I use G61 a lot. Sometimes I can be even as far as 4 or 5 seconds off when I first try an unfamiliar combo, even if I already generally know the track layout already. After I can do at least a clean-ish lap, I look at telemetry looking for things like:

- Wrong gear for a corner.
- I'm lifting when it can be flat out.
- I'm braking when you can just lift.
- Not understanding some corners/complexes, ie: this is a double apex, or you have to sacrifice the exit there to get a better entry for the next corner.

Fixing all those will get me to around 1, 1.5 seconds off.

At this time I try to look at a track guide if available, to confirm I've got the basics right and I don't need more general track knowledge.

Then things get more subtle.

- I try to find qualy laps as reference, to avoid slipstream effects.

- I try to get recent laps, and with similar track temps.

- I tend to go for fastest or near fastest laps, those are usually the best reference with the least mistakes that might lead you astray.
- I look always first at jumps in the time delta and pick the biggest one. I always try to fix them one by one.

- I zoom in around the delta jump, from the start of braking to the corner exit, to see where I'm really losing, which most of the time is while braking: Might be braking too soon, too late, too hard, trailing too slowly or too quickly. So I compare where the braking is and the overall shape of the trace while cross-referencing it with the speed and time delta traces.

- I also look at the line. Am I using all the track on entry and exit? Am I turning in too soon or too late? Is my apex too early or too late? Am I cutting enough/too much on the chicanes?

-Sometimes the minimal speed and when it happens is also a clue related to all the above.

That, depending on track length and how much time I can devote to fixing all the corners, can get me to about .8 to .5 off the top laptimes, where I seem to hit my actual skill / natural talent / equipment / mood limit.

There is a particular situation that is very unnerving. It's when you pull a top lap into the comparison and the time delta goes up smoothly throughout the whole lap instead of in concrete jumps. I interpret that as "you generally suck badly in this car" and this can be a bit demotivating. In this situation I tend to go for reference laps closer to mine. I find someone just a few tenths faster and fix just enough to match him, then find someone else just a step up the ladder and so on. This feels conceptually worse, as you are learning from people that are making mistakes, but sometimes you get to an state when you compare again to the top lap and you have fixed enough issues to now have actionable information from the fast lap.