Speed Differences

[Spriddler]

Ah, but if you're descending (or ascending) a long hill your speedo will measure the exact distance travelled over the ground but won't the satnav show only the forward/horizontal distance that the vehicle has achieved, thus under-calculating the vehicle's speed?

 

[Captain Yoghurtpot]

Wish my van could go that fast that I need to worry about speeding!

50 mph flat out, downhill with a tailwind.

 

[gus-lopez]

So by your own post it’s inaccurate.

Yes, I've never said anything to the effect that they are in any way accurate?

I was meticulous/curious/sad enough to actually do this, timing and counting the motorway distance marker posts while cruise was deployed at several different speeds, on an otherwise tedious journey some time ago.

I do it all the time.

 

[Widge]

I use a speedo app on an old android phone because trying to read the tiny red Kph on the ducato dial is almost impossible and I can set the digital app display to mph or kph, whenever I've passed a illuminating speed sign they've agreed spot on, while the vehicle speedo has been showing high between 3mph at 30 to as far out as 7mph at motorway speeds.

I live in france and have a UK Rav4, same problem with speedo lettering, also my wife uses hand controls, so the steering knob hides half the speedo anyway, so i installed one of these.

Universal 3.5’’ Car HUD Head Up Display Multi-Colour WindScreen Projector | eBay

The car battery. 1x Reflection Film. Type: Head up display. 1x Non-slip Mat. 1x OBD II Head up display. 3.Over-speeding alarm: Alerts you if you exceed the speed limit by setting over-speed buzzing alarm.

www.ebay.co.uk

Not this exact one but very similar, you can display various bits of info, and swap between mph/kms.

Apart from very bright low sun, it works brilliantly.

But you do need a compatable obdii port for it to plug into.

 

[Widge]

Ah, but if you're descending (or ascending) a long hill your speedo will measure the exact distance travelled over the ground but won't the satnav show only the forward/horizontal distance that the vehicle has achieved, thus under-calculating the vehicle's speed?

Satnavs work in three dimensions, so it ought to be correct, but i am not going to do a Thelma and Louise to try it out.

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[Widge]

Wish my van could go that fast that I need to worry about speeding!

50 mph flat out, downhill with a tailwind.

Sounds like my old 1981 transit

 

[Oldrat]

Sorry Gus, I can’t be ar5ed. Pasting multiple posts to show the opposite is proving too technical. But hey, life is too short.

 

[xad388]

My speedo reads faster than satnav speed - e.g. my speedo indicates 60mph but the satnav shows 52mph.
For a given distance 15" tyres have to revolve more than 16" and I wonder if the original base vehicle had 16" tyres which have subsequently been changed to 15".
Can the ducato speedo (one of the ECU's) be re-programmed to reflect current tyre size?

 

[G-RMPS]

Satnavs do work in three dimensions, however the 3D geography of the satellites means that the horizontal components of any track on the earth are likely to be more accurate than vertical. To all intents it doesn’t matter in a road vehicle especially if multiple satellites are visible.

Planes also need to consider the vertical dimension but are not so constrained by buildings and trees

 

[Widge]

The last sentence in the 'Source' that you quote states that both speedometer and odometer can be inaccurate, which is logical in that both are reading information from the circumference of the tyres.

But the key word is can be, the manufacturer will aim to get the system as accurate as it can, but then adjust the speed reading to display high to insure that all the allowed for tolerance ends up under the indicated speed, but that doesn't necessarily mean that the odometer is incorrect.

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[Spriddler]

They are using different technologies so it's inevitable there will be anomalies. I only use my speedo for speed so as long as it doesn't under-read I'm satisfied with it.