I make a lot of my own concrete, and while a 4 bag mix is considered average, a 3 bag mix is considered weak, because my gravel pit has a bit of loam to it, I go with a 6 bag mix.
A six bag mix (or 3 or 4) is just the number of bags Portland cement in a given cubic
yard. Assuming you want to go with an Earthcrete Mix depth of six inches, a 12 x 30 foot floor would take 40 bags of Portland cement (pure Portland cement in 94 pound bags, and not "premixed" bags that have gravel already mixed in).
To make life a bit easier for you, rather than use a cement mixer that would require a lot of shoveling, you can try mixing up your earthcrete with a rototiller. What you do is, add soil to make sure the garage floor is somewhat level and smooth, the spread out your 40 bags of cement over the floor. Then take a rototiller set to till at 6 inches, and rototill the earth and cement together, making as many passes as you need to so that it is thoroughly mixed up.
Then add your water, using the rototiller again to mix the earth and Portland cement, now with water. You will want to work an area first, and then work out, probably using a helper to spray the water with the garden hose as you rototill.
Then you just trowel it to the finish that you want, smooth or with a broom finish.
I would think with 6 bags of Portland cement per cubic yard, you would have plenty of strength to jack up a car.
Assuming again, that because your building is narrow and long, your car is going to travel in the same tire tracks every time you drive in, if a test earthcrete patch did not prove to be strong enough, you could do as John suggests, and take some logs, or 4x4 beams, or even 2x4's laid on edge and nailed together to make runners for you to drive on. You would not have to do the whole floor just where the wheels (and thus jacks) would go, probably (2) strips 18 inches wide? You could then pour your floor over that.