We need to put our bike in fifth gear.

[tufguy]

1) The bike is going too fast. Bring a gear down.
2) The bike is going too slow. Bring a gear up.
3) We need to put our bike in fifth gear.

Please check my sentences.

 

[Tarheel]

In American English we "shift" gears. So I suppose you could shift up or down (to a higher or lower gear). Or shift into fifth gear.

 

[bhaisahab]

In British English, we generally "change gear" up or down.

 

[5jj]

Are you riding a tandem, tufguy?

 

[Barque]

1) The bike is going too fast. Bring a gear down.
2) The bike is going too slow. Bring a gear up.
3) We need to put our bike in fifth gear.

Please check my sentences.

You don't necessarily have to shift down to slow down. It also depends on whether the bike can cruise in more than one gear.

1. You're going too fast. Slow down.
Or
You're going too fast. Shift down.

2. You're going too slow. Speed up/shift up.

3. Shift to 5th.

I assume the speaker is the pillion rider.

 

[emsr2d2]

1) The bike is going too fast. Bring a gear down.
2) The bike is going too slow. Bring a gear up.
3) We need to put our bike in fifth gear.

Please check my sentences.

1 and 2 don't work at all. They use completely wrong terminology for changing/shifting gear (in/on any vehicle). Sentence 3 is fine as long as, as 5jj said, you're riding a tandem (a bicycle where two people pedal). Even then, only one of those people is responsible for the gears.
What's the context for this unlikely sentence? Who's talking? Who are they talking to? What are they riding?

 

[tufguy]

Are you riding a tandem, tufguy?

No, I ride my motorcycle.

 

[tufguy]

1 and 2 don't work at all. They use completely wrong terminology for changing/shifting gear (in/on any vehicle). Sentence 3 is fine as long as, as 5jj said, you're riding a tandem (a bicycle where two people pedal). Even then, only one of those people is responsible for the gears.
What's the context for this unlikely sentence? Who's talking? Who are they talking to? What are they riding?

It is about two friends that are riding a motorcycle.

 

[emsr2d2]

It is about two friends that are riding a motorcycle.

If it's a standard motorbike, only one of them is in charge of the gears. The other one is riding pillion. If the person in charge of the gears is speaking, they'd say "I need to change gear/I need to put the bike into fifth [gear]". If the pillion passenger is speaking, they'd say "You need to change gear/You need to put the bike into fifth [gear]".
Using "we" doesn't make sense. They can't both change gear on the same motorbike.