speedo accuracy

[LIBER8ED]

hey trendsetters, this thought was sparked by another post in here and i thought i would put it out to you all..

of course you know i recently upgraded wheel size by 2".. does this, if at all, effect my speedo accuracy??

the QLD's amongst you will know what im talking about, but the speed checks they have on the toll booths up here, say that im doing 67kmph when my speedo, (and cruise for consistency) say 60...

i could understand a couple of km's, but i dont know how accurate it now is..

anyone??

cheers

[Ric]

what were your tyre sizes before and after?
We can calculate the overall diameter, and therefore the circumference from that.

[LIBER8ED]

we can do that?? ha ha cool..

they were the stock 17's on the 2.5i 215\45\17 ( i think ) and i now have 235/35/19 on there.

[MY05WAGON]

useful link from ozSTi

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

according to the calculator, diameter of your original setup is 625.3mm and your new setup 647.1mm. Its says speedometer reading of 19's is 3.5% too slow compared to 17's

eg: when you are doing 100km/h on your speedo you are actually doing 103.5km/h

[LIBER8ED]

ok, might have to take my shoes off for this one..

so does that mean that whatever speed im doing, or what the speedo is showing, is actually 3.5% less that i am.. hang on...

so if im doing 100, ( as per my speedo ) then im actually doing just under 103??

[MY05WAGON]

i suppose it comes down to how often they have the equipment calibrated properly. Like speed cameras there is always great debate about the accuracy of them, I haven't had a speeding fine in ages (touch wood) but do they factor in a certain percentage for errors?

Maybe things like tyre pressures, wear and tear "might" have some influence on the readings?

But according to the tyre calculator a 215 size tyre on the 19's will give you a closer reading on your speedo to stock.

[MY05WAGON]

LIBER8ED check out the site and see what you think, it works everything out for you

[LIBER8ED]

But according to the tyre calculator a 215 size tyre on the 19's will give you a closer reading on your speedo to stock.

isnt 215 the width? how does the width effect the speed?

[MY05WAGON]

i dont know, im just going by the link (if its accurate) might need someone else to explain this one.

[Ric]

...
isnt 215 the width? how does the width effect the speed?

Because the tyre is specified by its width in mm (215), and the ratio of height to width (35%),
so if you pick a narrower tyre, the height also reduces.

That web page at http://www.miata.net/garage/tirecalc.html is great for playing with different sizes. I'll make a permanent link to it somewhere soon.

[OzSTi]

Although the miata site is the more user friendly, this other one;

http://www.carbibles.com/tyre_bible.html

Tells you everything you'd ever need to know about tyres.

I admit it; I've got a tyre fetish!