question about speedo being knocked out?

[BFD83]

i rung up bob jane this morning about the 18"s im gonna be getting off my mate and as my mate thinks the tyres are a little big too big i rung em up and asked them if the tyre size being 245/ 402ZR18 would it knock the speedo out, the guy said for me to ring ford up and tell em what im gonna do and they can do something with what controls the speedo and to change something so it will be right with the new size wheels.. does anyone know how much roughly it would cost to get this done?

[Abacus]

What did you upgrade from?

Tyre diameter for 235/45/17 = 2021mm

Tyre diameter for 245/40/18 = 2052mm

See http://www.tyremaster.com.au/ and go to Fitment Tips|Diameter Calculator

Difference from standard is only 1.5%. Should be fine.

[BFD83]

i haven't got em on yet, i would be going from 225 to 245?

[JC]

Originally Posted by Brent83

i haven't got em on yet, i would be going from 225 to 245?

Why have 2 threads on the same thing? Didn't like the advice in the other one?

To answer your line of questioning between the 2 threads, there is no problem running 245/40/18s. In fact, there is probably 50% of the cars on this forum running at least 18s. Most of those running 18s would be 235/40/18, but some would be running 245/40/18. The maximum you will be out running 245/40s is 3.5%. If they were 235/40s, you would have a max error of 1.9%. The ADR for speedo accuracy is 10% - around 3 to 5% is normal, and for the speedo to read higher than actual is definitely the norm. So let's say your speedo currently shows 100km/h on your 225/50/16. A handheld GPS would have your speed at around 95. So when you fit the 18s, you will end up with a 3.5% error on 95, which would take it up to 98. So at 100 on your speedo, you would still be travelling at slightly less than the indicated speed.

Here's a link to your first thread, just in case you need it:
http://www.fordforums.com.au/showthread.php?t=66581

[BFD83]

thanks JC, sorry for making another thread, i just figured hardly noone was replying to my other one i would make up another one with a different subject and more ppl would be able to tell me. sorry for bein a pain hehe. i'm really not dumb just illiterate about a lot of things! lol i understand it all now

[JC]

Last bit of advice from me - go the 18s!

I have 265/35/18 on the back of mine, and my speedo, according to GPS, is 1km/h too fast.

[BFD83]

haha sweet cheers!