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        General >> News >> Speedometer accuracy
        
(Message started by: Neil W on Mar 6^th, 2005, 5:49pm)

Title: Speedometer accuracy
Post by Neil W on Mar 6^th, 2005, 5:49pm

On the Scorpio simple.lists site there was a thread about speedometer accuracy, or rather a built in inaccuracy on all modern cars. So I decided to check mine using differential GPS, driving at constant speeds. GPS is, of course, very accurate over more than one refresh cycle.

The results were interesting:

GPS speed Speedometer speed
30 mph 33mph
60 mph 65mph
70mph 76mph

I tried them twice on a dual carriageway, once in each direction (to cancel out wind effect ;) )

Has anyone else found this?

Neil.
Junior member at last ;D ;D

Title: Re: Speedometer accuracy
Post by Mottsy on Mar 6^th, 2005, 5:54pm

I noticed that when travelling on motorways at an indicated 60mph, lorries were always slowly cating up with me. This lead me to believe that I was in fact traveling at around 55mph.

A check along a measured mile produced results consistant with those you have found.

Title: Re: Speedometer accuracy
Post by Geoff_W on Mar 6^th, 2005, 7:01pm

Yes Neil,

Those numbers back up my GPS experience.

Geoff

Title: Re: Speedometer accuracy
Post by Aardvark on Mar 6^th, 2005, 8:50pm

A speedo over-reading of +4% / +8% is typical for most cars nowadays, consistent with your results.

An "actual speed" code can be accessed from the ECU via most SEAT/Skoda/VW/Audi climate control systems. This usually bears out an over-reading of +8% .

However my Alhambra speedo only reads +4%, thus when driving that I usually gain on most other cars through 40mph speed restricted roadworks etc but not in the Leon R which over-reads 8%!

Title: Re: Speedometer accuracy
Post by proberjohn on Mar 6^th, 2005, 8:51pm

Same for me - a full 10% out when compared with the GPS. No difference with higher tyre pressures either. Real pain when trying to be legal but hurrying along.

Title: Re: Speedometer accuracy
Post by geraghtyg on Mar 6^th, 2005, 9:43pm

I checked this with GPS and got similar results. Max speed on motorways over here is 120KM/H, which is about 74.5MPH. However, I can drive at an indicated speed of about 81MPH and be actually driving @ 74.5MPH. Interestingly, the speedo in the advanced diagnostics screen (where you get litres of fuel left, etc) gives a fairly accurate estimation of the current speed as 81 MPH also shows up as 120KM/H on this.

Title: Re: Speedometer accuracy
Post by Thunderchild on Apr 1^st, 2005, 11:17am

Same for me with the GPS. Mine is out by a fraction under 10%. Generally now when cruising on the motorway I rely on the GPS and cruise control to keep me on the limit. Most cars still fly past me though but hey what do I care??? LPG and the equivalent of about 45 to the gallon from my cossie is good for me!!! That and loads of grunt when you want it.... lovely.

Thunderchild

Title: Re: Speedometer accuracy
Post by ChrisB on Apr 2^nd, 2005, 10:44pm

You get the same thing if you hook a ODB lead up and measure the VSS sensor which gives true speed. It appears to be adjusted at the speedo head to read slightly high.
At least it means your unlikely to get a photo unless your doing something silly ;D ;D

Chris B

Title: Re: Speedometer accuracy
Post by Dave on Apr 2^nd, 2005, 11:16pm

You can also press and hold the didgery stick (the trip distance resetter) and cycle through the functions, one of them gives you your actual speed in KM/hour! It's like having a digital dash! This also gives the more likely accurate figure, about 10% less than the analogue type speedo.

D

Title: Re: Speedometer accuracy
Post by Danny_R on Apr 4^th, 2005, 1:39pm

I've often wondered why the speedo's are not always dead on accurate. In America, all law enforcement vehicles like my police cruiser have certified calibration. I've tested it alongside a GPS unit and its bang on accurate at any speed. I wonder why they cant do the same in all vehicles?

Danny_R

Title: Re: Speedometer accuracy
Post by nic on Apr 4^th, 2005, 2:01pm

M8,

I had a Mk1 Granada 3.0s ex police, with a huge calibrated speedo on the dash.

I often wonder if it was right, if so lucky i never got caugh, although i did get wrong for not driving in a straight line, it went sideways if memory serves me right.

nic

Title: Re: Speedometer accuracy
Post by Scorpioen84 on Apr 6^th, 2005, 2:55pm

ive found that the DIGITAL speedometre (when holding the reset button og tripcounter for 10sek) is werry acurate, maby 1 or 2 km/t to fast...
100km/t on needle gives 92km/t on Digital...