I like www.viamichelin.com for online route planning, as well as the Michelin Central Italy map to take along. The fastest route only takes 3.5 hours by driving (or 5 hours and 2 connections by train). But the benefit of driving is to stop on the way (not to save 2 hours travel time). Are you doing this trip in one day and is it your only opportunity for seeing Tuscan towns? San Gimignano and Volterra are both good stops near the start of your drive. Montepulciano, Pienza, and Montalcino are closer to your destination (but back-tracking there from Orvieto would take about 1.5 hours each way).
Between Lucca and Volterra, I traveled on the smaller roads, straight as the crow flies, and the flat farmland and big-box stores on that stretch were not as interesting as the southern part of Tuscany, so sticking to faster roads as far as Volterra makes sense.