My take on how to obtain the XP is the quickest.
The Method
“Other”
“Exhaust problems / Car is loud / Loud when accelerating”
“Brakes”
Other – You have to replace the fluids.
Exhaust – You have to fix the exhaust system, which usually means removing 1-3 exhaust-related parts, replacing them with new ones. Very fast job.
Brakes – This is optional. You either gonna have to replace a few parts or it’s going to be a bigger job. You should always look at the % number next to the order. It tells you the lowest quality parts you can use whilst repairing the vehicle. I noticed, that the lower the number, the more difficult is the assignment.
You should also take notice of the orders that give bonus XP. They may be difficult and lengthy, but will give you a lot of experience once the order is finished.
Example of a 15 second job.
The beginning
You should buy:
“On-Board Diagnostics”
“Electronic Multimeter”
“Fuel Pressure Meter”
“Compression Tester”
Having purchased that, begin finishing the Story Orders and once every 3 minutes, check back on the orders (phone icon), to see if any “easy” orders have appeared. Once you get it, on the top of fixing the car (finishing the order), you want to use all of your tools on the car, so it examines its parts and gets you the XP. If you want to gain even more XP per car, you can also manually examine the parts, starting from the engine. This should get you at around 50 XP per car with just the tools, around 75 XP if used the tools, examined the car manually (not the entire car), and finished the order.
Whole process takes around 3-5 minutes, depending how efficient you are. That’s almost 100 XP per car, under 5 minutes.
Optionally you could invest in “Eagle Eye” skill, and make examining parts faster.
TL;DR
2) Look for the easiest orders to finish (Other, Exhaust, Brakes), all while working on your Story-line order.
3) Use Tools + Examine the car manually.
4) Finish up the order, get XP.
5) Repeat.