Ford focus. Fully inflated diameter = 24 inches. Wall height 4 inches (Say 1 inch when flat). Rolling circumference inflated = 75.4 inches, flat = 66 inches. Obviously to be safe it will be somewhere in between, but 10% is not impossible. Fortunatly deflation causes an apparent increase in speed.
As an aside I get indication on my dashboard of the speed limit, harvested from the live sat nav traffic data. It's very up to date, and also indicates temporary speed reductions due to long term roadworks.
I noticed last week it was even synchronising with the 'ad hoc' speed limit signs on the 'smart' portion of the M3 !
Displaying the current speed limit is a *must* in my requriements for sat nav. Which discounts the inbuilt Citroen sat nav (which "disappointment" doesn't cover. And they can FRO with their extortionate demands for updates). And indeed all Android satnav apps apart from HERE.
Much as my *one* requirement for a driving mode app for my phone is to divert calls to voicemail when I am driving. *Not* "make it easier". Something that Windows Mobile had built in.
Do you really consider 60 quid a year extortionate for a satnav that is always up to date and displays speed limits and indeed warns you them as well as permanent and temporary cameras?
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The 'rolling circumference' doesn't change! I.e. the distance round the outside of the tyre doesn't change.
If you think of something like a piece of tube being rolled along it covers the same distance per revolution regardless of how squashed it is as you roll it.
I guess the Citroen one doesn't do it because it's illegal in France (and I think it's quite strictly enforced).
Strange - I was taught rolling circumference is derived from rolling radius as in 2*pi*rolling r.
Manufacturers are only required to meet the prescribed accuracy with tyres at normal running pressure. No need to worry about the indicated speed if running on rims! It's all set out in the UN regs.
My car does that, but mainly by recognising the speed limit signs on one of the front cameras.
It will also, if requested, use that to set the speed limiter.
If your tyres are flat, I doubt what the speedo reads is important.
Then you were taught wrong. Radius has no meaning on a flat tyre. If you want to define a 'rolling' radius, its the other way round, its derived from the circumference divided by 2 pi.
What changes is the *stretch* on the circumference. A little.
Must have missed the time when Corbyn was in charge.
The tread length, ie circumference when inflated, is unlikely to change much when deflated. Otherwise I would expect the tyre to balloon when inflated.
So, I don't think the indicated car speed , will change much between the tyres being under or over inflated.
If your tyres are 10% smaller due to underinflation, you're not going to be driving far enough for speed to matter.
Dunno about that. My car is a Peugeot 308
Does it pick up '30 Zone' stickers on the side of Wheelie Bins ?
No I don't. Except the Citroen sat nav does none of that. (I won't go into it's lack of POIs).
Meanwhile free HERE does all I want.
Hardly a tough choice.
I bought my car in the UK. If Citroen can't manage to load UK appropriate software, my answer remains. They can FRO.
Anything like under inflated or worn tyres will make the speedo read high anyway - on the so called 'safe side'. So not an excuse for making the speedo read high normally. Only thing that will make it read on the unsafe side is a change of axle ratio etc, or bigger tyres.
I had a hire Ford last year that did that (with reference to a database of speed limits).
Have yet to hear any logical reason why all cars should not be like that.
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