Ducato speedo inaccuracies.

[Happy Jack]

Good morning all.
My Ducato 2016, 2.3 speedo reads 70 when the satnav says 65.
Running on 15" rims at the mo with 215/70/15 tyres.
Not a problem as I think I can adjust the overspeed warning.
But it would be good if it was accurate.
Added complication:-
Getting alloys, could stay with 15" and same size Falken 01, which are the tyres I fancy.
Load rating is 109/107, but speedo would still read high.
Other profiles are available but the size that puts speedo correct has poor load ratings, only just good enough.
Would like to go to 16" and 225/75/16 tyres.
Excellent load rating but speedo would read even higher!!!
Other profiles are available but again load rating goes down, although not as bad as 15" versions.
Really did want higher rating as it allows loser pressures to give a softer ride.
Axle weights in full trim are 1610Kg front, 1500Kg back
Can anyone think of any solutions?
Can the speedo be recalibrated and how ?
Have emailed 3 suppliers to for their recommendations.
Waiting for reply.
Am North East based.

 

[Spriddler]

My Ducato 2016, 2.3 speedo reads 70 when the satnav says 65.

But it would be good if it was accurate.

All OEM speedometers are required by law to under-read.

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.

 

[robnchris]

Most speedos read slower than the actual speed over ground, we never look at the speedo always go off the sat nav, the clock however is a waste of time always slow.

 

[gus-lopez]

My Ducato 2016, 2.3 speedo reads 70 when the satnav says 65.

It also depends on positioning of the sat.nav ,dash cam ,etc; I have two dash cams with gps & speed readings & a heads up display .Between the three there is a 5kph difference.

 

[Happy Jack]

All OEM speedometers are required by law to under-read.

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 50.25

Hi Spriddler, that means if my sat nav is correct then the speedo is only approx 7.5% overeading.
Will check what effect bigger wheels/tyres will have.
Adjusting doors alarm might be best option.

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[Happy Jack]

It also depends on positioning of the sat.nav ,dash cam ,etc; I have two dash cams with gps & speed readings & a heads up display .Between the three there is a 5kph difference.

HI gus-lopez.
Sat nav is slightly right of middle of dashboard, front window just 8" inches in front of it.

 

[Hettie's Crew]

I fitted 225/70 15 tyres in place of the original 215/70’s. The increased width with the same 70% profile increases the overall circumference of the tyre a little. Our speedo is nearer now, about 2-3 mph below what the sat nav says.
We now have 112 instead of the original 109 load and more comfortable lower tyre pressures.
We’re on Falkens now previously on Continentals, the original 215’s were CP tyres and rock hard.

 

[Happy Jack]

Hi Spriddler, that means if my sat nav is correct then the speedo is only approx 7.5% overeading.
Will check what effect bigger wheels/tyres will have.
Adjusting doors alarm might be best option.

Meant speedo alarm, spell corrector again !!!!!!

 

[Guigsy]

Tyre choices on 16" tend to be significantly taller overall than on 15" rims. So the circumference is bigger. So the speedo is generally more accurate on 16" rims.

 

[Happy Jack]

I fitted 225/70 15 tyres in place of the original 215/70’s. The increased width with the same 70% profile increases the overall circumference of the tyre a little. Our speedo is nearer now, about 2-3 mph below what the sat nav says.
We now have 112 instead of the original 109 load and more comfortable lower tyre pressures.
We’re on Falkens now previously on Continentals, the original 215’s were CP tyres and rock hard.

My present tyres are 108 loading.
Your set up may well be OK for me.
But, as spriddler says above, Speedos cannot underead; so your one could be illegal.
However sat nav positioning can produce slight wrong speeds as gus-lopez says above, so perhaps nowt to worry about.

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[pappajohn]

It also depends on positioning of the sat.nav ,dash cam ,etc; I have two dash cams with gps & speed readings & a heads up display .Between the three there is a 5kph difference.

HI gus-lopez.
Sat nav is slightly right of middle of dashboard, front window just 8" inches in front of it.

I don't think position will have a great effect on accuracy.....it will either see the satellites or it won't.
It will be the small variations in the electronic components which make the difference.

 

[Spriddler]

Del'd.

 

[Guigsy]

I don't think position will have a great effect on accuracy.....it will either see the satellites or it won't.
It will be the small variations in the electronic components which make the difference.

GPS positioning can jump around a bit. Better systems do filtering and use other sensors like accelerometers to reduce the jitter. And going around tighter corners, a good system will do you distance based on a curve rather than straight lines between the position readings. So at lower speeds in built up areas, GPS can have bigger errors. But on the motorway, the differences should be pretty minor.

 

[Silverman]

I changed from 215/70/15 to 225/75/16. My speedo now reads exactly the same as the gps.

 

[Hettie's Crew]

My present tyres are 108 loading.
Your set up may well be OK for me.
But, as spriddler says above, Speedos cannot underead; so your one could be illegal.
However sat nav positioning can produce slight wrong speeds as gus-lopez says above, so perhaps nowt to worry about.

Good point and me being a bit dumb, I did of course mean above not below, eg when we’re doing 60mph on the speedo
the satnav says 57/58.
Sorry about the confusion.

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[Marie-Claire]

Arent modern speedos controlled by the ecu from revs of the engine and the gear in use. If so the size of the tyres will increase or decrease your road speed but the reading on the speedo will not change

 

[Guigsy]

Arent modern speedos controlled by the ecu from revs of the engine and the gear in use. If so the size of the tyres will increase or decrease your road speed but the reading on the speedo will not change

Yes. The ECU has no idea what wheels you have fitted. And the Ducato has quite a range of approved sizes. So it ers on the side of caution. So with smaller circumference wheels, the speedo can over read by quite a bit.

 

[Howard B]

On my previous van I swapped from 15" to 16" wheels and the speedo was bang on with the satnav and waze.

 

[Happy Jack]

Good point and me being a bit dumb, I did of course mean above not below, eg when we’re doing 60mph on the speedo
the satnav says 57/58.
Sorry about the confusion.

nee bother.

 

[AJM2]

Most speedos read slower than the actual speed over ground, we never look at the speedo always go off the sat nav, the clock however is a waste of time always slow.

I disagree with that, I think most speedos will read faster than actual speed, they should never read slower

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[Happy Jack]

Had replies from a couple of suppliers.
Bigger tyres do get your speedo nearer correct if it was overeading.

 

[Happy Jack]

Tyre choices on 16" tend to be significantly taller overall than on 15" rims. So the circumference is bigger. So the speedo is generally more accurate on 16" rims.

Hi Guigsy.
just had replies from 2 suppliers confirming that the way the speedo gets it's info means that bigger tyres do increase accuracy.

 

[Happy Jack]

On my previous van I swapped from 15" to 16" wheels and the speedo was bang on with the satnav and waze.

Hi Howard B.
Much more confident now to go with 16" .

 

[Lenny HB]

On the 15“ wheels 225 section tyres improve the speedo accuracy.

Or by a new van the new digital speedo is accurrate to within 1 mph.

 

[Spriddler]

It's an intriguing look into others' thought processes as I've never even thought about it. In any case it really wouldn't concern me that although my speedo read 70 I was actually travelling at 65mph.

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[Nirvanauk]

It's an intriguing look into others' thought processes as I've never even thought about it. In any case it really wouldn't concern me that although my speedo read 70 I was actually travelling at 65mph.

Finally a bit of common sense to this thread. ,changing to bigger wheels indeed.
Thank you Spriddler.

 

[jerryz]

My 2010 Nissan Note says 70mph actual on GPS is 65mph The 2009 Fiat Ducato says 70mph its 65 on GPS
My 2022 Kawasaki Z900 says 70mph actual on GPS is 68mph so very accurate but still just under reading
My 3 Old Ducatis 23 to 36 years old with old mechanical speedos when showing 70mph actual is only 63mph
My last Harley when at 70mph the speedo was only 61 on GPS but its was a 1976 shovelhead

 

[Happy Jack]

On the 15“ wheels 225 section tyres improve the speedo accuracy.

Or by a new van the new digital speedo is accurrate to within 1 mph.

New van next, just need to win the lottery

 

[Happy Jack]

It's an intriguing look into others' thought processes as I've never even thought about it. In any case it really wouldn't concern me that although my speedo read 70 I was actually travelling at 65mph.

Never concerned me but it would be nice for it to be right, and I wouldn't be annoyed by the overdosed alarm.
Will have to get off my bum and adjust the overdosed alarm

 

[Happy Jack]

Finally a bit of common sense to this thread. ,changing to bigger wheels indeed.
Thank you Spriddler.

My van needs new wheels and since the tyres are near the limit then I'm looking at options, which include bigger wheels/tyres which will allow lower tyre pressures to improve the ride.

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