Thread: Incorrect Speedo reading TD5 D2a Auto

It's possible that a previous owner may have replaced the hubs with aftermarket, some of which have 55 teeth on the reluctor rings instead of the factory 60 teeth , try and count them
BCU = body control unit

Originally Posted by TAHAIC

I have 265/60 R18's - my D2 came out with R18's

When my dash speedo say 100km I am actually doing 110km.

Originally Posted by TAHAIC

.....

I guess what I am on about is that I don't understand why there is a 10% difference from what the speedo says and actual road speed when my tyre size should only be making a 5% difference at the most......

Different tyre brand/model/type?

It'd be strange if you went from the same brand/model/type of tyre of the old 255/55 to the same brand/model/type in the 265/60 tyre too.
You'd think that a manufacturer would be consistent in their tyre size.

But if you went from a 255/55 road highway tyre to a more A/T type 265/60 tyre, with deeper tread blocks and so on, then the 10%(instead of 5%) discrepancy can make sense.

Tyre inflation can also make a difference in rolling diameter too if it's massively underinflated.

Does the Nanocom allow you to adjust the speedo for the variation?

Originally Posted by TAHAIC

Thanks Strangy, as normal good info!

I guess what I am on about is that I don't understand why there is a 10% difference from what the speedo says and actual road speed when my tyre size should only be making a 5% difference at the most.




The engine ECU road speed is the same as the speedo in the dash (10% less than actual)



My logic tells me that if the speedo is being driven from the wheel speed sensors then should it not be 5% less than actual road speed?

Which signals are used for determining shift points exactly? Engine ecu signal or slabs ecu and BCU signal?

What is BCU? what does that stand for?

I am not sure why you are getting the discrepancy. i'll think bit more.

Road Speed readings from the TD5 engine ECM is derived from the SLABS ECU, therefore should always match the Speedo indication, also derived from the same SLABS source. So regardless of tyre size the speedo reading should match the Road Speed from the engine ECU.

The engine RPM signal, derived from the engine ECU, is compared against the auto trans output shaft rpm signal by the EAT to determine shift points.
So while tyre sizes will affect the overall performance and actual speed related to shift points, the shift points are only influenced by engine RPM and trans output shaft rpm.
Actual or corrected road speed is not considered as an input for this. No input from SLABS wheel speed sensors is derived for the shift points.

Body Control Unit = essentially the box which looks after everything else.eg, pwr windows,cruise control,Wipers, lights,sunroof alarm central locking etc

The BCU is able to determine if the Engine ECM signals EAT signals and SLABS signals, are outside of "normal" parameters and precludes some functions operating correctly (commonly Cruise Control) Though this isnt your problem.
Doesnt really solve the 10% but may help.

Originally Posted by discorevy

It's possible that a previous owner may have replaced the hubs with aftermarket, some of which have 55 teeth on the reluctor rings instead of the factory 60 teeth , try and count them
BCU = body control unit

^This is possible.

Is there any chance the previous owner had fitted a speedo calibration device which hasn't been set correctly?

By law (ADR) a speedo must not under read, and can't over read by more than 10%.

The D2s average the speed of each wheel using the ABS sensors to generate a signal used to drive the speedo. So if 55 tooth hubs have been fitted, as Discorevy has already suggested, to all four corners you have got a 8% difference straightaway.

With your tyre size being 5% bigger that's a total difference of about 13%. Most D2s seem to over read by about 5% from the factory so your current 10% under read isn't so strange if you have four 55 tooth hubs fitted.

Have you set the tyre co-efficient in the Nanocom ? I have mine set to 1.06

I have 265/70*16 fitted which are 5.7% oversize. Checking the Nanocom against the GPS at a co-efficient of 1.057 was almost on the money but had to increase it to 1.06 so the Nanocom & GPS read the same at 100km/hr. The speedo reads about 106km/hr.

Previously had 255/70*16 fitted (3.8% oversize) and had to run the tyre co-efficient at 4% to make Nanocom and GPS match ... Speedo was about 103/104km/hr at 100.

Have original factory hubs fitted.

Sent by iPhone using two tin cans and Forum Runner

Originally Posted by discorevy

It's possible that a previous owner may have replaced the hubs with aftermarket, some of which have 55 teeth on the reluctor rings instead of the factory 60 teeth , try and count them
BCU = body control unit

Its unlikely, the previous owner had all services and repairs done by a Land Rover specialist and I have a copy of all paper work for whole life of vehicle. But anything is possible, how can I check this?

Originally Posted by strangy

Road Speed readings from the TD5 engine ECM is derived from the SLABS ECU, therefore should always match the Speedo indication, also derived from the same SLABS source. So regardless of tire size, the speedo reading should match the Road Speed from the engine ECU.

Yes, it does! Speedo reading matches road speed from ECU!

Originally Posted by strangy

The engine RPM signal, derived from the engine ECU, is compared against the auto trans output shaft rpm signal by the EAT to determine shift points.
So while tire sizes will affect the overall performance and actual speed related to shift points, the shift points are only influenced by engine RPM and trans output shaft rpm.
Actual or corrected road speed is not considered as an input for this. No input from SLABS wheel speed sensors is derived for the shift points.

Then maybe it's not affecting my shift points. I am tending to make that assumption because I expect the tire size (5%) to be affecting it and knowing that there is an additional 5% error I'm thinking shift points must be further effected.

I think I need to just stick to working out why I have an extra 5% discrepancy in the speed and consider the shift points of the auto to be normal for the moment rather than complicate the issue.

Originally Posted by Fluids

Have you set the tyre co-efficient in the Nanocom ? I have mine set to 1.06

No, not yet I have left it alone as not sure what affect it will have or how to use it exactly.

Originally Posted by kelvo

Is there any chance the previous owner had fitted a speedo calibration device which hasn't been set correctly?

I doubt it very much, all work by previous owner done by land rover specialist and I have all the paper work.