As of 08/03/2021 I think this is the quickest method to gain xp.
Obviously before this you should finish all of the championships, multiplayer and miscellaneous achievements before grinding this. You'll probably be at around level 40 which is about 66% of the way to level 50.
First of all upgrade the Lawnmower so that it has the race engine (Money shouldn't be an issue)Map:
Motorcity Circuit Vehicle:
Very Hard (If you want to change shifting back to Automatic you can and according to comments this doesn't affect the XP earned)
With the engine improvement you should be the fastest vehicle on the grid. When the race starts accelerate into the lawnmower in front of you and then try to hit as many after that as you can (If using manual shifting press
to shift up when the revs go into the red). I would normally hit around 3 or 4 in basically a straight line. Then just crash into the wall to wreck yourself. It should take 10-20 seconds.
If you crash straight into the wall you'll get around 12k xp for 10-20 seconds work.
If you hit a few other lawnmowers before wrecking yourself you'll get around 20k xp in the same amount of time (Depending on what contact bonuses you get credit for (Spin out, Crash out, Wrecked etc.)
Depending on how fast you are this is 4-8 times quicker than the 2 minute deathmatch method for the same amount of xp!