Many have discussed this . . . over & over again.
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In the last few years, Disney has upped the ante to encourage folks to NOT rent a car (and spend more of their time & $$'s on WDWorld property) by:
a) raising parking rates - this affects off-property guests who do NOT have AP's; and of course,
b) Magical Express to eliminate what used to be a fee for coach, limo or town car ride to/from MCO.
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Also, during the last year or so, rental car rates have increased quite a bit as there aren't as many extra-surplus manufactured automobiles being dumped into the rental car fleets. Florida taxes keep going up as well, so what used to be a $270/week "deal" is now a $410/week hit for a rental car. Plus some tolls and, of course, gasoline.
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All that being said, it's still a question of how you value and want to spend your time. I take public transportation most days to/from NYCity and since the congestion and parking is terrible, I don't mind waiting for the train and dealing with the uncertainty of delays and not getting a seat. Driving around WDW is not a problem so I love the freedom of an automobile in WDW.
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It also depends on the size of your party -- since the more people, the more total time wasted, waiting for the bus. The Unofficial Guide has some estimates of the time it takes to travel by automobile vs. Disney transportation between resorts - parks - downtown disney and it's not pretty. Typically, a group will require an extra 10~20 minutes for each trip. That's twice per day for the standard resort - park and then park - resort transportation. More if a group is park-hopping or going to another resort or downtown disney at nite for a meal or entertainment.
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The only exception is getting to/from the Magic Kingdom where driving is interrupted by the TTC while -Disney- buses go to/from the park entrance. In fact, the only time we did NOT rent a car was the first time we stayed at the Poly and our son was young (meaning we spent most of our time at MK).
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Generally, we rent a car because our vacation time ends up costing approx $30~35/hour ($3000 total travel/parks/meals/souvenirs cost for a "week" or 7 x 14 hrs of vacation = $3000/98 hrs). So if we use up an extra hour each day, more on the arrival and departure days, waiting -- that's 8 hours wasted. 8 hours x $32 = $260 . . . or maybe a little less than what a rental car would cost.
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So it depends. How many people in your party? Will you want to go off-property once for groceries or a much less expensive meal? Will you be going to one park a day or doing a lot of hopping? Is your resort proximate to the park(s) you'll be going to most.
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Either way, have fun -- there's lots of ways to do Disney and most of them are just perfect!