Boxer light chassis tyre sizes, speedo error

This is interesting. I did some testing today to work out how accurate the speedometer is in my Symbol. The answer is: not very. In fact at an actual 60 mph, the speedometer reads 65 mph. This is not from glancing at the instrument, as that would be open to interpretation, but instead using the cruise control digital readout, set appropriately to achieve an actual 60 mph.

This is an over-read of 8.33%. Now, the standard wheel and tyre size for the light chassis is 215/70R15. But the wheels and tyres I have been considering are 225/75R16 and these would adjust the speedometer: by 8.32% in the opposite direction. In other words, they would make the speedometer accurate.

I did wonder how Peugeot accommodated the two wheel/tyre sizes on the light and maxi chassis. Well, it seems they don’t bother. The speedometer is geared for the more common maxi chassis, so it’s way out on the smaller wheels.

These BFGs are looking more and more likely to be fitted

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My 225 15" tyres give an almost exact speedo reading , perhaps over-reading by only 1 or 2 mph. I can't compare how the original 215 tyres read as the 225 were already on when I bought it. Since owning a sat nav (probably over 15 years) and owned at least 10 cars in that time, almost all have over-read by around 7 or 8%. I did have one car that was subject to a recall because the speedo was under-reading and could  have resulted in speeding fines . The manufacturer offered to pay any fines but they wouldn't  have got the points on  license removed! Luckily I didn't get any.

I don't think it's an urban myth that speedometers are deliberately set to over estimate your speed by about 10%.

This is so that if you get done for speeding you haven't got a leg to stand on. Mr Plod will know at once that you are telling porkies if you insist you were doing (say) exactly 70mph, because your speedo would have been showing 77 or 78 mph!

Hi

v8oholic wrote:This is interesting. I did some testing today to work out how accurate the speedometer is in my Symbol. The answer is: not very. In fact at an actual 60 mph, the speedometer reads 65 mph.

Gromit wrote:I don't think it's an urban myth that speedometers are deliberately set to over estimate your speed by about 10%.

Humm don’t know about that but it’s not a new thing. My pre-historic vehicle’s speedo gives exactly the same error (reads 65mph at 60mph) according to the sat nav.

Brod

For context, I have four vehicles and the largest error in the other three is 1 mph over at 60. One of them is bang on. The law stipulates that speedometers must read between 100% and 110% of the correct value at 30 mph. That does not mean they all read about 10% over. That is an urban myth. With today’s digital speed sensors and electronic speedometers there is no technical reason why a speedometer cannot be 100% accurate.

v8oholic wrote:With today’s digital speed sensors and electronic speedometers there is no technical reason why a speedometer cannot be 100% accurate.

The main reason is that the vehicle's management system has no way of knowing how worn your tyres are - that alone can make a difference of several percent. They cannot rely on external methods such as GPS, because there are situations such as tunnels and very heavy rain-soaked tree cover where GPS can't reach. And GPS cannot easily account for actual roadspeed on a slope. Hence they still have to rely on a system that (in effect) counts the number of wheel rotations, and works speed out from there!

When I was a lad doing my mechanic apprenticeship (early 80's) I clearly remember this coming up in a lesson and yes, speedos are calibrated to read higher than the actual speed. I mentioned this in a speed awareness course not so long ago and it was reluctantly confirmed by the course tutor too, and was a surprise to most people on the course.

Roopert wrote:

v8oholic wrote:With today’s digital speed sensors and electronic speedometers there is no technical reason why a speedometer cannot be 100% accurate.

The main reason is that the vehicle's management system has no way of knowing how worn your tyres are - that alone can make a difference of several percent. They cannot rely on external methods such as GPS, because there are situations such as tunnels and very heavy rain-soaked tree cover where GPS can't reach. And GPS cannot easily account for actual roadspeed on a slope. Hence they still have to rely on a system that (in effect) counts the number of wheel rotations, and works speed out from there!

Several percent due to tyre wear is another myth, thanks for the reminder. The diameter of the new tyre is 744 mm. Deduct 5mm x 2 from that to allow for tread reduction, you will find it is a difference in diameter (and therefore circumference) of only 1.3%. An incorrect tyre pressure could perhaps make a small difference too.

This is the regulation in force. It isn’t quite the case that it’s allowed to overread by 10%, it’s actually worse! This complicated formula:

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Is basically just saying it can’t underread, but can overread by up to 10% + 4 kph (2.5 mph). So at 60 mph, a reading of 68.5 mph would be acceptable. 

Another thing I noticed. You know those gear shift lights that everyone ignores, which all manual vehicles have now? Another pointless regulation.

v8oholic wrote:
Several percent due to tyre wear is another myth, thanks for the reminder. The diameter of the new tyre is 744 mm. Deduct 5mm x 2 from that to allow for tread reduction, you will find it is a difference in diameter (and therefore circumference) of only 1.3%. An incorrect tyre pressure could perhaps make a small difference too.

So... it's not a myth then, is it? Deep treaded tyres can have 10mm of tread depth, so that gives 20mm of total wear, on a typical diameter 680mm, gives a difference of almost exactly 3% - but you say that's not "several" percent?

Another very common factor is people fitting aftermarket wheels and tyres. Because people tend to go for bigger wheels with a larger rolling radius, it's not uncommon for this to amount to several percent too. IIRC even the two "standard" wheel/tyre combinations that VW fit at the factory on my van have almost 2% difference between them.

There are genuine reasons why they allow speedos to under-read from the factory, though I suspect that the original reason was that when the legislation was originally made, mechanical speedos were likely to be one of the biggest causes of inaccuracy themselves...

Perhaps we will have to agree to disagree about the meaning of “several”. (10 - 1.6) x 2 = 16.8 mm difference in diameter, still only 2.2%. I am basing this on my own van tyres fitted by Peugeot which start with 10mm tread, or 8.4 mm legally usable. Most passenger car tyres start with less, typically 7-8 mm. Some high performance tyres only start with 5 mm (Michelin Pilot Cup 2).

IanH wrote:surely GPS should be the way to go?

Much as I like GPS, and I have a separate GPS-based readout that's independent of the speedo in my own van, it does have some serious limitations.

One, as already mentioned, is the problem with inaccuracy on slopes. The problem there, as I understand it, is that altitude is measured at lower accuracy within the original GPS system, and most GPS receivers don't account for altitude change anyway when calculating speed.

Another is inaccuracy caused by interpolation. There is a limit to how frequently your position can be updated, and the receiver therefore has to "join the dots" to be able to work out speed. This can introduce inaccuracies - especially on bends, where a sequence of straight lines does not give exactly the correct result.

And, of course, there are places where there is no GPS reception (or where GPS reception is deliberately interfered with - it's easy to do). So in that case, the system would have to fall back to some other method of measuring speed.

Are we just trying to make excuses for exceeding the speed limit?

GPS in conjunction with an accelerometer would be effective in almost all scenarios. But you’ve got to remember that the current legislation was drafted before the iPhone was launched, and the people involved had no knowledge that the technology was just around the corner.

v8oholic wrote:GPS in conjunction with an accelerometer would be effective in almost all scenarios.

I find it hard to imagine how GPS with an accelerometer could cope with a long tunnel? And possibly the biggest problem with the security of GPS - and each of the satellite based successors - is that the signal is really weak. It is almost trivially easy to overwhelm or to spoof, over a relatively big area.

It does work surprisingly well even in long tunnels, as I have discovered in the Alps. Perhaps it could be the primary mechanism, but fall back to magnetic pulse sensors as a secondary in the event of disruption or loss of satellite lock. It's all about to become more of an issue, as all new cars from next year will have a speed limiter. Vehicles will be limiting their speed to each different limit, without accurately knowing how fast they are travelling anyway. Expect a lot of vehicles thinking they are doing 70 mph on the motorway when they are really only doing 62 mph.

IanH wrote:

As to Paul's post, the reason we go to these lengths is to avoid breaking the speed limit. We set CC at 2 or 3 KPH below the local speed limit (displayed on our sat nav) and leave the computers to get on with it!

That was my point. Does it really matter if we are doing 66 in a 70 limit or 27 in a 30. Why is it so crucial to know you're  doing 70 in a 70. As long as we are within the limit, who  cares? If changing wheel sizes takes you too close to limits then don't change size. Plus when you sell are you going to tell the unsuspecting buyer that the van has non-standard size wheels?

Paulmold wrote:

IanH wrote:

As to Paul's post, the reason we go to these lengths is to avoid breaking the speed limit. We set CC at 2 or 3 KPH below the local speed limit (displayed on our sat nav) and leave the computers to get on with it!

That was my point. Does it really matter if we are doing 66 in a 70 limit or 27 in a 30. Why is it so crucial to know you're  doing 70 in a 70. As long as we are within the limit, who  cares? If changing wheel sizes takes you too close to limits then don't change size. Plus when you sell are you going to tell the unsuspecting buyer that the van has non-standard size wheels?

Because travelling long distances can be boring enough without driving artificially slowly in blissful ignorance. Or because the new wheel size enables much better suited tyres to be fitted, and what’s important is that the speedometer does not under-read. I would put the standard wheels back on to sell it anyway ‍� None of this has anything to do with breaking speed limits. In fact I was being diligent to avoid that possibility. I’m not sure why you even thought that was relevant.