Speedo Calibration

[H18 ENF]

Afternoon all,

This has been sort of bugging me for a while and as I have 20 seconds to spare I thought I'd see what response it gets on here..

I recently bought a Focus ST and was astonished when I drove past one of those Speed awareness laser detectors (the ones that flash up telling you your speed at the side of the road, normally by schools etc, you know the ones...) and discovered that despite that my speedo was telling me I was doing, near as dammit, 30 I was in fact doing 24.

The next one I came across I thought I'd see what speed I actually have to do to hit 30. The answer it turns out is about 35/36. I went through one at 40 at about midnight last night and it triggered a reading of 33.

Which kind of begs the question what speeds am I doing when I think I'm doing 60 or 70?

Does anyone have similar experiences, be it with a Focus or any other car? Is there a chance that my Speedo is not calibrated correctly? Without taking it to a Ford dealership to get it checked I thought I'd hear your thoughts...

[HustleRussell]

Speedos are not permitted to under-read by law. It is perfectly normal for a speedo to under-read by 10% or so. In fact, I have never driven a standard car on standard wheels which doesn't have an over-reading speedo.

ETA: also, those 'speed indicator' signs are rarely accurate.

[s_zigmond]

Most speedos are out by aprox 10%. Also do you have the standard tyre size on it?

[Papa Hotel]

Your speedo will consistently be reading up to about 10% over your actual speed. Those stupid roadside signs, fk knows what they're reading. Their readings are not legally binding or enforceable in any way, I'd imagine they're rarely, if ever, calibrated and most are reading way out.

[Fire99]

Ignore those roadside things. I regularly pass a couple that frown at me saying I'm speeding when I'm under the limit. Compared to a GPS, my speedo over-reads by about 5%

[8potdave]

Fords used to be notoriously bad for it and I think they still are. My wifes C Max was reading almost 90 when she followed me back in the Jag and mine was saying 80 (ironically thats a Ford too!)

[Captain Muppet]

I pass three "30" flashing warning signs on my way home, they all trigger at different speeds. 30, 28 and 26ish as indicated in my MX5 with the wrong diff and the wrong tyre size (which agrees with the GPS I used when I was first worried about using the wrong diff and the wrong tyre size).

[Dave Hedgehog]

most of the recent audi's i have driven have been bang on or at worst 1mph out at upto 70mph

[RenesisEvo]

Dave Hedgehog said:

most of the recent audi's i have driven have been bang on or at worst 1mph out at upto 70mph

my ('05 but dating from '98) Audi is largely spot-on when benchmarked with the sat-nav unit. My previous Ford was about 3-4mph out consistently across the range.

Edited by RenesisEvo on Tuesday 4th September 13:48

[JimmyTheHand]

The speed indicated can be up to 10% high + 4kmph - Link

My last BMW read about 3mph higher than the GPS was showing, current one is about 2 mph (but 4 new tyres, so potential to change slightly)

[LuS1fer]

Check it on your sat-nav. The Mondeo generally under-reads 3-4mph at 70.

[FiF]

Those flashing your speed type of things almost always over read, sometimes by 10%

Find yourself a measure mile, my speedos are calibrated on the measured miles used by West Mercia.

eg If you get down the M5 as far as Worcester.

Along the A449 Eastbound , from the entrance near to Hindlip towards M5 J6.
start marker 1 mile to end marker

Also along the M5 N and Southbound each side of J6.
start marker on M5 N so you know what to look for, black white edge post, white line on emergency lane.

[aw51 121565]

Motorway telephone boxes are a mile apart. Or they used to be.

At 60mph, a mile takes a minute to traverse *ahem*, so drive at an indicated 60mph and time how long it takes to travel between phone boxes, then do the maths (if it's less than 60 seconds, you were travelling faster than 60mph ).

But be aware that some phone boxes might be 1km apart (38 sec at a known 60mph) which will be a surprise when one first comes across it . But it's a significant difference you can't really miss .


Alternative, GPS as others have stated.

[Expatloon]

Don't forget too that an over reading speedo will give apparently better fuel consumption by an equal amount.

Here's a interesting poser:

You have a new car (with warranty) and the speedo over reads by a typical 5%.

Your 1st service is due at say 20,000 miles but for whatever reason you go over by 500 miles (20,500 on the clock but only 19,500 real miles).

Something goes bang.

Your dealer says because you have gone over the service interval mileage your warranty claim is invalid ?

[CapriV6S]

Interesting post above by Expatloon.

Added -
Could be lots of legal issues here, would be interesting to hear comments. Ultimately tho, don't think a dealer would be too fazed about attending to a customer who was only "500 miles over" warranty.

Edited by CapriV6S on Thursday 6th September 08:39

Edited by CapriV6S on Thursday 6th September 08:41