Speedo Gear

[Abacus]

I've gone from the stock Ghia 16" 215x65 wheels to XR spec 17" 235x45.

Anybody know off the top of their head what speedo gear I need to get installed?

Thanks.

[bodes-sh]

are you sure about the tyre size? according to the data on aufalcon.com the factory tyre size was 215/55r16. just to be sure check the tyre chart on the car...if thats the case you dont have to get anything changed as the rolling diameter is only 10mm longer with the 17s

this is where i got that from http://www.tyremaster.com.au/fitment_tips/diam_calc.htm

[Abacus]

Originally Posted by bodes-sh

are you sure about the tyre size? according to the data on aufalcon.com the factory tyre size was 215/55r16. j

Yep you're right. Sheesh, I can't believe I typed that. I've been working all day and there's too much blood in my alcohol stream.

So, after looking at the groovy Tyremaster calculator it appears, indeed, that I don't need to do anything.

[Sleeperau]

Originally Posted by Abacus

I've gone from the stock Ghia 16" 215x65 wheels to XR spec 17" 235x45.

Anybody know off the top of their head what speedo gear I need to get installed?

Thanks.

I hate to quote another forum site but there is a full calbration chart in the ford mods forum that caters for tyre & diff variant etc.

[AUXRVIII]

You should be ok leaving the speedo drive as it is. The 235/45/17 wheel/tyre combo is slightly taller, but the speedo will already be reading high so now your speedo will most likely be more accurate.

[Abacus]

Originally Posted by Sleeperau

I hate to quote another forum site but there is a full calbration chart in the ford mods forum that caters for tyre & diff variant etc.

Thanks, I don't need it now, but for future reference the link is:

http://www.fordmods.com/forums/documents.php?doc=30

[jonbays]

215/55R16 and 235/45ZR17 are very close in rolling radius I wouldn't worry about correcting for it as speedo error will be greater than the difference anyway.

Thats for an AU Fairmont Ghia right?

[Abacus]

Originally Posted by jonbays

215/55R16 and 235/45ZR17 are very close in rolling radius I wouldn't worry about correcting for it as speedo error will be greater than the difference anyway.

Thats for an AU Fairmont Ghia right?

Yes, that's right.

In fact for all intents and purposes the circumference of a 215/55/R16 and 235/45ZR17 are so close that they may be regarded as identical. A 1.26mm difference, or 0.06% difference according to the fordmods calculator. You would get more variance than that with a couple of months wear on a tyre.

[XR6-VCT-2000]

Originally Posted by Abacus

Yes, that's right.

In fact for all intents and purposes the circumference of a 215/55/R16 and 235/45ZR17 are so close that they may be regarded as identical.

Correct , If you look on the sticker on the inside of the front drivers door aswell (the tyre pressure sticker) , u'll see that the 2 standard sizes mentioned are 16" rims with 55 series tyres and 17" rims with 45 series tyres .
( thats on my xr anyway )

[jonbays]

There is nothing in it!

AU Falcon BTR 4 Speed AutoGearing ChangesKmh/1000rpmDiffTyresFirst SecondThirdFourthSpeeds in gears RPMRatio122030463000-5000-5750rpmat 110Kmh2.3901.4501.0000.680First SecondThirdFourth3.45235/45R1714.023.033.449.14270806911513210016719222423.45215/55R1613.923.033.349.0427080691151321001671922244

[jonbays]

that didn't work so much for WYSIWYG

[howesy]

why dont you just go to jaycar and get the speedo correction device for $44. That is of course if the speedo is a wire job which it should be.

[jonbays]

Originally Posted by howesy

why dont you just go to jaycar and get the speedo correction device for $44. That is of course if the speedo is a wire job which it should be.

Beacus waht i was trying to show i sthe speedo would be changing by next to nothing at all.

AU Falcon BTR 4 Speed Auto



Kmh/1000rpm

Diff Tyres First Second Third Fourth Speeds in gears RPM

Ratio 1st 2nd 3rd 4th 3000-5000-5750rpm at 110Kmh

2.390 1.450 1.000 0.680 First Second Third Fourth

3.45 235/45R17 14.0 23.0 33.4 49.1 42 70 80 69 115 132 100 167 192 2242

3.45 215/55R16 13.9 23.0 33.3 49.0 42 70 80 69 115 132 100 167 192 2244

[howesy]

Originally Posted by jonbays

Beacus waht i was trying to show i sthe speedo would be changing by next to nothing at all.
[/size]

Sorry but it wasn't directed at you. I apologise if you thought I was goading you. I was addressing his original post where he seemed to want to know which speedo gear to change to. If he were going 2 do that it seems to make more sense to get the device and then your set for future diff mods and furthe tyre changes that will put the speedo out.

[Abacus]

Originally Posted by howesy

Sorry but it wasn't directed at you. I apologise if you thought I was goading you. I was addressing his original post where he seemed to want to know which speedo gear to change to. If he were going 2 do that it seems to make more sense to get the device and then your set for future diff mods and furthe tyre changes that will put the speedo out.

What happened was I was just too lazy to get my facts right first.

I assumed that there would be a change of some sort if you went from 215/55/16 to 235/45/17. Clearly, there isn't. I should have checked first, but then again, that was the entire point of the thread.

I posted the fordmods link referred to by Speeperau just so that there is something in the database that might help somebody doing a more significant change.

[Mechan1k]

No need for a speedo correction device ... AUXRVIII was spot on the money really ... I found after changing from 215/55R16 to 235/45R17 that my speedo because dead on accurate ... prior to the change it was showing about 2km/h slower than actual speedo.

[Abacus]

This should also work if you are thinking of putting on G220 style 225/50/17 rubber.

The circumference of this tyre is about 2% bigger than a 215/55/16, so at a given indicated speed you will be travelling about 2% faster on the 225/50/17 than you would be on the 215/55/16.

If, as AUXRVIII and Mechan1k say, the AU speedo on 215/55/16s is over-reading by about 2km/h, at 100km/h the speed should also be accurate on 225/50/17s.

[Mechan1k]

I tested it with the GPS ... both on flat open road with cruise control set.

[ClassicAU]

i dont know if this helps anyone, but i just corrected my speedo with a different plastic trans gear. i recently changed my diff ratio to 3.73 and also have 235/45/17's on it. i managed to get it fairly close with a number 22 speedo gear. it previously had a number 19 and was about 15km/h too fast.

[Mechan1k]

Good to know as I am looking at similar diff gearing for the ute ... and I know I have my speedo right now ... so at least i now know what I will need when changing the diff over at a later date.