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Venice and Florence, two of Italy's big three art cities, are joined by a fast and efficient rail service, as well as a fast toll road called the Autostrada. The distance between Venice and Florence is about 258 kilometers, or about 160 miles. See also: Four-Day Tour of Venice, Florence and Pisa. Popular Stops en Route
Driving Rome to Venice is all motorway (not for free, unfortunately). It is excellent road, but it may get hectic during peak season, on weekends and in certain other periods of the year (e.g. Easter holidays). It is actually about 600 Km from Rome to Venice, which means at least 5 1/2 hour non-stop. I think - which you probably already did - you should take this as an opportunity for seeing a bit of the country as well.