Calibration of speedo after fitting SO-2

[MATHEW]

I have just fitted a set of 225 50 ZR16 SO-2to the rear of my griff 500.
The chap fitting the new boots said they were a different size to my old 235 50 ZR16 SO-1 and said my speedo clock will now have to be re calibrated due the tyres being smaller.
Is this correct?
How much will it cost to calibrate the speedo?
Is it a long job?

I would appriciate much help as I gather if the speedo is reading high then the milage will aslo be clocking high
Ta.

[CleG]

hmm that size seems very odd my griff 500 is 245 45 16" at the rear.

How old is your griff? I have the later wheels on mine .. maybe the earlier ones where smaller?

Mine are on a 1996 500 ..
if the tyre hight has changed from 45 to 50 etc, then the speedo will be out a bit BUT not if the width has been changed ir 235 to 225, so a thinner tyre like you have had fitted won't alter speed

>> Edited by CleG on Thursday 2nd May 15:24

[M@H]

hmm..

the difference between 225/50's and 235/50's is not much though.. you're looking at 5mm wall height difference... taking that into the context of the overall wheel diameter being apx.52.39 cm with 235's or 51.89 cm with 225's this gives a difference of 0.09% or in speedo terms .9 of an mph at 100mph.

I don't think this is really much to worry about.. especially as a worn set of 235's will be the same size as a new set of 225's

Cheers
Matt.

[MATHEW]

My griff is a 94 500.
The tyres on the car when i got it were 50 series.
So i got the same again but was unable to get 235 width in a SO-2 style.
I am now thinking should I have got 245 45 SO-3.
Ta,

[gazzab]

Sorry - am I being thick? But the difference here is one of width of tyre and not height. Surely only the overall height of tyre and wheel dictates the travel for a 360 revolution and therefore only this can impact the speedo readings.

[CleG]

if they are the same ones as in my profile the tyres are 245 45 zr 16" toyo ones are ZR rated for the rear and can be picked up for £110 fitted per wheel at challenger tyres & exhausts

[gazzab]

225 does sound more like a front tyre width. Ask a dealer!?!?

[M@H]

Good point.. I'll get my calculator out again.. I still don't think is would be significant though.... hang on..

[CleG]

yeah the tyre hight is the same but in theory be slighty less as the tyres are been stretched in a bit to fit smaller width tyres on a rim designed for 245 ones.
(thats if he has the same wheels as mine)

[CleG]

fronts are 225 45 zr 15's

[M@H]

OK
a 225/50/16 has a rolling circumference of 163.02 cm
a 235/50/16 has a rolling circumference of 164.59 cm

difference is 1.57 cm over 164.59 cm so 0.095% different..
again not significant

Cheers
Matt.



>> Edited by M@H on Thursday 2nd May 15:32

[CleG]

M@t you are forgetting that the rim what probably supposed to take 245 width tyres and 225 will deff be reduced in hight coz of the tyre walls beeing pulled inwards instread of been verticle.

get your geometry calculator out

>> Edited by CleG on Thursday 2nd May 15:41

[MATHEW]

Thanks for all the help.
My fornt tyre size is 205 50 ZR15
I have no Power steering.
Can anyone tell me how much it will cost to have the clock re calib.

[M@H]

quote:
M@t you are forgetting that the rim what probably supposed to take 245 width tyres and 225 will deff be reduced in hight coz of the tyre walls beeing pulled inwards instread of been verticle.

get your geometry calculator out

>> Edited by CleG on Thursday 2nd May 15:41

erm.. well that may be the case but then again what size are the rims themselves..? a 225 tyre is nearly 9" wide and I don't believe the car as 8"-9" rims. therefore I think the 10mm difference in width at a size that is already over the width of the rim itself would make this difference would it?
..if it did, it would again be really insiginficant.. we'd have to take tyre pressures and allsorts into account if we were going down this route

[gazzab]

Dont recalibrate get the right tyres on the rear.

[M@H]

quote:
Dont recalibrate get the right tyres on the rear.

And don't bother anyway as the differences are not significant enough to warrant the expenditure.. even a 5% difference is only 3.5mph at 70. and were talking about around 1% here..

Cheers
Matt.

[MATHEW]

Thanks M@H
you have given me some good info on this.
I am not going to change the tyres as they cost me £80 each.
Ta
Matrhew

[Neil Menzies]

quote:
OK
a 225/50/16 has a rolling circumference of 163.02 cm
a 235/50/16 has a rolling circumference of 164.59 cm

difference is 1.57 cm over 164.59 cm so 0.095% different..
again not significant

Cheers
Matt.

My arithmetic turns out a little different

Tyre	Radius	Circumference
225/50/16	315.7mm	1983.6mm
235/50/16	320.7mm	2015.0mm

Difference is 21.4mm in circumference, over 2015mm, or 1.06%.

Still not worth bothering over...

[M@H]

Yep you are right.. my mistake I only added the tyre width in once, not twice for the other side D'oh..

still as we both agree.. its an insignificant amount..

Cheers
Matt.

[kevinday]

quote:
Sorry - am I being thick? But the difference here is one of width of tyre and not height. Surely only the overall height of tyre and wheel dictates the travel for a 360 revolution and therefore only this can impact the speedo readings.

Gazza, the 45 or 50 figure is a percentage of the width, therefore a 235/50 is marginally taller than a 225/50.