This is an easy fix. Speed should be based on the GPS system. Problem solved.

 

I think you have missed the point. Clearly the built in GPS navigation could know the speed but it is clearly not asked for it by the car. The car knows, that is, assumes that it knows, the speed from either the rotation rate of one wheel or an average of all 4 wheels, it is unclear which. The GPS navigation speed on the pop up display is the speed on the dashboard and over reads. It is not independent.

it would be a trivial piece of software to calibrate the wheel speed correction constant from the GPS so that changing tire sizes quickly made no indicated error. Apparently that is not done.

if I can find a willing dealer, and pay money, they can adjust the wheel rev/mile constant so that the car reads true for the current tires. The odd thing is that my constant is 777 rev/mile when it should be 753 or so. However, that discrepancy is in the wrong direction and too large. All I can assume is there is a hidden fiddle factor in the management computer for speed. Therefore, I cannot easily just tell the dealer a new figure, instead I would need to try a number, drive the car to see what happens and then try a new one until reality and displayed were correct.

In the old days I could probably get the technician and laptop to sit in the car as we drive and do things real time. I suspect that COVID protocols forbid this now. Thus the cost of the fix just went up. Just accept it and shut up seems to be the standard protocol.

Alternatively, I could duct tape a $30 GPS receiver to the dash and get an accurate speed.

Philip

 

My point wasn't that it was not available within the cars system. My point was that it was not being used. Make them independent. Use the GPS for the cars speedometer and the tire rotation for the odometer.

 

If you know of a way of doing that I would be very appreciative. Is there a menu or option in the sat nav for this?

philip

 

Mine reads 2 mph slow at 40. Go figure, but can't use it as an excuse when doing 20 over.

 

Just to close this out, at considerable expense, AM have confirmed that my speedometer is within specification for this model, drive train and tire size (stock). They will not create a new software flash to recalibrate the reading for ‘insurance’ reasons.

This is as good an admission as you will get that the speedometer is designed to over read by about 5% on this 2015 model. I obviously cannot comment on other years or models. I guess there is little downside to making it read incorrectly, lower mileage, more frequent services, an owner who thinks their car is faster than it is and no chance of being sued for any speeding fines or citations issued.

Oh well.

 

On The Porsche list this was discussed at length. (3-5% MPH over)
When the area of increase mileage was brought up the general consensus was that even through the speedometer was designed to read faster than the actual speed of the vehicle, the odometer was correct.

Not sure I believe that :-)

 

2009 Vantage 255/35R19 front and 305/30R19 rear and my speedo is showing the exact same as my GPS and the electronic speed on the street :-)

 

There is a "speed offset" on the VH-platform cars, but it depends entirely on what sales market the car was spec'd for. Here in the United Kingdom and also in mainland Europe, there has to be an offset by law. On cars built for the United States of America, there is no speed offset. The DIM (Driver Information Module) a.k.a. the instrument cluster, gets its speed messages from the EBS (ABS/DSC) module, and these are sent to the DIM via the CANbus network. If the car requires a speed offset due to local law, the DIM software then does the appropriate calculations and adjusts the speed shown to the driver. This is one of the reasons why different countries require different software in the DIM. I can explain the offsets if anyone is interested.

 

Sorry to say but all my cars bought in North America have speed offset for sure and that include my 2009 Aston Martin and my current 2021 Audi A4 S-Line

 

Hang on, haven't you contradicted yourself? Either the speedometer shows the same speed as GPS and "the electronic speed on the street" or it has an offset. Surely common sense would dictate that it can't be both the true speed and have an offset.

Just for clarification, the vehicles speed comes in the form packaged data CAN frames which are broadcast to the DIM (Driver Information Module) a.k.a. the instrument cluster, from the AMT Selespeed transmission control unit a.k.a. Sportshift or EM-L transmission control unit a.k.a. Touchtronic and the EBS (ABS/DSC) control unit. This calibration defines the accuracy and/or offset of the speedometer, with United Kingdom and European Union calibrations deliberately over-reading speed via the offset as a legal requirement. Speedometers for European Union markets were calibrated with the following offsets: 40kmh with a 3kmh offset, 80kmh with a 4kmh offset and 120kmh with a 5kmh offset. Speedometers for the United Kingdom and other markets which use miles, were calibrated with the following offsets: 30mph with a 2mph offset, 50mph with a 3mph offset, 60mph with a 3.5mph offset, 70mph with a 3.7mph offset and 120mph with a 3.9mph offset. The United States of America used a different system as there is no equivalent legal requirement for an offset to be applied. Instead the true vehicle speed in miles is applied based on the original packaged data CAN frames broadcast to the DIM. Canada, along with all other target sales markets not previously mentioned, also uses the true vehicle speed in kilometres with no offset applied, again based on the original packaged data CAN frames which are broadcast. The DIM receives this calibration and offset in the form of packaged CCF (Car Configuration File) data CAN frames which are periodically broadcast on the low-speed CAN multi-master network by the CEM-P1 (Central Electronics Module - Platform 1), allowing the DIM to correctly configure itself in real-time. If this calibration or calibration and offset CCF data CAN frame is not received by the DIM after 20s, then an amber warning lamp is illuminated. Additionally, if this calibration or calibration and offset CCF data CAN frame is not received and there is a problem with the DIM EEPROM, then the speedometer ceases functioning. On top of these calibrations and offsets, the speedometer functionality tolerance was +/-3kmh and +/-1.86mph.

 

if you go up a few post you will noticed that I am now running 255/35R19 Front and 305/30R19 Rear vs stock which was 235/40R19 and 275/35R19, and the stock setup has an offset but my new setup is bang on the true speed according to my GPS and Electronic Speed sensor on the street. And yes all car company will display higher speed on speedo to protect themselves from people wanted to say that the car was saying that they were doing the correct speed and get out of a ticket

 

I don’t know where you live but have you asked the local police department to check your car with a radar gun. Some PD will do this as a courtesy if you inquire. My care is a 2007 and there is a radar to remind you to check you speed. My speedometer is correct to the radar. My car is completely stock.
my Lotus Europa and Triumph GT6 were slownby at least 7 miles. The Europa was off even more.

 

Right side of all my post

 

Apologies Handyman, I miss understood what you said. However, I still have to polity disagree with you on the following...
...as I've already explained how and when Aston Martin apply speedometer offsets to the VH-platform cars, and how the system works. I really can't explain it any clearer than that. I can't comment on other car manufacturers, but as I said, that's what Aston did on the VH-platform cars.