Harry's GPS Suite Forum

During a 1st gear run up to red line, I get some incorrect values reported. Attached is a spreadsheet and some images from my iphone5s.
After the GO line I did not add throttle (rpm at about 2100) then full throttle until red line shift to 2nd and then I did not accelerate any more.

Background: my Lexus RC-F has a G meter and it shows a peak of .6G linear during the run up full throttle.
I am using iKKEGOL vgate icar 2 mini obd2, wifi connected.
I have the iphone5s mounted in the standard portrait mode (no video) The laptimer mount was adjusted to get exactly 0 acceleration
in both directions when the car is level. The iphone has a clear view of the sky through the top of the front window from just right of the
mirror and the face is at about 25 deg pointing towards the driver and with the 5 deg tilt to get the 0 0 acc reading at rest.

The errors:
Laptimer charts shows max .48G Linear, it also shows a incorrect peak 70mph instead of around 60-62mph just after a shift to 2nd. Then laptimers shows a peak RPM of 8842 (worst error) for one sample (jumps from 5542 back to 5665) in the cvs file and this is way before the red line of 7337. My car can to get anywhere near this value of course. I then tried a laptimer recalculate step and the G for linear G went up to .66 which is a little closer to the .6 Lexus reported. I attached the recalculated spreadsheet with come cells colored.

So would adding some splits every few hundred feet in the run up help.
Is this ODB2 device just too slow or error prone?

Try adding a higher update rate GPS to your system. When using the internal GPS it updates at 1Hz. A certain amount of extrapolation is required in that case. You're going to get errors at 1Hz that you won't see at 10Hz.

My 10hz 818XT Qstarz will not pair with iphone. Not Mfi (Made for iPhone) certified I guess.

Guess I am out of luck on accuracy without spending some MORE $$ after
buying Laptimer $30 and a phone Mount $20 and power cord $10 and ODB2 $50 (which included Carista interface).

That's correct about the Qstarz. But it's quite well known.

You may have more problems than that. There's quite a discrepancy between the GPS speed and OBD speed. Are you sure that you're getting good GPS reception on your phone?

As gplracerx wrote, these are artefacts from the low GPS update rate and the comparable low accuracy of internal GPSes. The general rule is: the internal GPS is fine to record lap times, but once you start doing data analysis, you need a 5 or 10 Hz external GPS.

- Harry

thanks guys.