That is a great link! Toronto is just over an hour away from Buffalo so using Buffalo as the starting point can work. I like how they let you choose your vehicle and I guess they just take the consumption ratings to plug into their calculator.
I've never driven down but perhaps the simple math is figuring how many kms you get to a tank to figure out how many tanks of gas you'll need as long as you adjust for the less expensive cost of gas in the US.
The other option is figure it out by time. Loose estimate is 21 hours to Orlando from Toronto. I can get about 4 hours of driving before I need to fill up again. So 5 tanks at $75/tank is $375. Figure the extra hour adjust cost the cheaper cost of gas in the US. Good luck!
We went last May in a Chrysler Town & Country minivan. I do not remember the trip home but on the way there we spent $120 US on gas and that included the almost full tank we had for driving around Orlando while at WDW. Plus we used it to drive to the parks too.
When I used the above link it said it would cost $210 from Buffalo.
Oh yeah and in the vehicle was just me, DH, and DD. and we took the i79/77.
$410.00 leaving our house in Oakville to having a full tank arriving back at home 15 days later. Oh we drove a 2004 Suburban with a 5.3. doing with the flow of traffic. Uaually just over the posted limit. Most of the limits are 65 to 70 mph. We got excellent gas milage for the veichle we were using. It cost is about $60 less when we used our 2001 Astro 2 years earlier.