audiophool

I know this E60 of mine has an optimistic speedo (they all do), and I also know the computer uses true speed to calculate economy and distance for the odometer. Is there a way to code the system so the speedometer and HUD read true speed all the time? You know those Honda Civics with the big digital speedos? If I'm pacing one and my speedo shows 70, his shows 63 maybe.

I'm tired of having to mentally calculate: "well, the speed limit is 70kph, and I think it's optimistic by 6kph, and the popo usually let a less than 10kph over slide, so I can go 86 indicated and I should be fine".

spazzz

This is the closest I got but.....
https://www.m5board.com/threads/corr...coding.430570/

I lost the HUD and digital so I just coded back.
There was something posted on the BimmerGeeks Protools facebook page about this.
I said what happened to mine and he confirmed there was other reports of this.

I said screw it and gave up like a Nancy.

So I found the read and copied it..................


Let me preface this post by saying that everything that follows is relative to my 2012 LCI E70 with the 255/50R19 tires that are factory standard. YMMV.

While changing the value of BC_V_KORREKTUR in KOMBI to "nicht_aktiv" does nothing in US models, it is still possible to code changes that will permit your analog speedometer to reflect the true speed.

The K_ZAHL_TACHO and K_ZAHL_TACHO_KOMPL (also in KOMBI) contain values for the number of pulses per kilometer that determine the speed shown on the analog speedometer (and the digital speedometer if BC_DIGITAL_V_KORREKTUR is set to "aktiv".)

Also, the K_ZAHL_WEG and K_ZAHL_WEG_KOMPL (yes, also in KOMBI) contain values for the number of pulses per kilometer that are used by the odometer (and, I presume, the true digital speedometer).

The quick fix is to copy the values from K_ZAHL_WEG and K_ZAHL_WEG_KOMPL to K_ZAHL_TACHO and K_ZAHL_TACHO_KOMPL respectively.

For my X5, the default values work out to 699.72 tire rotations per mile for K_ZAHL_TACHO and 708.95 tire rotations per mile for K_ZAHL_WEG. While 708.95 TRPM is closer to the actual value for my tires (714 according to the manufacturer), it's still a little low, so both my speedometer and odometer were incorrect.

I'll take you through the calculations to determine the correct values using my tires.

PPK = 714 TRPM * 0.621371 miles/kilometer * 48 pulses per rotation = 21,296 pulses per kilometer (rounded to the nearest whole number).

Convert to hexadecimal: 21,296 dec = 5330 hex
Split the hex value into two 2-digit numbers and reverse them: 30,53
These are the two values for K_ZAHL_TACHO and K_ZAHL_WEG

Find the 1's Complement of PPK: 65,535 - 21,296 = 44,239
Convert this to hex: 44,239 dec = ACCF hex
Split and reverse: CF,AC These are the two values for K_ZAHL_TACHO_KOMPL and K_ZAHL_WEG_KOMPL

I don't know if ProTool is capable of making these coding changes, but if not, it can easily be done with NCS Dummy and NCS Expert.

You can check the current values in KOMBI and reverse the calculations for a quick check to see if the numbers look correct. The only real variable here is the number of pulses per rotation, but I'm pretty certain that 48 is correct for most BMWs.
4

• Since I already have a spreadsheet for doing the calculations, I'll save you a little time for the two tire sizes in your post.
  
  Size 1: 806 revs/mile = E8,5D for _TACHO and _WEG, and 17,A2 for _TACHO_KOMPL and _WEG_KOMPL.
  
  Size 2: 797 revs/mile = DB,5C for _TACHO and _WEG, and 24,A3 for _TACHO_KOMPL and _WEG_KOMPL.
  
  The actual numbers may vary slightly, depending on the specific tire you buy. Check the manufacturer's information to get the correct revs/mile for your specific tire. It can vary a bit based on a number of factors beyond the size of the tire

FormerE30Owner

I compared my speedometer to my Garmin GPS and the difference is constant across the speeds, at the speedometer reading 4 KPH higher than the GPS. IOW, the zero on the speedometer is off by + 4 KPH. This with the original size tires. If you could find a way to move the speedometer pointer to be 4 KPH lower that should do the trick. I ignore the difference and just use the difference as a safety margin for the popo.

tstiglich

My true speed is 2-3 MPH slower than what is shown on the speedometer. I always thought that my car was a fluke, but now I am led to believe that it is a design intent. Do other car brands do this? What could possibly be a rationale for doing so?

JayArras

Don't forget to take into account if you have non-standard wheels (e.g. 17 or 20-inch rims).

FormerE30Owner

Probably done for liability reasons. The owner can't sue the manufacturer for misleading him/her about going above the speed limit etc., since the indicated speed is really "off" by 3-4 km/h "high". All bets are off, however, if not using proper sized tires.

audiophool

I'm using 225/50R17 on OEM wheels, and 245/40R18 on aftermarket BBS. Nominally, they should be 780 vs 784 revolutions per mile, a half percent. Might go to a 255/40R18 next time for the 3-season tires.

audiophool

For liability sure. But what about the Honda? Surely their legal beagles would be similarly concerned.

audiophool

Not going to pull the needle. Too many horror stories about farking the clocks!

audiophool

Thank you for the comprehensive info. Filed for when I have my PC all set up.