Hi,
I believe this sentence is correct, but Microsoft Word tells me it's incorrect. The issue is with the word "were"
--Further analysis of the high end speeds indicated that 2% or less of the vehicles recorded were traveling at 35 mph or greater.--
I wouldn't rely too much on MS Word for grammar guidance.
The real, meaningful subject is 'vehicles', so 'were' is OK.
MS Word should be suggesting 'fewer' to you as an alternative to 'less'.
Best wishes, Clive