So it does't hurt mileage.
If you have your mileages from intown and out of town see if you pull it out what happens.

I had a person that put one in his VW VR6 and got 2 miles per gal better so he went from 28mpg to 30mpg about 7.1% Hwy.

So on a Disco that gets 13 mpg intown it would be .92 better not much to see.