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Oh, it certainly isn't flat. 1.5 at the edges, close to one mid-band. Again, it worked well enough for my needs - I was able to reach the 2M club repeater on a 100 mile loop through mountainous terrain. My APRS reports made it through on most of the route (there are known holes where the club puts up temporary digipeaters as needed for events).
Would an NMO mount be better? Sure. Do I want to take a hole saw to my fender? Nope.