c5 2014 Hydractive 3+ tyre pressure monitor issue

This is the Forum for all your Citroen Technical Questions, Problems or Advice.

Moderator: RichardW

User avatar
GiveMeABreak
Forum Admin Team
Posts: 26065
Joined: 15 Sep 2015, 19:38
x 2992

Re: c5 2014 Hydractive 3+ tyre pressure monitor issue

Post by GiveMeABreak »

But it's all relevant to the sizes fitted! So a 16" tyre fitted to a 16" wheel has a tolerance for the tyre wear - so does a 17", 18" and 19" - but NOT the same as the wear tolerance on a 16" tyre to a 19" tyre.

My point is, as usual, that if it didn't matter then why would they go to all the trouble of writing the software and designing the ECU to accept these values. I'll leave it there as this is going off topic now - make your own decisions, this is only advice, take it or leave it.
giochel
Posts: 17
Joined: 02 Dec 2018, 14:10

Re: c5 2014 Hydractive 3+ tyre pressure monitor issue

Post by giochel »

Paul-R wrote:
27 Jan 2020, 13:57
But the thing is that there is virtually no difference!

I did the same calculations some years ago when I was looking at going down a size exactly as the OP is doing. I also found that the difference was less than 1/2 a percent. The difference was minimal between 17", 18" and 19" tyres however you measured it - turns per mile or tyre radius. ISTR that 16" wheels were an option and here there was a significant difference though.

I don't have the maths to prove it but my gut feeling is that difference between tyre sizes is actually less, or comparable with, a worn tyre compared to a new tyre.
<rim diameter> * 25.4 + 2 * (<sidewall ratio> * <width>) - this is what I have in a Python shell since I was looking for that space saver :D But there's also this page: https://www.tyresizecalculator.com/tyre ... dimensions

Anyway, here are the outside diameters:
16 * 25.4 + 2 * (0.6 * 225) = 676.4
17 * 25.4 + 2 * (0.55 * 225) = 679.3 (+0.4% over 16)
18 * 25.4 + 2 * (0.5 * 225) = 682.2 (+0.9% over 16)
19 * 25.4 + 2 * (0.4 * 245) = 678.6
20 * 25.4 + 2 * (0.35 * 245)= 679.5
20 * 25.4 + 2 * (0.35 * 255)= 686.5 (+1.5% over 16).
User avatar
Paul-R
Donor 2020
Posts: 5443
Joined: 07 May 2009, 16:24
x 804

Re: c5 2014 Hydractive 3+ tyre pressure monitor issue

Post by Paul-R »

These are my calculations. I've used the same web site for the T/Km figure for the tyres. My original reason was, as I said, to go down a wheel size exactly as the OP is doing and so that's the first column %ge difference. The second %ge column references the 16" tyre as a base.
Tyre size Turns per Km %ge reference to 18" %ge reference to 16"
225/60 R16 471 0.21 0.00
225/55 R17 469 -0.21 -0.43
245/45 R18 470 0.00 -0.21
235/40 R19 475 1.05 0.84
As you can see the biggest difference is going up from 18" to 19" but that wasn't one of my options and it's only here for completeness. Going from 18" to 17" was only a -0.21% difference, i.e. negligible.

I haven't been able to find a definitive answer WRT the difference between worn and new tyres but here are a couple of links to have a look at.

https://www.cartalk.com/content/how-muc ... er-reading
https://www.busboss.com/info-tire-size

The tyre calculator reckons that the 19" tyre has an outer diameter of 26.42". Taking the information from the second link for a larger outer tyre diameter (36") will read 1 mile less over 158 miles i.e. 0.63% difference.

As they say, a smaller tyre will have a greater %ge difference so this confirms my feeling that there is probably more difference with a tyre as it wears than between these different size wheels and tyres. Certainly going from 18" to 17".

So, what does this mean in for the OP? The option to change the wheel size is there in the BSI but in practice, providing there is no other reason that these options are listed (and I can't think of any), I wouldn't worry about changing the wheel option in the BSI unless it's an easy thing to do at the time.
RichardW
Forum Treasurer
Posts: 9652
Joined: 07 Aug 2002, 17:12
x 493

Re: c5 2014 Hydractive 3+ tyre pressure monitor issue

Post by RichardW »

Taking 470 turns / km gives a rolling circumference of 2.128m, and therefore an effective diameter of 0.6773m. New tyres are about 8mm tread, min tread allowance is 1.6mm, therefore the drop in diameter is 6.4mm x 2 = 12.8mm / 0.0128m. Which is 1.9% of 0.6773m, i.e. twice the max difference you have listed above.
User avatar
Paul-R
Donor 2020
Posts: 5443
Joined: 07 May 2009, 16:24
x 804

Re: c5 2014 Hydractive 3+ tyre pressure monitor issue

Post by Paul-R »

As I said in an earlier post, I haven't done the maths on this, it was my gut feeling. If what you've worked out is correct (I don't doubt it BTW) it may be because a smaller tyre will have a greater percentage difference between worn and new tyres. This may explain the larger figure you have. Regardless it backs up what I said about tyre wear being comparable to the difference in the different size wheel/tyre combinations.

I've been thinking some more about speedo accuracy. It used to be the case that a speedo could be ±10% accurate and this gave rise to the oft quoted "tolerance" of speeding prosecutions being +10% plus 2mph. Some 30, 40 or more years ago this changed to +20% and -zero% - i.e. the speedo couldn't over-read at all. This may be the reason for the options on wheel sizes in the BSI. Marc says that PSA wouldn't have put the option there for no reason and this may be it. In order for the speedo (and odometer) to read more accurately it cannot be allowed to over-read at all.
furriegurrie
Posts: 130
Joined: 07 Dec 2012, 16:21
x 9

Re: c5 2014 Hydractive 3+ tyre pressure monitor issue

Post by furriegurrie »

Interesting! This option is also available on the C6 tpms. I’ve tried to change this when I switched from my 18” summer wheels to my 17” winter tires but no matter what I tried it wouldn’t store the change. So I’ve left it and found no issues.
giochel
Posts: 17
Joined: 02 Dec 2018, 14:10

Re: c5 2014 Hydractive 3+ tyre pressure monitor issue

Post by giochel »

After seeing this discussion I startd thinking of going from 17 to 16, because on small frequent bumps it doesn't feel as good as I would expect; it is better than most cars, but I would say not as good as my old Renault (195/65R15). I don't think there's any issue with my suspension, as I've driven other C5s and mine seems about the same with those. Is this a good idea or am I going overboard with it?
User avatar
Paul-R
Donor 2020
Posts: 5443
Joined: 07 May 2009, 16:24
x 804

Re: c5 2014 Hydractive 3+ tyre pressure monitor issue

Post by Paul-R »

ISTR that cars with 16" wheels have a different steering rack (a larger turning circle?). You also need to check that the smaller wheel fits over the brake disc and calliper.
User avatar
GiveMeABreak
Forum Admin Team
Posts: 26065
Joined: 15 Sep 2015, 19:38
x 2992

Re: c5 2014 Hydractive 3+ tyre pressure monitor issue

Post by GiveMeABreak »

Yes, correct Paul, the racks are dependent upon engine type - either 180 or 184.
giochel
Posts: 17
Joined: 02 Dec 2018, 14:10

Re: c5 2014 Hydractive 3+ tyre pressure monitor issue

Post by giochel »

Thanks. I guess I'm OK with respect to brakes. It's the 2.0 gasoline and I have 16 inch steel wheels which I use in January-February. This winter though the weather was too good to mount them, so I can't make a comparison in comfort right now.