Just a thought have you tried switching bikes you never know but that would eliminate the possibility of it being the way each of you ride.

 

Are you both getting the same amount of petrol when filling up? How many kilometers per liter of fuel for both bikes?
The low fuel lights are known for coming on early with two or more gallons of fuel still in the tank. Compare kilometers per liter to see actual mileage to know if there's a problem or just an early low fuel light.

 

^ Stated his buddy adds 12L and the OP adds 15L

 

It's the tune. Evidently they are running different A/F ratios

 

Uhhh, we're Americans, you're expecting us to do all of these conversions from kilometers to miles, liters to gallons, for Christ's sake!

 

^ That`s some funny stuff !!

1 k per L = 2.35 mpg

 

What’s the weight difference between the two of you?

 

The comparison of fuel actually used is idea #1.
Switching bikes is idea #2 (as riding style braking, accelerating, shifting) can make a huge different in MPG's.

If #1 and or #2 doesn't resolve the fuel consumption question, it may be time to take it to the shop and have then check it out.