The rules for Rush Jobs and Careful Repairs don't work. In almost all cases, Rush Jobs only reduce repair time, while Careful Repairs only increase the time. The +30/-30 penalty is insignificant compared to the +/- 50% time. For example, a 48h job will only take 27h if done as a Rush Job (24/2 + 3*1).

That seems like a reasonable tradeoff to me. What is the problem with these rules? +-30 are quite hefty modifiers unless you are very skilled in repairing vehicles you'll try to get every modifier you can.

Your point? A rush job will take less time but it won't be as good a job as if you took your time and did things right?

The problem with rush jobs is that you might not get the job done at all, not that it may take longer.

That being said, there is a certain amount of lacking clarity. To quote (page 211&212):

"A successful [emphasis mine] Repair Test reverses the Damage Condition
or repairs the Structural Integrity or Critical Damage the
character attempted to negate."

"Gaining additional Degrees of Success on the Repair Test
can reduces the Repair Time, but additional Degrees of Failure
can add to the Repair Time. It is possible for multiple Degrees
of Failure to result in failed Repair Test, which might mean
that the Repair Test must be reattempted (and the time already
used is wasted) or, in some cases, further damage can be caused
to the component the character had attempted to repair."

So… the latter part and the rest of the chapter read like it's possible to sucessfully (if somewhat slowly) repair a vehicle even if your test is not actually successful , contrary to the first quote. It might be helpful to remove the ambiguity here.

However, there's a balancing issue that has to do with Rush Jobs - there are multiple kinds of repairs that have no penalties for failed tests beyond the increased timeframe. For example, if one repairs a lightly/heavily/critically damaged vehicle, it's always preferable to rush the job - you spend half as much time and the -30 doesn't matter because apart from upping the time by an insignificant amount there's no drawback to a failed test. It might be helpful to institute a general "If you fail the test by X degrees, the repair does not succeed and all time is wasted" rule.

Aaand finally, there's the oddity of Fire Damaged. You can repair it with the rules on page 213 - which is all fine. The only question is: What happens if you don't? Being fire-damaged does in no way inconvenience a vehicle as soon as the fire is put out. After repairing the structural damage, you can have all the molten cabling, torched seats and burnt consoles you want - there are no specific penalties for it.
A solution might be to say that no other repairs other than Lightly Damaged ones can be attempted before the Fire Damage isn't fixed.