Are you a TRUE Wheelman? Follow this guide and do Vin Diesel proud!
There are 7 different side missions, with 15 levels each. Most of these side missions are unlocked as you play through the story (I believe "made to order" is the first).
In order to unlock the next level(s) of each side mission, you have to get an "A" rank or better to move on. I believe you have to A rank all but one or two levels to unlock them all. Each side mission (with an A rank) will give you a bonus upgrade of some sort.
So without further ado, here's the side mission list:
Made to Order
Upgrade: Garages Unlocked (lose a wanted level by driving into one)
Recommended Vehicle: Superbike
These are easy... kind of. If you have the superbike (you'll know because of its speed and the license plate says "spy hunter") it's super easy to catch the vehicles. Just airjack them, and drive them back AS CAREFULLY as possible. Don't rush it and you'll be fine.
The only exception to this are two levels: Avinguda Accumulation and Stealing from Sagrada. These require you to steal convertables and superbikes, respectively. Since these are the fastest vehicles in the game, and you have to grab a new vehicle each time you bring one back, luck plays a bit of a factor. You NEED a superbike for these, unquestionably. So get one ASAP.
Upgrade: Taxi Point (warp instantly to any taxi location in freeroam)
Recommended Vehicle: Taxi (duh)
You CANNOT leave the taxi. That's a given. Other than that, just drive from point A to point B, and be sure to check the GPS (back) to find the best route. Try to AVOID driving into destructables (like benches, tables..) and stay on the side of the road to avoid collisions. These may take a few tries, but you'll get them down.
Upgrade: Higher Vehicle Attack Damage
Recommended Vehicle: Anything EXCEPT a bike
These are tough. Be sure to immediately hit every "bonus multiplier" object, and get your multiplier up to 8X (though 6X will work). Then, drive on the SIDES of the roads. Parking meters and bus shelters are worth 15,000 a piece, and will add up fast. Additionally, tables are worth a surprising amount.
If you know some good spots (with things like lots tables and the like), try to beeline for those, if you have enough time. One that works for me is right above the contract mission "Persecution at Passeig De Gracia," where there are tons of tables and merch booths (as I call them). If you can't get here, just look around for a place that works. It WILL take practice.
Upgrade: Faster Vehicle Top Speed
Recommended Vehicle: See below
As soon as the race begins, STEAL A BETTER VEHICLE. Either a superbike or convertable will do the trick most of the time. Don't worry so much about attacking the other drivers, as I found it doesn't help much (shooting out the tires sometimes works though).
One thing to be careful is that these are kind of glitchy, in the sense that you'll be in 2nd, and it'll say your in 6th, OR the guy in first somehow finishes the race while you just finished the first lap. You want to stay near the front to avoid this, is all I can say.
Upgrade: Increased Focus Gauge
Recommended Vehicle: Convertable
Contracts are like Made to Order, except you have to destroy the vehicle in question. There is never more than one, and you have to use vehicle melee. The best thing to do is play the level a few times, to learn the pattern of the AI driver, and just get close and pound the shit out of them. Quick hits will lead to a "finisher" opportunity, leading to an instant kill.
I'd recommend doing all the Rampage missions first, since they increase your attack power, leading to a successful result faster.
Bikes are useless, since they are so small. The convertable is almost as fast, so you should use that. I found that the Taxi of all things is pretty effective for these missions.
Upgrade: Weapon Caches
Recommended Vehicle: Superbike
The superbike. All you need. Learn to make e-brake turns and hit the nitrous (X, then A) as soon as you pick up or drop off a package to keep your speed up. Practice these a few times, to plan yourself the best route.
One exception to this is the mission "Pick up from Parc De La Ciutadella," where you have to collect packages on foot in a hedge maze. Trial and error is key here. Look at the minimap CLOSELY to see where each path leads, and make sure not to sprint (A) for too long, try tapping the A button instead.
Upgrade: Increased Vehicle Health
Recommended Vehicle: ANYTHING
These involve running from a group of enemies to get to a certain destination. I found that the time-limits on these levels are pretty lenient, and you have a decent room for error to get an S rank.
Whatever you do, DONT fight back against your enemies. This will just slow you down and more of them will swarm you. If you want to try to shoot at tires or motorcyclists, this is probably your best bet.
Other than that, stay away from them so they can't melee (if they attack from the left, swerve right), and don't be afraid to use the nitrous to get away. Jumps work great for these missions.
Using a motorbike during Fugitive missions is a gamble, since their speed vs their stability (you get hit hard enough and Vinnie goes flying) is a trade off. Just remember you can steal vehicles at any point, so swapping between bikes and convertables seems to work well.
As of now, I have only 15 side missions without an S rank (and the majority are Rampages), so I think my methods will help you out.
One tip is to take the upgrades from each mission into consideration and if they'll help out in another (for example, doing the street showdowns and getting increased speed may help for fugitive missions). Focus on getting "A" ranks first, to both unlock more missions and get the upgrades.
This is NOT an easy task. But if you can do it, you will be a true wheelman.
And of course, you'll unlock this along the way: