having lived thru switching from miles->km you learn a few simple rules of thumb: 100km==60miles 80==50 50==30 It helped that that also covered most of our po

having lived thru switching from miles->km you learn a few simple rules of thumb: 100km==60miles 80==50 50==30 It helped that that also covered most of our posted speed limits - the US with its penchant for speed limits ending in 5 would find it harder going