Speedometer re-calibration?
Discussion
I have two near identical cars, one a Jaguar XF-S and the other a Jaguar XFR. Both 60 plates manufactured within 3 months of each other. The XF-S has a bang-on speedometer - reading +3mph through the speed range until well into silly figures. The XFR over-reads between +3 and +9mph and differently within the speed range. All checked on Waze, with (for example) a true 70mph on one car being 73 indicated and on the other is 80.
It has been back to a dealer this week, who have checked all sorts of things like error codes, wheel diameter, wheel bearings, etc. Nothing amiss. But they say they cannot re-calibrate the speedometer.
Anyone know if this can this be done elsewhere? A local rolling road said they could not do it even though they could adjust ECU software if I want it chipped. It is weird that two cars so similar can be so different.
It has been back to a dealer this week, who have checked all sorts of things like error codes, wheel diameter, wheel bearings, etc. Nothing amiss. But they say they cannot re-calibrate the speedometer.
Anyone know if this can this be done elsewhere? A local rolling road said they could not do it even though they could adjust ECU software if I want it chipped. It is weird that two cars so similar can be so different.
there are tolerances on a large quantity of components such as wheels tyres, speed sensor where it picks up the speed be it wheel bearing or from the diff.
then there are the electrical tolerances in the voltage/current readings generated by the relevant sensors
All these add up to make it very difficult for two vehicles to be the same.
Even differing levels of tire wear can effect the speed readings
then there are the electrical tolerances in the voltage/current readings generated by the relevant sensors
All these add up to make it very difficult for two vehicles to be the same.
Even differing levels of tire wear can effect the speed readings
Buffalo said:
I have two near identical cars, one a Jaguar XF-S and the other a Jaguar XFR. Both 60 plates manufactured within 3 months of each other. The XF-S has a bang-on speedometer - reading +3mph through the speed range until well into silly figures. The XFR over-reads between +3 and +9mph and differently within the speed range. All checked on Waze, with (for example) a true 70mph on one car being 73 indicated and on the other is 80.
TLDR: I suggest it's not worth worrying about.The UK law says the speedo must not under read, and can over read by up to 10% + 6.25 mph.
It seems to me that all the figures you listed above fall within the allowed range of accuracy. There's nothing that says any two cars need to be consistent with each other as long as they're both legal. The accuracy will be affected by the tyre size and condition, and may even be slightly affected by temperature and pressure and road conditions and so on, so it can vary over time and under different conditions.
It is technically possible to intercept the signal from the gearbox to change the reported speed, and that might be necessary if you're changing the transmission gearing or wheel size, but it's not at all common - even when it's done, this would typically only be a simple scaling and not trying to make the speedo completely accurate under all possible conditions.
Buffalo said:
It has been back to a dealer this week, who have checked all sorts of things like error codes, wheel diameter, wheel bearings, etc. Nothing amiss. But they say they cannot re-calibrate the speedometer.
You say "all sorts of things" have been checked & mention wheel diameter, but did they also check tyre size/profile as a range of different sizes can be fitted to the same rims which will give vastly different speed readings.I would say tyre pressure and tyre temperature changes are having the biggest effect here, and is effecting the rolling circumference at different driving circumstances. Very little you can control (except set the most suitable pressure for general driving), so live with the variation. The speedometer will not under-read.
Both cars have Michelin tyres of the correct size, and like i say are so similar in other ways I can't see it being solely those sorts of variables being the issue. That is what led me to think either sensors (those were checked, at least visually) or the speed unit itself.
It bothers me only on 2 points. The first is the difference between gps and indicated varies depending on speed on one car and is a static difference on the other, which doesn't make a lot of sense to me. And also because dialling in 80mph on cruise to do GPS 70mph is disconcerting. The other car achieves GPS 70mph at 73mph indicated, which 'feels' more comfortable. I can live with it but if there was a means to make it more precise I would prefer to do so.
I might swap all the wheels over one day if I have nothing better to do, just for an idle curiosity.
It bothers me only on 2 points. The first is the difference between gps and indicated varies depending on speed on one car and is a static difference on the other, which doesn't make a lot of sense to me. And also because dialling in 80mph on cruise to do GPS 70mph is disconcerting. The other car achieves GPS 70mph at 73mph indicated, which 'feels' more comfortable. I can live with it but if there was a means to make it more precise I would prefer to do so.
I might swap all the wheels over one day if I have nothing better to do, just for an idle curiosity.
I too like an accurate speedometer, having driven trucks with calibrated speedos for years its annoying if my private vehicles overread by anything more than the 2 or 3% even trucks vary as tyre wear occurs.
We have plug in GPS speedos from Amazon in both cars which sit on the dash (not HUD), they're not perfect but as accurate as a typical satnav, bought originally after wifey picked up a speeding ticket and she much prefers the speed reading being on rather than in the dash display.
Food for thought?
We have plug in GPS speedos from Amazon in both cars which sit on the dash (not HUD), they're not perfect but as accurate as a typical satnav, bought originally after wifey picked up a speeding ticket and she much prefers the speed reading being on rather than in the dash display.
Food for thought?
Not everyone's cup-of-tea, but I have used both of these on various cars for years now.
https://www.ebay.co.uk/itm/124208202939?_skw=speed...
https://www.ebay.co.uk/itm/176133573251?_skw=autoo...
Both are made by Autool & the data they display is configurable & the speedo is also calibratable in use, so you can get it to match GPS quite easily.
They work extremely well & put your speed etc. exactly where you want it.
https://www.ebay.co.uk/itm/124208202939?_skw=speed...
https://www.ebay.co.uk/itm/176133573251?_skw=autoo...
Both are made by Autool & the data they display is configurable & the speedo is also calibratable in use, so you can get it to match GPS quite easily.
They work extremely well & put your speed etc. exactly where you want it.
.. and amazing that some people do not know the difference between a wheel and a tyre. Between old and new, tyres will lose about 1.7% in rolling circumference as a nominal reading, and may differ by 2% between front and rear on staggered wheel/tyre combinations and old and new. Some cars can be adjusted to fine tune the speedo reading, often by a hidden menu. Not really worth it in my mind.
Pica-Pica said:
.. and amazing that some people do not know the difference between a wheel and a tyre. Between old and new, tyres will lose about 1.7% in rolling circumference as a nominal reading, and may differ by 2% between front and rear on staggered wheel/tyre combinations and old and new. Some cars can be adjusted to fine tune the speedo reading, often by a hidden menu. Not really worth it in my mind.
Could be as much as 3% on wear alone - difference between new and old.. depends on wheel size, smaller wheel...higher %.Richard-D said:
Are you saying a reading of 80 at a speed of 70 is within spec?
Yes. At 70 mph the speedo must read no less than 70 mph and no more than 83 .25 mph, to be legal in the UK.In my experience it is very common for speedos to read about 10% high and extremely rare for them to read exactly accurately. If they read accurately, the car would be on the verge of being illegal, and I don't imagine manufacturers would want to risk that. I expect they would aim to be roughly in the middle of the legal range most of the time. 80 mph is slightly above that but only by 3mph, and that's not enough to imply there's anything wrong.
GreenV8S said:
Richard-D said:
Are you saying a reading of 80 at a speed of 70 is within spec?
Yes. At 70 mph the speedo must read no less than 70 mph and no more than 83 .25 mph, to be legal in the UK.In my experience it is very common for speedos to read about 10% high and extremely rare for them to read exactly accurately. If they read accurately, the car would be on the verge of being illegal, and I don't imagine manufacturers would want to risk that. I expect they would aim to be roughly in the middle of the legal range most of the time. 80 mph is slightly above that but only by 3mph, and that's not enough to imply there's anything wrong.
Pica-Pica said:
Minus 0 to +10% + 4kph is the UNECE regulation 39. So at 70 the speedo could read 70 to 79.5mph, however modern cars are usually much closer than that.
The UK law is based on the EU standard but does not match it exactly. The UK law allows the speedo to read up to 10% + 6.25 mph over the actual speed. See Motor Vehicles (Approval) Regulations 2001 Schedule 3 if you want the gory details.It would be a non-issue if you had one car which was inaccurate (but within the guidelines/legislation) but having two so similar cars, yet with the 9% difference, is super annoying.
I am sure the internal design of the instrument cluster is such that it receives raw data in the form of pulses from the speed sensor(s) then converts this using a factory-configurable parameter into a derived speed, then displays this using the mechanical needle (and digital readout on the trip computer if you wish too). The electronics module and software will be similar for all XFs and may be shared with other cars in that era too eg LR, other Jags. I don't know if ALL XFRs overread or that you just have a bad one.
By sheer chance I have a Jag XF too, and had it plugged into the diagnostic scanner this morning (I was looking for a date/version of the nav map), but since mine is the plebeian 2.2 diesel, I can't help much further.
I am sure the internal design of the instrument cluster is such that it receives raw data in the form of pulses from the speed sensor(s) then converts this using a factory-configurable parameter into a derived speed, then displays this using the mechanical needle (and digital readout on the trip computer if you wish too). The electronics module and software will be similar for all XFs and may be shared with other cars in that era too eg LR, other Jags. I don't know if ALL XFRs overread or that you just have a bad one.
By sheer chance I have a Jag XF too, and had it plugged into the diagnostic scanner this morning (I was looking for a date/version of the nav map), but since mine is the plebeian 2.2 diesel, I can't help much further.
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