the speedo in relation to each other then put this theory to the test. Plus I want new wheels for the car too

Tristan

Baker93 · Post by **Baker93** » Tue May 03, 2016 6:50 pm

Hi. Your deduction is correct. This is relatively easy to calculate. Many years ago we developed a basic Excel spreadsheet to do these calcs. We can change any parameter we want. Tire size, diff, gear box and RPM to calculate speeds in any gear. This has been used extensively over many years to ensure the correct gear is installed for any ratio particular track or tire combo. Ultimately i used to run a different diff ratio for every track in the North Island and even for wet and dry tires.
Obviously it is only as good as the information you give it

reliablejunkie · Post by **reliablejunkie** » Tue May 03, 2016 9:26 pm

Ah cool, I was thinking of putting together an excel sheet and coming up with a few examples for the theory side of things. Once I do this and hand in my paper, I would love to check out the work that you have done if you still have it around.

Baker93 · Post by **Baker93** » Tue May 03, 2016 10:27 pm

Hi
I definitely still have it and refer to it all the time, it contains a lot of reference info on tire sizes and various gearbox ratios. It also contains a lot of historic data on some reasonably well know NZ race cars. Once upon a time this info may have been considered a little privileged, but a few years have passed and i doubt this is now the case.
I am happy to share this if you want to send me your email address.

reliablejunkie · Post by **reliablejunkie** » Wed May 04, 2016 7:03 pm

Thank you very much, I do appreciate it. I will be in touch through PM.