Guys, since my feeble brain had forgotten whatever it knew about mechie engineering-
I'm looking to build my own speedo-
how do speedos usually work? measure speed of what? wheel rotation?
in which case, how fast will a 15" wheel rotate at e.g. 60 mph?
Basically, Im trying to determine what scan rate I'll need when i design the program for the speedo...

Hi

It will depend on how your sensing the revolutions, i've fitted 4 magnets onto the propshaft and a reed switch. Prop shaft revolutions = Number of magnets x differential ratio x tyre revs per mile. You'll need to no the rolling circumferance of your wheel and tyres to work your tyre revs per mile

Hope this helps

Adrian

First what you need to work out is the wheel dia to the tyre tread and from that get the circumrenece

tyre size =195/50x15"
To find wheel dia
((195*0.50*2)/25.4) +15 =22.68"
Circimfrence = dia*22/7 = 71.82 or 1.98 yards
giving
889 wheel turns per mile or 889 wheel rpm at 60 mph