Harry's GPS Suite Forum

I read this thread. For what I understand, there is no (semi-)automatic calibration of ODB speed to GPS speed. So here's another feature request: You drive a few hundred meters at different speeds, and HLT automatically adjusts ODB multiplier to match GPS speeds - for the currently selected vehicle. :-)

Also, this could optionally be done on startup or even during racing on straight parts of the track, this way it would eeeeven include tire milage, ie when "your flaps are wearin' thin" to quote a famous rock band. ;-)

Caaaaalibrate good times, come on!

There are all kinds of sensor synchronizations and calibrations one can think of - this one included. Other examples are acceleration and GPS, multiple GPS sensors, or audio (video) and rpm etc. I have that on my list but didn't find the time so far. I need to say it is sitting there for 2 years already

- Harry

I've been wondering how you do what you do ever since I started using HLT, considering that you seem to have a regular daytime job as well. ;-)

Keep it up though, it's very good work!

That changed in July, I'm fully committed to LapTimer now Interesting enough, it didn't have too much influence on the to do list

- Harry

NRGDEAD wrote:I read this thread. For what I understand, there is no (semi-)automatic calibration of ODB speed to GPS speed. So here's another feature request: You drive a few hundred meters at different speeds, and HLT automatically adjusts ODB multiplier to match GPS speeds - for the currently selected vehicle.

Also, this could optionally be done on startup or even during racing on straight parts of the track, this way it would eeeeven include tire milage, ie when "your flaps are wearin' thin" to quote a famous rock band.

Caaaaalibrate good times, come on!

Basically you're talking about sensor fusion where the data from the accelerometer and gps are combined. There's also a three axis rotation rate sensor and, for most smart phones, a three axis magnetometer. The DIY drone community uses all this data to fly model planes and helicopters, so the calculations can be done in real time on a small computer. Roll (lean angle on a bike), pitch and yaw (drifters would like this) would be calculated as part of the process. But to do this in HLT would likely require completely rewriting the core of the program. I can understand why it's not a high priority.

Calibrating the wheel speed vs the GPS speed is, in my opinion, something that doesn't need to be done internally. How is the program going to tell, for example, if the rear wheels are spinning? Given the precision of GPS, a few hundred meters isn't really far enough. Turn on your GPS and compare GPS distance traveled vs odometer distance traveled, calculate the correction factor and enter it into the program. The longer the distance, the more precisely you know the factor. I would say a minimum of 10 km is required and 100 would be better. And it isn't going to change unless you change your wheels and tires or the drive axle ratio. Don't use the indicated speed on the dashboard to do the calibration. You will very likely find that the OBD wheel speed is different from the the dashboard speed. For a quick and dirty calibration, compare OBD wheel speed to GPS speed on the HLT GPS page.

That would be more precise, yes. But just yesterday, I drove with HLT and XGPS for a few kilometers at a steady speed, and GPS speed didn't change much. And it was a lot more precise than my speedometer. So I would still appreciate an automatic or assisted calibration of GPS to ODB speed. Of course, it can be done manually, so it's just a "nice to have".

I got my BT1 yesterday, and so far, it works fairly good.
I noticed, however, that ODB speed is mostly higher than GPS speed (see screenshot) - but not as much as I figured it would be.

Readings (in kmh) be like:
Speedometer: 105
ODB: 100
GPS: 93

These are my theories so far.

1. ODB and speedometer are the same, HLT does calculate some correction factor, but "not enough".
2. ODB and speedometer are measured differently, HLT does not neccessarily correct the factor, or "not enough".

Ideas and solutions appreciated. ;-)

PS: Eww, can I somehow scale down that screenshot? I figured it would be just a link.. ;-)

It is #2. Measured speed is always raised on its way to the dashboard to make sure the speed displayed is never smaller than the real speed (this is a requirement for all cars sold for public roads). When I attached an OBD the first time I thought the OBD speed is the raised speed - but this is not necessarily the case. Without insights into the design for a specific car model, you can actually say not too much about the relation between these two values, except:

• Speed displayed by your car's speedo will always be wrong and will always be too high.
• OBD speed *can be* the measured speed, but needn't.
• For a drive at constant speed, GPS will always be the best and most accurate value.

The good thing is most track visitors do not need to compare absolute values mostly. Usually it is more on "is this line faster or slower". Only in case you want to compare cross cars this becomes critical. From my PoV it is best to calibrate OBD to match GPS speed using a constant drive and adjusting the Expert Setting (speed multiplier). Afterwards, it is best to use OBD as it does not miss peaks like GPS does. Finally, keep in mind OBD covers speeds up to 255 km/h (158 mph) only. LapTimer will switch to GPS for higher speeds

- Harry

NRGDEAD wrote:PS: Eww, can I somehow scale down that screenshot? I figured it would be just a link..

You need to scale it down before uploading it. Both when using the [Img] link or the attachments forum feature. On a MacBook Pro Retina display it looks great at this resolution

- Harry

NRGDEAD,

The OBD speed should be calibrated to the OEM tire circumference (revolutions/mile). If the tires you have on the car don't match that circumference, then the OBD speed will be wrong. I have a tuner that allows me to change the drive configuration (rear axle ratio and revolutions/mile) so I can get the OBD speed and the odometer to match the GPS to the resolution of the revolutions/mile or 1 part in ~750. On my car, at least, the odometer is equal to the integral of OBD speed. The speedometer reads a little higher for the reason that Harry details above.