D4 Speedo v GPS

My D4's speedo now shows proper speeds - yeh! It shows 100 when the GPS shows 99 with CC setspeed at 100 on a long, flat straight. The speedo market option of 2% + 0 kph has been selected. The tyres size is set to the fitted 245/70-17 so the odometer should be correct again but will be able to confirm on Monday. These tyres are reported to be 30.8" instead of the nominal 30.5" so the odometer may still understate marginally, as evidenced by the speedo not being a full 2% optomistic. The fuel economy factor has been set to -6% because changing from the original +7% to 0 still showed about a 6% discrepancy on a 500 km brim to brim check (10.1 display vs 10.67 actual).
Edit: the individual % & kph fields have been set to the same as the market option 2% + 0 kph - for CC use it seems.

I'm still working away at getting the specs right. The odometer doesn't seem to honour the tyre size specs because its still reading 3% low. Perhaps it uses the rolling radius specification which has so far been unchanged but now increased by 10 mm to reflect the slightly larger tyres. Fuel consumption was pessimistic by 7% on 1 refill (495/510 kms) and 4% on a 2nd (450/463 kms), so the economy factor now set to -2%. No 950 km days again soon so it might be a while before any more worthwhile results.

Graeme, what device are you using to change these settings? I would very much like to get my speedo (and fuel economy) reading correctly.

Thanks,

Paul

I'm using a Faultmate MSV2. The rolling radius had 1 for the most significant (left) byte and 116 for least significant (right) byte, making 372 (1 x 256 + 116) which is 9-10 mm less than the nominal radius of the standard tyre. It now has 1 & 126. Immediately the ABS ecu is reloaded it reports several faults presumably because its specifications have been changed, but none once the ignition is cyced. I then clear the faults and no more occur.

The speedo still shows 99-100 with 100 on the GPS after the increase in rolling radius, so the speedo parameters seem set. Its likely to be some time before I can check the odometer against freeway odometer check posts to confirm the rolling radius change is having the desired effect, but the odometer at least agrees with a 1000 m distance to destination check on the navigation device so I haven't particularly upset it.

Thanks Graeme,

Cheers,

Paul

I have also been trying out some settings in nmy RRS 2010 model. As standard it comes with 275/40/20 tyres but I have fitted the L322 rims (identical to my FFRR) with 255/50/20 tyres. Without any settings changes the speedo is now absolutely spot on at all speeds but the ODOmeter is around 4% low (for 100 miles on the GPS it reads 96 miles).

Also the vehicle is equipped with adapative crusie control and the fitting of larger tyres is claimed to affect this, although mine seems fine, very ocasionally I get "radar signal blocked" and then it disables the "forward alert function" until a recycle of the ignition. I don't think I get any more of these than before but I do know it uses the ABS singals along with the radar.

My tyre settings are MSB = 1 and LSB = 106 and the FFRR fitted with these tyres as standard is MSB = 1 and LSB =115.

Both have market settings of 3.5% + 2kph, but the RRS has individual settings of the same value and the FFRR has the individual settings as 0% and 0 kph.

Out of interest the trip computer fuel ecomomy setting is also +7%.

So I think I need to adjust the RRS to match the tyres and what you have discovered that the settings actually do is very interesting! What I have not found out is what the actual tyre circumference setting does and the tyre size setting as it seems to have no influence (in my case 2320mm).

If I find out some more I will post here.

My D4 CCF does not appear to have a tyre circumference.

I have deduced that the tyre width, aspect and rim diameter are used to create the actual speed signal but that the speedo adjusts it according to the market specification of +3.5% & +2kph or +0% & +2kph as the only options. The suspension ecu seems to use the actual speed generated by ABS that's broadcast on the HS canbus, not the market specification adjusted speed shown by instruments.

I have deduced that cruise control uses the speed signal generated from tyre size and the individual values matching +3.5% & +2kph or whatever is selected, noting that 0% & 0kph are available. Setting these values different to the speedo values results in CC speed different to the speedo. (It looks odd to set CC and see setspeed quite different to the speedo whilst travelling at a constant speed.)

I have deduced that the dynamic rolling radius Most Significant Byte and Least Significant Byte are used for the odometer. The decimal values are converted to hex then the pair of hex bytes formed by the MSB and LSB in hex (now a word) are converted to decimal to produce the dynamic rolling radius in mm. Hence 1 & 106 results in 362mm dynamic rolling radius and 1 & 115 results in 371 mm. Note that the radius is dynamic rolling which allows for tyre deformation due to the vehicle weight so doubling the result is 5-10mm less than the nominal tyre diameter.

Thanks for the reply! The tyre circumference in the RRS is in the EUCD section of the config, in my case 2320mm.

I find that the MSB and LSB figure alters the speedo and odometer. If I change mine from 106 to 116 the odometer now suits the larger FFRR tyres and the speedo goes back to being optimistic from being spot on. This can be corrected by setting the region to 0% a already documented above!

What fun, we are slowly getting there.

I hadn't detected the relationhip between the rolling radius and the speedo but my fiddling wasn't exhaustive, being content with achieving the correct speeds and distance. Its also possible that ecu logic has changed parameter usage since I played with the parameters.

Perhaps BBS have updated their software with more fields displayed since I saved my original data which does not show the circumference in the EUCD section. It might be time to save another copy, having recently updated to the latest VE version.

The only way to check an accurate speed out of your car is to stick it on a certified set of rollers. When I say certified, I mean from those measures and standards people (I think the automotive societies/associations have them and hopefully your LR mechanic). GPS is unreliable and as such is not used in court (sorry for all those wanting to challenge speeding tickets off of GPS)there have been a number of cases around the country that have proven this (unless the lawyers in the crowd wish to challenge or support this). Obviously with tyre wear it varies the outcome and GPS is very very close but not classed as accurate. To answer the original question, no it isn't something to worry about, go off of your speedo and not the GPS and make sure your dealer calibrates it when you go in for a service.