INACCURATE SPEEDOMETER?

[WXL]

Having checked the speedo in my car against the speed indicators in Waze, Google Drive and several electronic roadsigns, it seems my HUD and the cruise control are registering my speed at 3mph slower than everything else.  If it's true, is this a major recalibration job? 

[Britprius]

53 minutes ago, WXL said:

If it's true, is this a major recalibration job? 

All car speedo's over read the cars speed.
The reason is that the law will not allow a car speedo to under read the cars speed because of the implications of drivers getting speeding fines, and then blaming the car manufacturer.

There is a scale of speedo accuracy in the construction and use regulations that the speedo has to comply with. This can be found on line, but 10% of actual speed at 30 mph is within that range. From memory at 80 mph the speedo could be read 90 mph.

Because of factors that affect the reading such as tyre ware different makes of tyre being slightly different in rolling circumference, and even tyre pressures it is impossible to get an accurate reading all the time.

John. 

[Herbie]

Exactly as John says:

https://www.thecarexpert.co.uk/how-accurate-is-a-car-speedometer/

[Barry14UK]

Whilst I agree with replies,  the OP said his HUD was reading about 3 mph slower than other indicators not faster.  With standard size tyres correctly inflated I would expect the HUD to read faster than other devices.  When comparing with satellite devices, it is important that comparative figures should be obtained on level road as hills will make a difference but not to the cars HUD.  I have done this exercise on my car and the speedo and HUD which agree show a slightly faster speed than my fitted TomTom or the back up small Garmin.  This is what I would expect and also find on my BMW.  So in short, if compared as I said, the OP's HUD is suspect and does not comply with the margin speed indicators supplied on the car are meant to show.

[Herbie]

Good point Barry - just shows how the mind can jump to conclusions:

A priest, a minister and a rabbit walk into a pub
Barman - "What can I get for you?"
Rabbit - "Don't ask me - I'm only here because of Autocorrect"

A bit like what my brain did when I read the OP

[dilbert]

If it is truly underreading then that is an issue for Lexus to deal with as it is illegal for a speedometer to underread.

The main reason for speedometers overreading (up to 11%) is to allow the fitment of various option wheel and tyre combinations that the manufacturer may offer.

[ColinBarber]

10 hours ago, WXL said:

Having checked the speedo in my car against the speed indicators in Waze, Google Drive and several electronic roadsigns, it seems my HUD and the cruise control are registering my speed at 3mph slower than everything else.  If it's true, is this a major recalibration job? 

What tyre sizes do you have?

[Bluesman]

7 hours ago, ColinBarber said:

8 hours ago, dilbert said:

If it is truly underreading then that is an issue for Lexus to deal with as it is illegal for a speedometer to underread.

The main reason for speedometers overreading (up to 11%) is to allow the fitment of various option wheel and tyre combinations that the manufacturer may offer.

When my speedo is showing 76 mph the sat nav which is probably more accurate shows 70 mph. Tyre's 225 60 16

 

[flookyk]

7 minutes ago, Bluesman said:

 

That is within the limits, they are allowed to read up to 10% over but not under.

The only vehicle I have had that the speedo was accurate was a Honda ST1100 Pan European that has an RS calibrated speedo fitted due to it previous use