rebel son

i need some help about my bike. ive got a gz250 and when i ride i come form dead stop and go thru all the gears and at fifth gear im pretty much maxed out about 50 if i go to 55 it will just rev becuase its gone thru all the gears and its killing my mpg and thats being filled to the tip tip top like that fine line before spilling out but enough room for it to breath and i do that just to make sure ive got my moneys worth. so if theres any ideas or reallitys that you have or had please be my guest and share. thakns