GPS vs OBD speed calibration

[karacticus]

I collected a bunch of data today to see if I needed to tweak the OBD speed multiplier-- ran several steady state speeds and then ran a .csv output, comparing the parameters labeled SPEED_MPH and WHEEL_SPEED_MPH. On export and comparison, these appear close enough that there isn't much point point in adjusting the multiplier, which is a bit of a surprise based on how optimistic BMW speedometers are generally though to be, so I'm wondiering if I'm doing this correctly.

In the GPS Tweaks section of the settings, speed is set to Measured. Do I need to set this to calculated and repeat?

[Harry]

The GPS Speed measured is what you usually should use - but it is not related to OBD speed.
In case you want to calibrate your OBD speed to "reality", I'd suggest to do the following:

1. Drive on the highway at constant speed (in case you have a cruise control, use it).
2. Now, start a manual recording (use the Classic View or turn the feature on in general).
3. Record a few minutes and go back home for analysis.
4. Open the lap's data recordings and change to the Charts area, select the Speed page.
5. In a range where you had constant speed, select any point and write down gps speed and wheel speed.
6. Set the OBD Speed Multiplier to: new_multiplier = gps_speed/wheel_speed*old_multiplier

Step #1 is important as GPS is highly accurate - at constant speeds. The outlined approach will give correct wheel speeds for future recordings. Especially in the speed range you calibrated it with (in the US 55 mph, in Germany 200 km/h ), this speed will be pretty accurate and by far more responsive than GPS speed.

Btw. you can use a similar approach to sync OBD and GPS speed (just compare the gap between peeks in the Speed Chart). To adjust this gap to zero, change OBD sensor delay in LapTimer's Expert Settings.

Last item: please do not assume the speed you see on your dashboard is the same speed you get from OBD. At least for my 911, GPS speed, OBD speed, and the car's speed gauge all show different values while doing an exercise like above... Car manufacturers have several speed sources available (double checking, speed by wheel, etc.) and will feed calculated values to the speedo.

- Harry

[karacticus]

Thanks!-- I'll work off the charts then.

[gplracerx]

If your speedometer and odometer are run by the engine management computer, the most accurate way to determine a correction factor for OBD vs GPS wheel speed is to use the odometer. Compare GPS miles traveled vs. odometer miles traveled. The correction factor is then the GPS miles traveled divided by the odometer miles. The further you drive, the more precisely you can determine the correction.

[karacticus]

I'll see if I can give that alternative a try as well. Best I can make out from GPS vs OBD speed, I'm looking at at the OBD being something just under 1% optimistic.

[gplracerx]

1% is pretty good actually. I can change the tire revs/mile in the engine computer so I can calibrate to one part in about 750 or ~0.13%. Of course I need to drive about 75 miles to check the accuracy to that level. You can eyeball at lower miles by watching when the GPS display changes compared to the odometer. But you probably shouldn't as you're not keeping your eyes on the road.