There's three things I can think of, but I'm not sure which one will do it, maybe all these will somehow...

Projectile, as in Specialeffect>projectiles>ArrowDaedricProjectile for example. And altering the speed, gravity and range there.

Or

Settings > fArrowSpeedMult & fArrowMinVelocity/Power. Also there's gravity settings.

Or

There was an Ini. line you could add which affected arrows, what I have atm is

fMagnetismStrafeHeadingMult=0.0
fMagnetismLookingMult=0.0
f1PArrowTiltUpAngle=0.3
f3PArrowTiltUpAngle=0.3

Under combat.

If I just wanted to adjust the speed, gravity and range which would be the best to alter? Note this is just for a personal mod, not anything I plan to release.

All of them would do the trick I guess, but I'd use first option, because it allows you to set exact numbers. Also because I did this myself and it worked.

Ok thanks. I didn't want to alter all of them, just the one that would give the best results. Altering all of them would probably result in supersonic arrows, which is actually an option in projectiles, must be from FO3/NV.