Microsoft Streets and trips does a bit of a better job of this, but of course it isn’t online (that I know of) and isn’t free. I picked up a prior year copy of it at Best Buy for like $29. You can enter specifically your mpg, the speed you intend to average, the road type you prefer, how often you plan to stop, etc. That being said, cost to drive was only about $10 off from Streets and Trips for the 929 mile round trip I was thinking of taking for Christmas, so I guess most can be forgiven 
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