You can make reservations for the Uffizi and other related museums online in advance.
Before you make reservations consider if your bicycle tour includes a visit of, or transfer to Florence and how it will impact your availability – see your tour specific itinerary for more details.
The Uffizi Gallery is generally free on each first Sunday of the month, which means crowds. If your Florence visit on our Bike Across Italy trip falls on such a free day, then it is better if you make your reservations for the Saturday after 3 PM, which will give you enough time for a good Uffizi visit before dinner.