I'm planning to set up my acewell using my satnav, but could do with a starting point for circumference. I have 15" wheels with 195/50 tyres and a 3.62 diff [I think - came from 1.8 cvh sierra]


Thanks

Stu

mark a point on the tyre, roll the wheels forwards 10 times and measure the distance.

far more accurate than any mathematical solution as it accounts for the deflection in the sidewalls, tyre pressures, car weight all that stuff.

tom

That's what I was hoping to avoid!

Once I've got a rough figure I'll use the satnav to get it spot on.

Stu

^^^ that is measuring rolling radius - you really want to have the drive in there too!!

but why bother? You want to be inaccrucate so that thje speedo reads faster than you are actually traveeling hence static diameter is good.

a rule of thumb for a lot of speedos is 1,000 turn per mile.

diameter 195/50 R15 (like mine) is 0.576
so circumference is pi x 0.576 = 1.81 m

numnber turns per mile = 1,600 / 1.81 = 884

allow for diff = 884 x 3.62 = 3200

if you are taking signal from prop, that is your nuber; put in 3170 to allow slight over-read.

now multiply this number by the number of magnets you have.

with the same size as you and 1 prop magnet its around the 513 mark if I recall correctly. Overread about 4mph at 60 against my sat nave

Thanks

Stu

out of interest
my toyo 888's have a rolling circumference of 68.94 inch = 919 revs per mile
whereas my yoko 539's have a rolling circumference of 71.27 inch = 889 revs per mile
both are 195x50x15

Tony

Sorry to resurrect an old (but helpful) thread. I am just in the process of setting up my Veypor dash and it asks for the circumference of the wheel in inches or millimetres depending on if you are using mph or kph. The instructions are aimed at bike set ups.
My sensor magnet is on the sprocket adaptor on the ‘blade engine. How do I use the calculations above to find the correct format? I also have 195/50/15 tyres and a 3.38 diff.