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Sat-Nav Speed Read Out.
d2d4j:
Hi
That’s definitely a smaller wheel, but you changed the rim and tyre to smaller
I would ensure you have informed insurance company of change
Our car did not have a spare wheel, just one of those puncture sealant kits.
It is worth noting though, if you are with AA or RAC (others may also offer same), and have a puncture. They carry correct spare wheel to get you going and you repair your tyre, then they collect loan wheel
Many thanks
John
Ronski:
Many cars do not now have spares, some have space savers, only if you're really lucky do you get a full size tyre and somewhere to store it. Some cars we have pass through have huge 22" wheels, we removed some 315/35R20 from a company X5 to reduce cost, no idea where you'd put one of those!
You've gone lower on the wheel diameter, and a lower profile, the profile is the height of the tyre as a percentage of the width. End result is the rolling circumference will be quite a bit less.
As John states you do need to inform the insurance company, as it counts as a modification and they could refuse to payout if any claim was made and it came to light.
I have a VW Tiguan and the speedo needle reads about 5 mph faster at around 60 than I'm doing. My dash has a digital option for displaying the speed and this tallies with the GPS in my phone. IIRC the speedometer can read faster than you're going (within certain limits) but not slower. You can get various phone apps, some that even use an external GPS which is a lot more accurate.
Another thought is if the tyre size you have gone with is standard fitment option for that vehicle then the main dealer may be able to change the settings to suite that tyre size.
4candles:
--- Quote from: d2d4j on October 24, 2017, 01:20:29 PM ---Also, sat nav are not calibrated so the speed cannot be relied upon as an accurate speed measurement
--- End quote ---
The satnav derives its time from the GPS satellites, and calculates its position by triangulating the time of reception of the signal from three or more of them. If the device in question accurately displays its actual location, then I can't see why any 'calibration' is required to display an accurate speed.
d2d4j:
Hi
Many thanks
Please see this link or google sat nav accurate speed to show why sat nav speed cannot be relied upon
Many thanks
John
https://www.driving.co.uk/car-clinic/car-clinic-which-should-i-trust-my-speedometer-or-my-sat-nav/
sevenlayermuddle:
If the new wheels and tyres now fit the spare wheel well, and the old ones didn’t, then the new ones must be smaller. Hence speedometer will over-read. Be very mistrustful of anything on the web relating to ‘equivalent’ tyres & wheels, there’s an awful lot of dangerously inaccurate data published.
Also worth bearing in mind, no speedometer that is driven from wheel sensors can be very accurate, at least not all the time, simply because it will drift by a percent or two as the tyre tread wears down (less tread means smaller circumference). Legally they are allowed to over read a little, but must never under read.
I think satnavs can usually be trusted, I suspect that Sunday times article is just plain wrong about how Sat Navs work, in relation to speed measurement. I would assume they measure speed by detecting phase shifts and Doppler changes in the Satellite signals rather, than repeated location sampling.
As others have said, make sure your insurers know, else you might be accused of ‘non disclosure’ if you ever need to claim. They may want to increase your premium, the argument often is “different wheels might make the vehicle more attractive to a thief”.
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