From a post I made 8 years ago on the subject, but yeah, I've played with it My guess would be IPT as stated by the manufacturer is based on what they consider a full spool, and simply a single actual measurement one time.
"I read Tom's comment with interest and had some time to kill last night, so I grabbed a reel and started measuring. I had a Shimano Castaic CA200 sitting around that I won't be using any more before things freeze up over here. Capacity is listed at 125 yds of 14# test. I had a nearly full spool of16# fluorocarbon on it. It has a 6.2:1 gear ratio, and the tested IPT was around 24.5"
The initial starting diameter of the line filled spool was 1.233" (radius = 0.616"). After stripping off about 40 yds of line, the effective gear ratio only dropped to just above 5:1. That would be about as long a cast as I'd normally ever make with that outfit. I kept stripping line off, passing the 82 yd mark that is factory placed on the line and continuing until I had nearly 100 yds stripped off. At this point the filled diameter was just 3/4 of an inch, with a spool radius of 0.385". I was well into my backing and likely only had another 20 or so yds of line left as I could see my arbor knot starting to appear. The calculated "effective gear ratio" at that point was 3.9:1 which was an IPT of 15", a 37% decrease from initial starting capacity.
Your mileage will certainly vary, as every reel is different, and a lot of variables affect the overall outcome. In general though, I'd say a typical long cast will likely decrease your overall effective gear ratio by at least 1+ spool revolutions per handle turn (a 7.x:1 becomes a 6:1, etc.). This would only happen during the initial part of your retrieve. As the line gets built back on the spool, the effective gear ratio continues to climb back to "normal." Wider spool reels would probably show less of an effect because they hold more of the line on the "outside" of the spool, while heavier lines, because they are thicker diameter, would likely exaggerate this effect.
I also carried out the "shoe test" at all stages (as suggested in one of my other posts). At full spool and attached to a 6' musky rod, it was a big struggle to lift and wind a 10.4 oz. shoe off the ground using only the reel handle and the drag clamped down. After stripping off the 100 yds of line and retesting, even though the gear ratio is still technically 6.2:1, since your IPT has decreased so much, the effective gear ratio of 3.9:1 made the task a pretty simple affair. A very noticeable difference and considerably easier to accomplish with much less effort in winding."