How accurate is your speedo?

cajonlardo

A couple of replies on that speeding ticket thread reminded me of this.

I've read/heard it said that car speedometers are usually a significant per cent over reading. For example you are actually doing 60kph but your speedo may say 65kph. About 2 years ago I used a gps to check 3 different cars just out of idle curiousity and all 3 were very accurate.

So, just wondering if my little sample was just luck? have any of you checked yours?

BTW, the cars I checked had the origianl wheels and tyre sizes and changes to either would obviously affect the speedo reading.

Skatedude

using gps speedo on straight road is very accurate. and i always test if i change wheels and tyres.

my old 93 bmw740 was spot on reading 120km on dial and gps.

mothers z4 also spot on reading 120km on dial and gps.

my bmw840 was spot on reading 120km on dial and gps with stock wheels and tyres , but i recently changed wheels and tyres & deliberately fitted a slightly wider tyres so i knew the speedo would be off and dial now reads 116km instead of 120km which is as expected with a slightly bigger tyre then stock.

it's easy to check with any gps speedo phone app.

CiniO

I haven't seen a car yet, which would be accurate.
Most cars I had were overreading by about 5-10%

GM228

CiniO wrote: »

I haven't seen a car yet, which would be accurate.
Most cars I had were overreading by about 5-10%

Indeed I have never had an accurate car, most average 4km below the actual limit when at 100km so the slower the car is going the more accurate it is.

GM228

savagethegoat

depends on tyre wear too surely, New tyres will have a larger circumference

Trollhättan

Not very accurate at all....120 on the speedo in my old 900 shows about 109/110 on the gps.

ironclaw

Went for a spin in the car one day, and thought 'that seems shade quick', was accurate but in MPH

Mehaffey1

My cars speedo doesn't work, so I'd say extremely inaccurate.

pete4130

My Forester speedo says about 117kmh compared to my GPS @ 110kmh (max legal speed here in NSW Australia).

Same GPS in my 22 year old Landcruiser, they are the same.

efb

my speedos keep my balls snug

cajonlardo

How many of you posting a difference are using original size wheels and tyres?

GM228

cajonlardo wrote: »

How many of you posting a difference are using original size wheels and tyres?

Mine have all been original wheels.

When changing wheel size speedo needs to be re-calibrated, has anyone ever actually done that I wonder?

GM228

savagethegoat

cajonlardo wrote: »

How many of you posting a difference are using original size wheels and tyres?

mostly not I'd say. Speedos have to be calibrated so that they will never read under so it is inevitable that they will have to overread a good bit at some stage in the tyre life (not bright enough to know whether that would be when new or worn)

my3cents

On a couple of long runs in the UK recently (hence mph and not kph) with the speedo at as near as I could keep it 60mph the sat nav was showing 56 or 57mph.

GM228

savagethegoat wrote: »

mostly not I'd say. Speedos have to be calibrated so that they will never read under so it is inevitable that they will have to overread a good bit at some stage in the tyre life (not bright enough to know whether that would be when new or worn)

It's not just that they overread at some stagr in tyre life, they seem to overread from day 1!

I buy a new car every two years and the GPS has always shown a slightly lower speed than the speedos from day 1!

GM228

edburg

Dials on my 2 read over compared to sat nav GPS speed, closer difference at 110-120km/h, however the digital read out on both are just about accurate maybe 1 km/h out. both cars on original wheels and tyre sizes.


On GM cars you can update the on board computer/ecu so it knows you changed from 18 to 19 or vice versa. Only if you change to manafacturers wheel settings though, i.e optional extra wheel size BUT NOT if you go from 16" to 20" spinners lol

Melodeon

GM228 wrote: »

It's not just that they overread at some stagr in tyre life, they seem to overread from day 1!

I buy a new car every two years and the GPS has always shown a slightly lower speed than the speedos from day 1!

GM228

This is deliberate and is specifically required by EU legislation:
http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CELEX:32000L0007:EN:HTML

savagethegoat

GM228 wrote: »

It's not just that they overread at some stagr in tyre life, they seem to overread from day 1!

I buy a new car every two years and the GPS has always shown a slightly lower speed than the speedos from day 1!

GM228

it would need to overread at all stages really, from a very small amount as a safety cushion to a larger amount dependent on tyre wear

CelticRambler

cajonlardo wrote: »

How many of you posting a difference are using original size wheels and tyres?

Have a Renault Mégane and a Fiat Ducato (campervan). The Mégane's on original spec tyres and reads 82-ish kmh on the GPS for 90 on the speedo and about 115 when supposedly at 130 by the speedo.

The Ducato was delivered with "big" tyres because I'd asked for a weight upgrade but I never asked/checked to see if the speedo had been re-calibrated. On a long run recently, GPS was giving 100kmh for 110 on the speedo.

K.Flyer

Checking against the flashing speed warning signs around the area and our Mondeo is reading 5kph over on 60 and 50 zones.
There were no other cars around at the times I checked, but who is to say the warning signs are accurate as well.

GM228

K.Flyer wrote: »

Checking against the flashing speed warning signs around the area and our Mondeo is reading 5kph over on 60 and 50 zones.
There were no other cars around at the times I checked, but who is to say the warning signs are accurate as well.

Anytime I have passed those signs they usually show less (same as GPS) suggesting they are usually accurate!

GM228

jimgoose

According to GPS, the speedometers in both of the cars here over-read by about 6%. This appears to be consistent throughout a range of observed speeds from 40 - 100 km/h. This would be perfectly normal:

http://www.boards.ie/vbulletin/showpost.php?p=87171246&postcount=52

*Kol*

I accelerated towards two of those signs yesterday and it didn't register my speed at all on both occasions? Do they only register your sped if it is constant? I could see that they were working in the distance as other cars passed by them.

highdef

2013 Opel Astra: 50 reads as 53, 60 reads at 64, 80 reads at 85/86, 100 reads at 107/108, 120 reads at 108/109.

2015 Nissan Leaf reads at 10% across more or less all speeds.

GM228

*Kol* wrote: »

I accelerated towards two of those signs yesterday and it didn't register my speed at all on both occasions? Do they only register your sped if it is constant? I could see that they were working in the distance as other cars passed by them.

Should work for all cars irrespective of constant speed or not.

GM228

_Kaiser_

Mine seems pretty much spot on from passing those signs and GPS - A6 with 18" alloys

my3cents

GM228 wrote: »

Should work for all cars irrespective of constant speed or not.

GM228

I can't remember where but I noticed one recently that seemed to only show your speed if it was over the limit.

kirving

S40 with 17's, and 205/50's reads almost exactly correct.

edburg

highdef wrote: »

2013 Opel Astra: 50 reads as 53, 60 reads at 64, 80 reads at 85/86, 100 reads at 107/108, 120 reads at 108/109.

2015 Nissan Leaf reads at 10% across more or less all speeds.

Is that last bit right for the Astra??

I reading that as you were doing 120 (GPS) car is reading 108/109. :eek: Hoping you mean reads 128/129

colm_mcm

The funny thing is, the odometer and trip computer is reading the correct mileage. If you go at 120kmph steady on the speedo, reset trip computer, then drive at a steady 120kmph, trip computer will usually say your average speed was 110kmph or so.

edburg

colm_mcm wrote: »

The funny thing is, the odometer and trip computer is reading the correct mileage. If you go at 120kmph steady on the speedo, reset trip computer, then drive at a steady 120kmph, trip computer will usually say your average speed was 110kmph or so.

Would that not be most likely down to fact your slowing down or stopping while you reset the trip computer, on board taking in to account that time spent at lower speed.

colm_mcm

no. If you reset the trip computer while doing 120kmph, it will within a few seconds say your average is 110 or so, even if you continue at this speed, the average speed doesn't change from what it was initially.

GM228

colm_mcm wrote: »

no. If you reset the trip computer while doing 120kmph, it will within a few seconds say your average is 110 or so, even if you continue at this speed, the average speed doesn't change from what it was initially.

Most cars onboard computers base average speed on the previous 50 miles driven AFAIK and when reset are not truly an average or accurate until 50 miles have past.

GM228