Speeders bikes in light cover (tile) would get what cover save?

"Various game effects, such as suppression tokens and the cover keyword, can improve a unit’s cover by one or more.
» When a unit that does not have cover improves its cover by one, it is treated as having light cover.
» When a unit that does not have cover improves its cover by two or more, it is treated as having heavy cover.

» When a unit that has light cover improves its cover by one or more, it is treated as having heavy cover.

» If a unit has heavy cover, that unit does not gain any benefit from improving its cover."
Rules Reference, p. 29-30.

Cover itself does not stack, but it can be improved by keywords like "low profile" "cover X" or a supression token. Since speeder bikes have the "Cover 1" keyword they would increase any kind of light cover to heavy cover.