This sort of discrepancy is pretty common and still within Legal Guidelines

 

How are you measuring the speed comparison?

 

By setting the cruise control momentarily which gives a digital readout that seems correct, judging by the other traffic.

Whilst you probably won't pick up a speeding ticket, I think your speedo could be out of calibration.

It certainly won't get me a speeding ticket, I'd be doing 20 in a 30! I'd just like to have a better idea of my actual speed.

How would I calibrate the speedo...?

 

Just for info - ACPO Guidelines on speeding and the UK law regarding prosecution are as follows:

"Vehicle construction and use regulations require a vehicle speedometer accuracy to be in the range of -0->+10%. The implications are that it must never under-read - for obvious reasons - but may over-read. As the cost of manufacturing a speedometer with -0% error would be very costly they all over-read by a few percent without exception. Even if speed is measured correctly the display may not be accurate, so a speedometer error is allowed. Because of this, the Association of Chief Police Officers (ACPO) have an official formula for calculating a speeding offence. It allows a leeway of 10% plus 2mph. In reality, most speed traps are triggered at higher speeds than this because if they were set bang-on those guidelines, the sheer amount of paperwork generated would overrun the police speeding departments."

Whilst you probably won't pick up a speeding ticket, I think your speedo could be out of calibration.

 

Just as an aside have you had any problems with the wheel sensors ? Some manufacturers rely on pulses coming from the ABS Sensors
which are converted by the ECU in order to display a cars speed electronically

 

I didn't know it had any. How would I know if they had problems? The brakes work fine though I haven't had to test the ABS yet.

 

Well I have been attempting to get a hold of you on another thread you started about your car in order to get more details as to where the car came from
and what sort of warranty you have if any . From memory of the other thread I understood you had been having some issues ?
These cars have a tendency now to turn up in all manner of Garages and in all sorts of conditions , the trouble is that these Garages often don't have the
Specialist Knowledge needed to put any problems right , pretty soon your warranty is up and your left holding the baby

 

Chiltern Jaguar have been mentioned before on this site; they aren't main dealers but anything 11 years old isn't going to be in that network anyway. They're not perfect but no worse than any other indy I've dealt with. They told me the car came from a man who had to give up driving due to dementia and his family had to dispose of it.

It had a faulty starter motor but that now seems to be fixed. It's generally in very good condition and 68K miles, much lower than my S-Type diesel (also 2006) which had none of these oddities. If I need to be worried about Chiltern, or anything else, please PM me!

 

My experience of Chiltern is that they are fair. I've bought twice from them and so far so good. They generally give a 6 month warranty, so if you have an issue why not contact them and give them the chance to resolve it. Be interested to know how you get on.

 

Good advice which is why I am attempting to find out Simpos position regarding his car and where it came from , it wants to be taken
back to the supplying Dealer for any faults occurring within the Warranty Period , its only in the last Post that the OP has confirmed from
where he purchased the car . Chiltern from my point of view must be doing something right to have been in Business selling second hand
Jaguars as long as they have , many on here speak highly of their Servicing Department , my only negative is that I have found that some
of the stock can be a bit rough and to me that gives an indication of the type of person the previous owner was . I should add I am talking
about in the main X350 and X358 models though a near neighbour purchased an X150 convertible in Silver which was very rough , my advice
as always is inspect carefully especially bodywork and this advice is by no means restricted to Chiltern .

 

By setting the cruise control momentarily which gives a digital readout that seems correct, judging by the other traffic.

It certainly won't get me a speeding ticket, I'd be doing 20 in a 30! I'd just like to have a better idea of my actual speed.

How would I calibrate the speedo...?

Not sure how the speedo can be re-calibrated. I do know that if you change a complete speedo binnacle you need to use the Jaguar SDD software to activate it to the vehicle. There may be some adjustment in that process. Normally this can only be done by Main Dealer because of the ability to change the odometer mileage.
My first action would be a hard reset - disconnect both battery main connectors and then touch them together - this clears any module memory which may be corrupt. If you do this - make sure that BOTH main connectors are completely dis-connected from the battery. You will then probably have to reset the electric windows, electronic park brake etc - these are all simple processes covered by the Owners Manual.

Having said all of that - as you have just purchased the car, I would go back to the seller to see if they will sort it for you FOC.

 

He could I suppose speak to some of the Mileage Correction Boys though if the vehicle is still in Warranty then back to the Supplier
you must go

 

a couple of things here....

at best, your speedo won't be accurate. Put simply, its not legal to sell a car with an accurate speedo. Manufacturers must get their cars type-approved and there's a formula for speedo reading vs. actual speed. Whilst the formula could allow you to be accurate at one end of its tolerance band, this doesn't take account of worn tyres or tyres of a differrent make. So manufacturers stick usually to the middle of the band, which means about 12% over-reading.

so, from above - your 20% could be possible with a non-faulty speedo

your actual speed reading is taken from the rear wheel speed sensors and interpreted by the ABS computer, whereby it is corroborated by the front sensors and further corroborated by the gearbox output speed sensor, before being squirted around the car on CANbus. In my opinion, it is very unlikely for the speedo to be faulty or poorly calibrated - there isn't a calibration procedure.

use GPS. This is accurate to just about 100% - it can't be anything different. Cruise control also relies on CANbus, but isn't even required to be any kind of accurate.

 

10mph difference at around the 20 or 30mph (indicated) mark is ridiculous, that cannot be considered acceptable. That said I'd want to measure the actual vs. indicated speed accurately with a GPS device. The car only has one source of road speed and that's the sensors in the wheels; the cruise control works off that, as does the speedo.

All that said, however, it could be a fault in the speedometer in the instrument panel so if a GPS device shows, say, 28mph while cruise is set to 30mph but the speedo reading is significantly different then there you have it.

@Simpo, I'd get yourself a GPS that shows speed, or better yet get a Bluetooth OBD-II adapter and something like Torque Pro (Android only) on your phone. That can show you the OBD speed data vs. GPS and will also let you read common fault codes etc. Once you can prove there is an issue get back to the dealer and make sure they do what's needed to put it right.

 

Thanks everyone for all the replies and ideas. Taking it back to the dealer is not quite the simple task you might imagine as it's a 2.5 hour drive away - and when the starter motor was dodgy they insisted I go back in the AA truck so I had a courtesy car (so they didn't have to deliver it back). So unless they can fix it while I wait, it's a two day event invariaby involving the M25 in rush hour.

On the plus side, the speedo is performing perfectly today! I have no idea what has changed to do this, but possibly related to battery disconnection and it takes a couple of weeks to shake out...? The wonders of too many electronics in a car. Equally today it started first time and the hood worked, so hopefully the gremlin has gone. And the problem with an intermittent fault of course is that you can take it back to the dealer and they can't find it.

I'll wait until the warranty has perhaps a month to go and see how the car is behaving then. In other words, let any other the issues appear first.

Thanks again

 

On the plus side, the speedo is performing perfectly today! I have no idea what has changed to do this, but possibly related to battery disconnection and it takes a couple of weeks to shake out...? The wonders of too many electronics in a car.

Thanks again

Hence my suggestion to carry out a hard reset - XK/XKRs are notorious for electrical problems associated with low battery voltage. This can cause some real wacky problems just like you have experienced. Make sure you have a good battery, carry out a hard reset and you are quite likely to see most problems disappear.

 

Thanks jhdee, I'll certainly have this top of the list if it plays up again.

When the car was in the local garage (the first time there were starting issues) they checked and recharged the battery and said they'd put it on some sort of reconditioning device which is supposed to clear out any clag etc. But it's a 'Planet' brand which I'd never heard of, so we shall see...

 

You want to watch yourself with those Local Garages 1) That they are competent 2) They don't invalidate your warranty

 

Fear not, it was taken there with the assent of the dealer, due to the latter being so far away. They removed and tested the starter motor but found no fault, only a blown fuse. Had the starter motor misbehaved then, the subsequent 2-day exercise a fortnight later after it broke properly wouldn't have been necessary!

 

I take it your round Dartford somewhere ? As long as your on the ball Simpo , theres nothing worse than seeing a member shafted
because he didn't have the foresight to ask on the Forum . Fingers crossed your problems are at an end , all the best mate

 

Essex/Suffolk border, but yes, whilst not a mechanic I learn fast... Thanks for your input and let's hope the teething problems are over

 

Just for interest - when a vehicle is subjected to a BIVA test, the formula used for speedo accuracy is as follows:

true speed + 6.25mph
---------
10

The wording of the BIVA is as follows :

1. The speedometer must not indicate a speed less than the true speed.
2. The speedometer must not indicate a speed that is in excess of the true
   speed plus the permitted tolerance in the chart below. (see Note 3.)
   
   

	Indicated Speed (mph)			Min True Speed (mph)
35 40 45 50 55 60 65 70	26 30 35 39 44 48 53 57

So it looks like your speedo would have satisfied the BIVA - the BIVA is Basic Individual Vehicle Approval - a set of rules which must be satisfied for a modified vehicle to be approved for road use. Its used for kit cars, hot rods etc. Can't confirm but, believe in this particular case, the rules are the same for mass produced vehicles.

 

If I should end up in court you shall be my Expert Witness sir

It is quite remarkable, though, that in these days of super-high-tech, the reading of a thing as important as a speedometer is basically irrelevant!

 

Always remember the definition for EXPERT --

Expert is made up of 2 words viz:

1 Ex - as in former or has been

and

2 Spurt - as in "drip under pressure " :-D:-D:-D

 

Ah but then again, in the land of the blind the man with one eye is king

 

the "production car" formula is indeed the same, but for a type-approval, you have "DFs" to take account of.

that's "deteriation factor" for the uninitiated - so you need to have done the test with new and shaved tyres, for instance

the IVA test is just a pared-down type-approval - we take out some of the less vital parts for single approvals and very small series