Speedo Accuracy

[Brains]

With most vans having a GPS unit as standard now, it's very easy to compare the GPS speed with the Speedo speed.
On my 2018 Hymer on a Mercedes Sprinter base, with the Hymer fitted tyres pumped up to the pressure listed in the manual my speedo is 4.5 mph fast.

I've set the GPS to KMPH and then tried it on the motorway between the KM posts, so I'm fairly sure the GPS is accurate to within 1kmph (less them 1mph)

What this means in practice is keeping to say a 30mph limit means I'm doing 25mph, which could be annoyingly slow to the vehicles behind me in urban areas. It also means that to cruise with the freight on the motorway who are doing 56mph, I have to set the 56mph cruise control at 61mph, and then get told I'm speeding by the speedo (but not the GPS).

On the plus side for every 1,000 miles I travel the milometer will only display 950 miles.

Can this be fixed easily ? Is there an adjustment screw somewhere ?

 

[Phill D]

It's deliberately set low in my opinion.
Manufacturers don't want to be in a law suit because they are reading slow and we all getting speeding fines.
No adjustments as far as I know

 

[DBK]

The amount they over-read tends to increase with speed. So if it was 4.5mph fast at 60 it might be only 2 mph fast at 30 mph. Did you do the test at different speeds? If you find a quiet straight road just compare it to the gps speed, which should be more or less spot on.

 

[chatteris]

fit some higher aspect ratio tyres or slightly wider same aspect ratio when you need new tyres

 

[TinaLes]

Fiat Vans:
Listened to a guy that has had 3 vans, fist two ran on 15" wheels and the speedo was 3.5/4 MPH faster than his GPS.
He switched wheels to 16" with higher profile tyres 75mm instead of 70mm, and he reckons its spot on now.
I can only speak from my experience in that running on 16" rims with 75mm Camper Tyres, my speedo is still 3mph faster than my GPS.
Would have though slightly better fuel economy maybe on 16" rims, whether noticeable or not who knows.
LES

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[suavecarve]

With most vans having a GPS unit as standard now, it's very easy to compare the GPS speed with the Speedo speed.
On my 2018 Hymer on a Mercedes Sprinter base, with the Hymer fitted tyres pumped up to the pressure listed in the manual my speedo is 4.5 mph fast.

I've set the GPS to KMPH and then tried it on the motorway between the KM posts, so I'm fairly sure the GPS is accurate to within 1kmph (less them 1mph)

What this means in practice is keeping to say a 30mph limit means I'm doing 25mph, which could be annoyingly slow to the vehicles behind me in urban areas. It also means that to cruise with the freight on the motorway who are doing 56mph, I have to set the 56mph cruise control at 61mph, and then get told I'm speeding by the speedo (but not the GPS).

On the plus side for every 1,000 miles I travel the milometer will only display 950 miles.

Can this be fixed easily ? Is there an adjustment screw somewhere ?

Your MPG is also now worst !

 

[Phill D]

I must admit I take little notice of the speedo, I almost always use the sat nav GPS.
Checked against the radar speed checkers it seems spot on.

 

[gus-lopez]

I run 3 gps 's & rarely do they all show the same speed. I usually set my cruise through road works at the lowest of them & am still bowling past people.

 

[Steve N Tracy]

The speedo's on most vehicles are designed to read about 10% below true road speed, this is mostly to allow for slightly diffirent rolling circumference sizes amongst diffirent manufacturers even for the same size tyres.

 

[peterc10]

The law gives a tolerance for speedos which requires that can read up to 10% too fast, but must not read too slow. Therefore, to cater for the different wheel and tyre combinations, they are are usually set to read fast in most circumstances. A GPS reading will be much more accurate, although they are at their most accurate when you are driving on a straight level road and are maintaining a steady speed.

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[Marmite]

Used to be UK standard that speedometers were permitted to read 3mph +10% over, but nothing under. Not sure if this still applies, but it reads about right for all my vehicles

 

[Despicable]

Having driven numerous HGV-1 units, particularly Mercedes, up to 2016, I found the trucks' digital speedos matched my Garmin sat nav; I therefore drive to that and not the vehicle's speedo. When my Peugeot Boxer based motorhome shows 60mph, the sat nav shows 56mph; my preferred cruising speed.

 

[Nasher]

A vehicle speedometer, which legally can read up to 10% faster - but must not read slower than actual speed, invariably takes it's reading from the rotations of the wheel

It will never be 100% accurate as it will have to take into account that the circumference of a tyre will change, not just with the tyre wearing, but also expansion of the tyre when rotating faster

 

[Marmite]

The UK law is based on the EU standard, with some minor changes. A speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph.

So if your true speed is 40mph, your speedo could legally be reading up to 50.25mph but never less than 40mph. Or to put it another way, if your speedo is reading 50mph, you won’t be doing more than 50mph but it’s possible you might actually only be travelling at 40mph.

To ensure that they comply with the law and make sure that their speedometers are never showing less than true speed under any foreseeable circumstances, car manufacturers will normally deliberately calibrate their speedos to read ‘high’ by a certain amount. As your satnav is not the designated device by which a car’s speed is measured, it does not need to incorporate any fudge factoring.

 

[Lenny HB]

Under the construction & use regs speedo tolerance is minus 0, plus 10% + 10kph(6.2mph) for the life of the vehicle.

Van speeds always appear worse than cars maybe because of the ratio tyres with deeper tread as they have to allow for ware.

Not uncommon for a Ducato speedo to read 68/69 when you are doing 60.

I've also noticed the speed reading is higher on a mph dial than on a kph dial.

Because of the poor visibility of the speedo on the x290 I fitted a GPS speedo, it has got an HUD but can't use that on my A Class. Just have it on top of the dash behind the Instument cluster.

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[kevenh]

My previous car, a 65 plate Mazda 3, had an accurate speedo.
It was quite a surprise as I assume if the Speedo says I’m doing 70 I can eek out another mph or 2.
I wonder if the speed displayed was driven by the onboard/built in Sat Nav and only the odometer is driven by a direct link to the wheels.

I run 3 gps 's & rarely do they all show the same speed. I usually set my cruise through road works at the lowest of them & am still bowling past people.

I use GPSr speeds to set the limit or cruise for road works. You do need to trust it though.
Like you say, you’re often quicker than the other drivers.
Tbf, they’re doing 50 according to their display - maybe only 47mp actual.

 

[Steve N Tracy]

My previous car, a 65 plate Mazda 3, had an accurate speedo.
It was quite a surprise as I assume if the Speedo says I’m doing 70 I can eek out another mph or 2.
I wonder if the speed displayed was driven by the onboard/built in Sat Nav and only the odometer is driven by a direct link to the wheels.

I use GPSr speeds to set the limit or cruise for road works. You do need to trust it though.
Like you say, you’re often quicker than the other drivers.
Tbf, they’re doing 50 according to their display - maybe only 47mp actual.

The speedo and the odometer use the same input signal.

 

[Lenny HB]

The speedo and the odometer use the same input signal.

The odometer is a direct digital readout, the speedo is an electric meter with the digital signal converted to an analog voltage to drive the meter coil. If you run some tests you will be surprised how much more accurate the odometer is than the speedo.

 

[Steve N Tracy]

The odometer is a direct digital readout, the speedo is an electric meter with the digital signal converted to an analog voltage to drive the meter coil. If you run some tests you will be surprised how much more accurate the odometer is than the speedo.

True having been involved with the design manufacture and fitting of kmh to mph converters for a number of years I have found that It will depend on the type of odo and speedo type, digital types will be more accurate the older analog needle type.