Speedo reading

[jacko52]

Hi All
I got my self a tomtom satnav the other week and i have noticed that the speedo in my car is about 3mph out slower then the satnav is this a common thing i would have thought the speedo would have been right saying as its electric
my car is a vectra 150sri
Jacko

[robbo8911]

yer most sat navs are out by 3-5mph in cars i drive coachs and hgvs and there spot on with mph in them don't no y tho ?

[Thomas1]

Car speedos are rarely - if ever - 100% accurate. Legally they're not allowed to under-read at all, but are allowed to over-read by up to 10%, hence they're set at the factory to over-read slightly to avoid ever under-reading. Generally when you're doing 30mph, the speedo will say 31; at 70 mph it'll read 72 or 73; at 100 it'll read 104 or 105

[Duncan]

Exactly

[MrT]

At 70, mine always read 75-76, until I switched to 225/45-r17 fronts when the reading went to 77. When these wear down I'll probably be illegal (78 or so at 70). Similarly at 30, mine is in the 32-33 range.

Someone mentioned once that it was possible to use TECH2 to calibrate the speedo to the new wheel/tyre size. Would love to know what I ask the dealers for to do this...

[jaxouk]

At 70, mine always read 75-76, until I switched to 225/45-r17 fronts when the reading went to 77. When these wear down I'll probably be illegal (78 or so at 70). Similarly at 30, mine is in the 32-33 range.

Someone mentioned once that it was possible to use TECH2 to calibrate the speedo to the new wheel/tyre size. Would love to know what I ask the dealers for to do this...

say "i changed my car tyres to 225/45`s. Could you reprogramme it with the tech2 please" or "i thinks my speedo`s slightly out,...you wouldn`t have a look at it"

[Duncan]

Someone mentioned once that it was possible to use TECH2 to calibrate the speedo to the new wheel/tyre size. Would love to know what I ask the dealers for to do this...

You should be able to ask them exactly that - to reprogram for a different wheel/tyre size. You can also simply enter the circumference of the wheel if the particular wheel/tyre size you have isn't on the list.

[MrT]

Yeah, I tried asking that a month back at Reg Vardy's / Evans Halshaw in Leeds and they looked at me like I was mental. Clearly some dealers know more about what they cando than others.

I'll be popping the car in for the handbrake recall soon enough, so that will be one of the things I'll add to the list (the other being the rear brakes - outer pads look fine 4mm or so left - but they're grumbling when used). Won't be going to 'Vardy's' though...

edit: I have factory 17"s on from new, and IIRC the Exclusiv is setup for 16" steelies, which may explain why things are so far 'off' to start with. Switching from 215/50 to 225/45 throws it out further by approx 2%

[big rich]

mine is out by a few mph aswell its a tomtom go 500

[m8internet]

It's better if you are driving at an indicated 70mph and in real-terms you are driving at between 63 and 70mph
If you do get stopped for "speeding", then get a copy of the original documents :
Road Vehicles (Construction and Use) Regulations 1986 permits the use of speedometers that meet either the requirements of EC Council Directive 75/443 (as amended by Directive 97/39) or UNECE Regulation 39. [12]
The Motor Vehicles (Approval) Regulations 2001[13] permits single vehicles to be approved. As with the UNECE regulation and the EC Directives, the speedometer must never show an indicated speed less than the actual speed.
However it differs slightly from them in specifying that for all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower that this), the indicated speed must not exceed 110%