When upgraded, it partially increases the amount of health you recover over time. It is recommended you upgrade this (as an alternative) if you don't have the "Max Health" upgrade. Try your best to avoid walls and enemies so your health can be replenished.
Mostly referred as an item. When you upgrade it, this will recover a large portion of your health. Recommended to quickly use if you are in a dangerous situation.
Racer, Piercer, Ambulamp, Sludger, Deprived
When upgraded, your max Boost capacity is partially increased. It isn't necessarily recommended because there is an upgrade like this, except it's twice as better, but you can use this as an alternative.
Racer, Flash, Hazard, Star, Deprived, SecretClass
When upgraded, your Speed stat is increased approximately by 1.5%. It is recommended you upgrade this for slow classes (Hazard, Sludger, Deprived) to outrun enemies, or if you're at the last stretch and someone is catching up to you on laps. It is a race, after all.
Racer, Piercer, Flash, Hazard, Sludger, Deprived
When upgraded, it partially increases the amount of Boost you recover over time. This is a better version of the "Max Boost", because you can always generate more boost without having to run out. It is recommended you use this if you run out of boost and desperately need some more.
When upgraded, it decreases the delay time to shoot a Buster/Sludger bullet. However, it is noted that this upgrade actually has no effect.
Mostly referred as an item. When you upgrade it, this will recover a large portion of your boost. Recommended to quickly use if you are in a dangerous situation or if you need to win the race more quickly.
Flash, Star, SecretClass
These are upgrades that only appear in specific types of classes. It is unknown if more will be added.
When upgraded, it partially increases the Buster's bullet damage. It is recommended you upgrade this if you want to defend yourself and/or kill high-upgraded classes.
When upgraded, the Hazard's damage will gradually increase. It is recommended to use this if you want to take out tough enemies effectively such as; Racers, Bullies, Busters, and heck, even your own kind!
When upgraded, the Hazard's body size will exponentially increase. It isn't necessarily recommended to use this as it doesn't increase damage, but you might be able to scare others and make it harder for enemies to dodge you (while they're racing).
When upgraded, the "ricochetiness" will partially increase, making enemies bounce off of you more further each upgrade. This is recommended if there are are lot of people driving awfully fast, because if they run into you (if this upgrade is at least Lv5), they could bounce into someone and/or a wall and perish.
When upgraded, the Ambulamp's healing bullets will heal slightly more than usual. It isn't necessarily recommended to use this unless you're just helping others, but you can actually heal yourself by shooting directly into a wall. However, this is only if you really need to win the race and/or help others.