[Untamed]

Ah ok, yeah. You -can- remove a few of the joists without any real structural damage or threat to the roof, however... the joists really are there for a reason. What you do to reinforce the missing sections is build a box out of 2x10's around the opening, T-ing the joists to the box with a combination of mending plates (metal plates with holes in them for nails) and metal joints (metal plates with spikes or cletes that you hammer into the wood). Don't just toe-in nails to joint the joists with the box. Remember that joists aren't compressing, so they wont push against the box, they run the width/length of the house, with other 2x10's attached to run the entire length.

Before you begin remember to check the joists for attached junction boxes or electrical wires. They will have to be relocated if necessary. Secondly, make sure there isn't any ducting beneath the insulation. If so... moving that is a pain and will definately take more time. The more bends in the ducting, the more disruption to airflow / chance for condensation, espcially if the attic will have a hole in it, exposing it to more outside air. Lastly, consider what is being stored up there (if anything) and what a hole in the ceiling in the garage will do to the items up there. While attics have venting on at least one side, if not two, the added humidity and generally cooler temperatures may affect the stuff up there. There aren't large holes in ceilings for a reason - not the least of which is exposed insulation which may fall down from time to time, spreading the fiberglass around. Not a good thing to have in the air when working on engine block internals.

Ok, here's the last thing - selling your house. When it comes time to sell your house, any home inspector with half a brain will stick his head up into the attic over the garage and look for roof warpage, leakage, mold / humidity signs and so on. If he sees the box, or if you;ve repaired it and have a few mending boards across the spot where the board was, it'll send up all kinds of red flags, and prompt all kinds of questions. Seriously consider the re-sale of your house before you start changing the structural integrity of the house. Removing the sophet over a load bearing, internal doorway is one thing - removing joists is a whole other thing.

Remember - these are ideas. I have never done this before but have seen it done on a smaller scale, for a second attic entrance. I would not recommend you do anything of what I posted above, however it's an idea for you to consider if you are hell-bent on doing it. Just consider the consequences, short and long term, before you decide.