Predictive lap timer accuracy

I've been using my AEM CD-5 Logger/Dash as a predictive lap timer on the track but I am finding very lacking.

All other issues aside, the biggest problem I have with it is the predictive lap timing which seems to be very inaccurate. I am assuming this is because the predictive calculations are distance based and uses just a start/finish line (it has no track database) rather than actual position on the track? There had been MANY occasions where my predictive time/delta would be over half second off near the end of a lap when compared to the actual time as I complete the lap. It is sufficient for a corner/corner comparison but not overall track time.

I am strongly considering purchasing another lap timer for predictive lap timing purposes and use the CD-5 just as a dash. I know Racelogic/Vbox uses position based comparison which according to them would provide much greater accuracy for the predictive timing but the Vbox products are much more expensive compared to the competition and feature limited too unless you buy their most expensive stuff.

What about the other brands? AiM Solo 2 DL is VERY attractive given how I could gradually add accessories to it but it does not mention what method is used for its predictive lap calculation.

I believe nearly all the predictive lap timing algorithms are based on time to the current lap distance. Inaccuracies are usually a result of either:

1) the reference lap used for comparison isn't correct. Some systems will allow you to specify a reference lap, that you can make sure is a good representation of the typical performance you can expect. If this is from a previous running at the track, you need to make sure the speed/distance is accurate for this lap. Other systems will use the fastest lap from the current session (or current day / event). This can be incorrect if say a chicane is short-cut or some other reason for the track distance data for that lap to be in correct (say you passed another car on the outside of a sweeping turn. Then the distance from that point on is compromised. Any wheel lockups or wheelspin on the channels used to capture speed/distance will also affect the track length/accuracy.

2) The current vehicle speed / distance has not been calibrated correctly. Say you change tires and the current ones have a different circumference. Measurement using that wheelspeed could then be off by a 1%. It only takes a 0.5% error to make a half second error in a 1m40s lap. If you are not using a wheelspeed input, then you need to closely examine the GPS data to make sure the speed is accurate, and not dropping out due to trees or building obstructions.

I find the predictive lap timing works best with drivers that can turn consistent lap times. If they make a mistake or improvement in one part of the track, their consistency will result in the predictive lap time matching the actual time at the end of the lap.

Solo2's are very good with the predictive laptime as long as you have good satellite reception at the track.

Motec dash has done a better job of this than Aim and VBox in my experience. I've not used AEM dash.

another thing to note is the calculations (while not given), can determine the viability of the accuracy. Motec for example has some pretty wild swings on predictive lap settings at the beginning of a lap, which would lead me to believe they're using a distance scaled model, vs a distance offset model

@David Ferguson. The Racelogic lap timer has the ability to do Delta T (difference vs reference lap) and Delta V (Difference in velocity vs same spot on the reference lap). Question, can Motec C125 be set up to do Delta V? Thanks!

I've never seen a deltaV output. What do I use, that is VERY helpful is minimum corner speed. This is a "latched" (remembered) value from the minimum speed in a corner -- it stays valid until the next corner. So I don't have to see this while in the corner (usually busy looking out), but can check it on the next straight. You develop targets for a few key corners and this lets you work up to that. BTW, they also do a "Max Straight Speed" that works for the peaks of the speed trace.

The Minimum Corner Speed feature is found on MoTeC and Pi/Cosworth dashes, and I have found it very helpful. Lap Gain/Loss (delta T) is useful for trying different lines or gears to see what works best.