Over the next 4 weeks, I will be driving close to 5100 miles not counting daily 40 mile commute and misc. miles. From Ames/Ankeny areas to Tulsa OK, Burlington IA, Estes Park CO, Madison WI, Minneapolis MN, & Overland Park KS. Mostly interstate miles. How many miles over the posted speed limit in IA, MO, KS, OK, NE, CO, WI & MN is generally safe to drive without getting a ticket? Every extra mph I can average will cut my driving time 1.5-2hrs roughly.
So, I'd look at my math on that.
(5100miles/80miles/hour)-(5100miles/70miles/hour)=9.1 hours. This assumes the entire trip is at highway speeds. So, you are talking 9 hours over 4 weeks. Depending on how you value your time, one speeding ticket can wipe out all your savings. I'd stick to 5 over and be happy with the 4.5 total hours you'll save.
Obviously this doesn't count your daily commute, but I'd imagine you already have your max figured out for that.