Higgs Boson, you are pretty sure it is there, but you can't put your finger on it. 
On a recent trip to Spain, I carried 8 lenses, 2 bodies, tripod, monopod and a notebook plus the little stuff in a Lowepro Pro Trekker 300 AW, weighing in around 15 kilos, but I also took along a Lowepro Fastpack 200 that takes a body, 2-3 lenses, a flash and a compartment for backpack stuff for walking around much lighter. This way I had a selection, but didn't always have to tote it all. I use the Fastpack the same here locally, just packing what I think I might use plus one, and If I find I don't have something I get creative and make do.
Your stated quandary is 85L or 100L. I have both and use both, they are quite different. I wouldn't part with the 85L for its characteristics, even if it sits around for periods, and I wouldn't part with the 100L for its characteristics and functions which are damn handy.
Now if you have both, either get some bag discipline, or keep the 100L cause it is handiest for the track, IS, macro and all.
I shoot cars too 
5D2+100L
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