bikeroutetoaster.com is fairly fast and gives you cumulative gain (unlike gmap). But it doesn't take enough points and tends to underestimate total gain by about 20%, especially if there's lots of up and downs along the way.
runpartner.com is better, but a little slower, though it gives you a closer estimate (probably about 10% low).
Most websites simply don't collect enough data points to give you an accurate estimate. Though if we're talking about 1500-2500 ft of climbing over 56 miles total, it won't be too far off. When you're trying to map epic 5 figure days though, you get a huge range of estimates across different sites.
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