I'm planning on giving the PCs in my game a landspeeder, and then have a combat encounter with enemies on speeders. Assuming the PCs can shoot at the riders (rather than their vehicle) with their personal weapons while travelling as passengers on the PC's speeder, I was wondering if the PCs suffer any setback or difficulty change to hit the enemy NPCs? What if the enemy speeders are faster than the PC's one?

Apologies if this is detailed in the core book but I can't see it.

I would handle it as vehicle to vehicle combat with setbacks added for cover (as typically about half of the target would be covered by the vehicle) ontop of those imposed by the driver for any manuvers that they have,

1 setback for shooting from a moving vehicle, 1 setback for target in cover

Additional setback if the pilot is doing evasive maneuvers.

Like above, you can get setbacks from lots of sources, terrain, speed, the unstable firing platform of a maneuvering speeder.

Targeting the crew specifically can either be setbacked by a cover bonus, a called shot, or both, depending on the encounter design and the speeders in question.

Personally I'd consider making it both for reasons of precedent. Make it too easy and the players will try and do it all the time and never bother shooting the vehicle.

So assuming the terrain isn't too rough I'd probably do something like:

Vs. a Landspeeder traveling speed 2: +1 for terrain, +1 for firing platform totaling, 2 Setback

Vs. a landspeeder's crew: +1 for terrain, +1 firing platform,+2 for cover, +2 for aimed shot totaling 6 setback.

Vs. a Speeder bike traveling speed 3: +1 for terrain, +1 for firing platform, +1 for speed, totaling 3 setback

Vs. a bike's crew: +1 for terrain, +1 for platform, +1 for speed, +1 for cover +2 for aimed shot, totaling 5 Setback.

2-3 setback isn't hard to overcome, and talents like Brace would certainly come into effect, so hitting the vehicle would be doable, but hitting the crew would be more challenging. On the other end, a talent like Call Em eliminates the aimed setbacks, but not the environmental ones, so even a fancy pants gunslinger will still be looking at a few setback. And of course a double aim would remove one setback from aiming.

Of course a Bodyguard/Gunslinger with Brace and Call em would be amazing at this, but if you want to invest that kind of XP into an effect this specific I guess it's ok....

Bottom line: Using both would generate a rather difficult check, but the setbacks are also able to be mitigated should the player want to invest in doing so.

To spice things up narratively, you may want to consider using difficulty upgrades (especially for evasive maneuvers) instead of a pile of setback dice as suggested in the above posts. In my (admittedly limited experience), too many setback dice just leads to a lot of "swingy" rolls and it just becomes frustrating for the players. The challenge dice provides the threat of despairs which will keep things interesting.

To spice things up narratively, you may want to consider using difficulty upgrades (especially for evasive maneuvers) instead of a pile of setback dice as suggested in the above posts. In my (admittedly limited experience), too many setback dice just leads to a lot of "swingy" rolls and it just becomes frustrating for the players. The challenge dice provides the threat of despairs which will keep things interesting.

Also a valid option, though I'd say increase, not upgrade, difficulty unless you feel the need for Despairs.

If there is a chance that the PC could fall or drop their weapon out then give the roll an upgrade, probably in line with the speed of the vehicle (if you have ever been on a fast moving small boat it feels unwise to stand up). Then having the 3-4 setback for cover/aiming as well. On top of that you could flip the odd destiny point for an upgrade or if the target has Adversary there is another upgrade

So a mix of increased difficulty (by 0-1) and upgraded difficultly (by 0-2) and 3 to 4 setback gives lots of options for interesting results as well as rewarding players with appropriate talents and skill.

I'd second both LD and GoM's suggestions, and maybe further suggest a middle ground between the two. Alternate between open ground and hard to navigate quarters through the course if the fight maybe. The former gives them the most open shots and relies solely on increased difficulty, while the latter makes them focus on maneuvers and upgrades the difficulty, giving them a chance to end the chase early but also adding the risk/reward factor of them crashing head first into a tauntaun convoy.

If there is a chance that the PC could fall or drop their weapon out then give the roll an upgrade, probably in line with the speed of the vehicle (if you have ever been on a fast moving small boat it feels unwise to stand up).

Now imagine the small boat doing an upredictable serpentine to avoid incoming fire.

I think it's warranted.