Classic cars forum & vehicle restoration.

oldtimer · Guest

I am curious about speedo factors, for example who knows how accurate/inaccurate their particular instrument is or if it is checked at any time in some way,like M.O.T or by the Police if it became an issue if one were stopped for speeding?
How much would it be influenced by tyre/wheel changes from the original or by fitting of overdrive gearboxes and/or swapping of differential gearing or a combination of some or all of these elements?
Are there firms who deal with such issues?

oldtimer

Roger-hatchy · Joined: 07 Dec 2007 Posts: 2135 Location: Tiptree, Essex

As to speedo accuracy, mine shows Urm something between this and this, usualy swinging up to 10 mph
25 mph on the speedo = 30mph on the road
30 mph = 40
after that who knows.
At 80, I know I shouldn't, it bounces between 55 and 60.

I ended up fitting a bicycle computer, more accurate, and cheaper than any fines.

Got to get it on a rolling road to fine tune it but at 31 on the cycle speedo the speed warning light at the end of our road flashes to slow down.

I tried using my satnav but over 40 the wind noise drowned it out.

Bayliss Thomas · Joined: 25 Sep 2009 Posts: 557 Location: SUFFOLK

On my modern daily runner the speedo is about 10% out reading fast, roughly the same as my 1928 vintage car!

peter scott · Joined: 18 Dec 2007 Posts: 7118 Location: Edinburgh

Jim.Walker · Joined: 27 Dec 2008 Posts: 1229 Location: Chesterfield

Don't rely on those flashing signs to check your speedo!
I regularly ride in a group on motorcycles. At constant speed it is quite amusing to see that the signs often change with almost every bike. Sometimes around + or - 5MPH.
Almost every Smiths or Jaeger speedo has a small number on the dial either below the trip window or almost hidden behind the bezel at the bottom of the dial. That is the number of turns the cable should make every mile for that speedo to register correctly. It does not take a genius to mark out an exact portion of a mile and roll the car over that distance counting cable turns. A cardboard flag on the disconnected cable helps. The turns per mile can then be easily worked out. If you send a speedo away to be calibrated that info will be required.
Fitting an overdrive designed for that vehicle is generally ok. I think the speedo drive is generally chosen to match. Others may not. Any change of wheel diameter or tyre section will mess things up as will a change of back axle ratio.
Been there! Done all that! I was lucky though. it was in the days when there were plenty of scrapped cars with similar speedos and I found one only 2 turns per mile adrift. I just swapped those innards into my speedo case to retain the matching parts of the instrument.
Most areas have a measured mile usually on a public road for the use of Police and for the Local Authority to check taxi meters. If you can find out where that is then of course a steady 30 mph should take 2 minutes. I know where ours is. I used to operate taxis.
Peter, you beat me to the Post! I must have been typing when you posted.You mention tyre wear. A London cab I used to drive on a regular 9.4 mile run on new tyres used to clock 9.5 when the tyres were worn. In fact the difference was well outside the allowed meter tolerance of two yards in a mile! In the end, every car went for a meter check on brand new tyres because we knew it would fail otherwise and it was easier than re-calibrating the meter.
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peter scott · Joined: 18 Dec 2007 Posts: 7118 Location: Edinburgh

47p2 · Joined: 24 Nov 2007 Posts: 2009 Location: Glasgow

Modern cars usually read about 2 or 3 mph fast, so no excuse for speeding Sad

My 2 Mercs are 2-3 mph fast, my last V-Class was 1mph slow and the Rover P2 is 100% accurate
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One of Britain's Fine Cars

MVPeters · Joined: 28 Aug 2008 Posts: 822 Location: Northern MA, USA

There's a forum thread from earlier this year which may help:

http://www.oldclassiccar.co.uk/forum/phpbb/phpBB2/viewtopic.php?p=54896&sid=9ccad49568e02ab7d720aace1397cbb1
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Riley Blue · Joined: 18 Jun 2008 Posts: 1750 Location: Derbyshire

Have a look here: http://www.carbibles.com/tyre_size_calculators.html

Scroll down and there's a calculator that works out the speedo error when using different size tyres.

You could use a SatNav too - my TomTom gives an instantaneous speed readout (best used on a straight flat road for maximum accuracy). GPS is as good a measure of speed that you'll get without having your speedo constantly re-calibrated.

The flashing roadside signs are not to be relied on; one near me is over 5mph out, another is 3mph out.

oldtimer · Guest

My thanks to all for your input.I never cease to be amazed at the wealth of knowledge on matters automotive and the readiness to impart same to other forum members like myself.
There are aspects which I had never even thought of,like Jim Walker's info regarding the implications for meters on taxis,and Peter's comments that even the difference after fitting new tyres could influence matters.I must now confess to Riley 541 that I am a true dinosaur,having neither possession nor knowledge of Sat Nav,Idon't have a mobile phone either!
However this could well have potential in this context and I thank him for it.
oldtimer