i got a 91 civic with a 5spd and i use it for work since i drive 50 miles one way it get 35 mpg and i want 40+ so i figure if i get a taller 5th gear that can go about 70mph at 2500 rpm ill get over 40mpg. just wondering who makes a product like this or what other cars may have this 5th gear....

 

Is it a dx or an si? if you have an si, you could run a dx trans or an hf trans to drop the rpm's in 5th