I did turn off the auto lap, but you wouldn’t have to. When you press the lap button you can set it to reset the mileage shown on that screen to the closest mile.
If you didn’t turn off auto lap, but then went ahead and pressed the lap button at the actual mile marker, it would still round off to the nearest mile. But then when you go into Garmin Connect after the race, you would have a lot of extra laps, and not really know what your average for a mile was... Garmin will still keep track of your total mileage even though Race Screen will ‘round the distance’ at each lap press. If you just used auto lap, and never ‘reset’ it at any of the mile markers, it wouldn’t give you a good ‘estimated finish time’, because the likelihood of ruining the exact mileage is pretty slim. That’s confusing... sorry.
TLDR: I like to only have one lap for each mile in Garmin Connect, which is why I turned auto lap off.
For Chicago, I would definitely recommend turning auto lap off, otherwise you will not have any idea of your pace or time per mile for several sections of the race. I think it had me going a mile in about 4 minutes during part of it. At least if you are manually pressing the lap button at each mile marker, you will know how long it took you to do that mile.
I’m also not sure how not having a good GPS signal would affect the Race Screen data field - if Garmin says you have gone almost ‘2 miles’ but you’ve really only gone 1 mile, when you press the lap button for that mile would it round to that second mile?? (I kind of think it would, because if you forgot to lap it for one mile, I think that’s how it has worked...)
I’m thinking that Race Screen may not be very good for estimated finish times for races that have bad GPS blocking areas, unless you are using a footpod or something to help with the actual distance when you can’t get a signal.
You guys could try it on a regular run. You would first need to go into the settings for Race Screen, and set up the distance you are running and anything else you want. Turn off auto lap, and try and watch for when you get near one mile, then press the lap button. See what it does, and what it shows you.
Here’s how my settings look for a marathon distance:
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