speddo recalibration ??????

IMDGEM · tinkerer Joined: 16 Oct 2001 Posts: 103

i have just put a set of 15' wheels on my wagon they have 50 series low profiles on them do i need 2 get my spedo recalibrated?

Westy · rice boy Joined: 16 Nov 2001 Posts: 41

hey man where u at??? some guy up the road has a wagon with 15's on is it u?

Racegem · backyard mechanic Joined: 30 Jan 2002 Posts: 987

What width tyre?
With 195 or 205's you'll read about 5kms low. So sit on about 95kp/h.

Yonnee.

Pete · backyard mechanic Joined: 05 Oct 2001 Posts: 502

nah man the track stays the same! i checked it. (195 50's vs stockers) the profile takes out the extra inches if you get what i mean

Racegem · backyard mechanic Joined: 30 Jan 2002 Posts: 987

Track has nothing to do with the rolling diameter! What are you measuring? (non offensive tone)

Yonnee.

Pete · backyard mechanic Joined: 05 Oct 2001 Posts: 502

sorry man wrong terminology, what i mean to say is that if you were to roll your tyre out flat (one entire revolution) it is the exact same distance, 13", 14" 15" and 16" rims. if you get the correct tyre. in this case, for 15's the correct tyre is 50 series, io know this bcoz that is the tyresi have and i have compared it to a stockie.

Racegem · backyard mechanic Joined: 30 Jan 2002 Posts: 987

But what WIDTH tyre did you measure?
The "50" in your, let's say, 195/50-15 is the tyre height from rim to outer diameter, and is measured as 50% of the width. A 60 series tyre is 60% of the width. So a 195/50-15 would have a smaller rolling diameter than say a 235/50-15.
Bare with me...without boring you with the maths, a stock 155-13 tyre has a 78series or aspect ratio. It has a diameter of 572mm, give or take a few for tyre wear. A 195/50-15 has a diameter of 576mm, give or take a few. Multiply pi times radius for rolling circumference and you get, over a km, a difference of roughly 6 metres, close enough not to worry. But do the calculations for a 215/50-15 and the difference is 32 metres, over three kp/h on your speedo, and take into account driveline and speedo error and the fact cops now book you for 1kp/h over, I'd sit on 95.
And just for interest, you'd have to have a 205/35-17 to get 575mm diameter.

Here endeth the math lesson, you can all now go to play lunch.

Yonnee.

Racegem · backyard mechanic Joined: 30 Jan 2002 Posts: 987

I f**ked up! That should read 0.6metres and 3.2 metres.

Yonnee.

IMDGEM · tinkerer Joined: 16 Oct 2001 Posts: 103

im in campbelltown its dark green and it has 15" imported astra chasers on it

IMDGEM · tinkerer Joined: 16 Oct 2001 Posts: 103

ive got 15' rims with 195-50 series on them ???????

Racegem · backyard mechanic Joined: 30 Jan 2002 Posts: 987

You'll be travelling about 3kp/h faster than your speedo's reading.

Pete · backyard mechanic Joined: 05 Oct 2001 Posts: 502

man the simplist thing like tyre ware will blow your theory way out the window

Racegem · backyard mechanic Joined: 30 Jan 2002 Posts: 987

So will tyre pressures. It's about acceptable limitations. Easiest and cheapest way to check your speedo, check against a known distance. Those highway markers are pretty close. Take a tripmeter reading at one, then another reading at 10k's. If you're tripmeter says 9.6 then you'll be doing 100kp/h when your speedo reads 96 and so on.