i checked my speedo with the phone sat nav speed and was quite suprised how accurate the cars speedo is,its within 1 mph or less. 2009 hse td4e manual,18 inch wheels.

I have compared mine at a range of speeds and its always within 1 or 2 mph.

Any other car ive been in has been further off.

What size piece of rubber do the 18 inch rims have? Procrastination, mankind's greatest labour saving device!

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Indicating 2mph slow error at 20mph wipes out the allowed 10% error for speeding calculations, you’ve only got the “grace” allowance and that changes by police force!

What I have noticed over the years is a fairly consistent kph difference regardless of speed.

With 255/60r18 I have about 2kph difference. Now the tyres are older, lower tread, about 3kph.


P.S. If you haven't noticed, the odometer adds km (or miles) even if you are reversing down a long driveway etc. Procrastination, mankind's greatest labour saving device!

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sid wrote:

i checked my speedo with the phone sat nav speed and was quite suprised how accurate the cars speedo is,its within 1 mph or less. 2009 hse td4e manual,18 inch wheels.

My speedo is always 2 mph optimistic as shown on my separate SatNav, and confirmed by these roadside 'flashing' signs.

GPS Speedo
20 22
30 32
all the way to
60 62
70 72
and allegedly beyond 19in standard fit

This helps you obey speed limits by making the indicated speed higher than the real speed.
I tend to use cruise control to help keep to limits on some of the longer stretches of limited (wide open) roads - set using the GPS figure of course.

Although I read that the Met Police are halving the 2mph (+10%) threshold to gain over double the amount of fines (revenue).

https://www.independent.co.uk/news/uk/crim...74896.html FL2 XS SD4 Auto 2010 2012-2017 (21k - 91k miles) (MY2011)
FL2 Metropolis SD4 Auto 2014 2017- (16k - 76k+ miles) (MY2015)

I would have put 245/70r17 on the car, but at the time to replace the 18 inch poodle rims with 17 inch & buy a second spare was not worth the price, so just have to be a bit more careful on corrugated rock strewn roads. Still reasonably happy with the 255/60r18. I have 2 spares for the car when traveling in the outback.

https://tiresize.com/tyre-size-calculator/ Procrastination, mankind's greatest labour saving device!

Acoustic insulation ARB TPMS 3xARB air compressors After cooler Air tank On-board OCD pressure air/water cleaning Additional 50L fuel Carpet in doors ABE 2x1kg Waeco 28L modified fridge Battery 4x26ah Solar 120w Victron MPPT 100/20 DC-DC 18amps 175amp jumper plug Awning 6x255/60R18

Lightwater wrote:

What size piece of rubber do the 18 inch rims have?