I've got an 98 Ex manual, and since I literally only do highway driving I am wanting to swap to a manual DX,HX tranny for the taller gears. I know it's more then likely plug and play, but would i have to re calibrate the speedometer?

 

no you wouldn't ...but still not worth it imo though

 

All of the 6th gens have lower gearing than the ex. So really your gains would be really minimal. Your best mileage gains will be had from low rolling resistance eco tires fully inflated properly and hx light weight wheels

The hx is a package of light wieght wheels, a low compression lean burn motor, and a mix of other sneaky light weight tweaks. The rear drums were lighter, the flywheels were slightly lighter,

I really think at best you would gain 1-2mpg highway at best if you drove 100% highway all the time. Seriously though get a set of hx wheels and inflate the tires to max sidewall pressure for best results