Current time now in Time Zone: America Denver (USA Mountain Time) USA & Canada clocks are now on. Daylight saving start: Sunday 12 March 2017 02:00 local time. Clocks go forward one hour. Daylight saving end: Sunday 5 November 2017 02:00 local time. Clocks go back one hour.
The shortest distance (air line) between Florence and Rome is 145.34 mi (233.91 km). The shortest route between Florence and Rome is 172.14 mi (277.03 km) according to the route planner. The driving time is approx. 2h 59min. Half of the trip is reached in 42.86562,12.03059.
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.