Speedo accuracy

[John Ratsey]

I'm not that clever with the maths but I wonder how the mph figure would change from new tyres with 7mm of tread down to 2mm ?

The overall diameter of the tyres is about 600mm (varies slightly according to which model Jazz). 5mm wear is 10mm off the diameter (ie 1.67% smaller). The wheels will therefore have to turn 1.67% more for the same road distance so speed and distance will both be over-reported by that amount.

[Expatman]

It’s about the same difference if you fitted 195/55 R16 tyres instead of 185/60 R16 tyres - and 195/55 tyres are much more widely available with a choice of summer, winter or all seasons tyres and at a lower price than the few 185/60 tyres you can get in the UK.
I don’t know what deviation there is between tyres in manufacture but I suspect it is probably within the same tolerance level as the wear variation!

[Kremmen]

My speedo always reads higher than the GPS. 30 mph on the speedo is 27 mph on the GPS, and 60 mph on the speedo is 55 mph on the GPS. I have three GPS displays (GPS speedometer, dashcam and satnav). The satnav reads 1 mph higher than the other two, which always read the same. The speedometer and satnav both indicate altitude as well.

My Jazz speedometer of really pretty crap in that they show 20 - 40 - 60 mph, with only a mark at 30 and 50. Not exactly accurate. I refer to my GPS speedo all the time.

Take your 3 displays and raise you a SatNav:

[Jocko]

My titchy satnav has gone and been replaced with a big one mounted in the CD slot! 

[sparky Paul]

Take your 3 displays and raise you a SatNav:

That looks like an MOT fail!

[Kremmen]

It was posted ages ago on a SatNav forum showing that of 4 TomToms, they couldn't make up their mind which way to turn at the end of the road.

[TiJazz]

My Model 3 displayed a 100% match to GPS speed.

The Honda is within ~2mph of Waze (GPS) IIRC.

[Westy36]

I must be the odd one out here. Other than a dashcam I have no incar tech whatsoever. My windsreen is filled only with what is ahead, no distractions at all.

I never use a satnav and no longer own one. My Citigo had a Garmin that fitted on top of the dash, and I never used its sat nav facility. It was set on car info mode.

[sparky Paul]

I must be the odd one out here. Other than a dashcam I have no incar tech whatsoever. My windsreen is filled only with what is ahead, no distractions at all.

I have a dashcam tucked away on the passenger side of the car's own camera, I don't notice it's there.

Sat nav is built in. I find that a satnav makes for a much more relaxed drive if I'm going somewhere unfamiliar, I don't find it a distraction at all. Mind you, it's not bonging all the time like some do.

[Kremmen]

My dashcam is on the passenger side behind the rearview mirror. It's a solid mount unit and the main body is slid onto the base one handed, perfect, not like my old Nextbase units on a ball joint that had to be realigned every time.

I have the built in Honda/Alpine unit but that only has RDS traffic that only covers motorways and major A roads and is pretty tame, updated traffic is very slow so I run a Garmin DS61 with Inrix digital traffic that is far superior. The Garmin sits on a lower shelf so isn't on the top of the dash.

[Jocko]

I have nothing in my line of vision. My satnav is in the CD player; the GPS speedo is below the windscreen, the dashcam is to the left of the mirror, and the Scangauge is on the steering wheel binnacle.

[springswood]

I use an OBDII reader (for mpg & temperature) and satnav on my phone. It's in a holder at the top of the driver's side vent. High enough up it doesn't block the vent or reflect in the side window.