Unlimited Time in Timed Missions
When you start most timed missions in the game, you will be told to get into your car, then the clock will start. Instead, don't get in your car, but run to the area you need to go, then find a phonebooth or borrow a car. When you get in it, you are pretty much finished, and you are already at the goal with seconds(or minutes) to spare.
Wrecked cars
After your car has exploded, you can still get in it and drive it. You are able to get the wrench and repair it, and drive it like normal again.