I changed the stock 3.69 ratio for a 3.07 ratio from a 300D turbo. My commute is around 40 miles several days a week and around here it's pretty flat and at fairly low altitude so I thought the rpm drop at highway speed would be more comfortable and not hurt driveability.
The first two gears (automatic) are a little too low anyway and now they are about right. Third is good to about 50 mph (gps) but once it shifts to 4th it struggles to speed. However, once attained, 65-75 mph is fairly easy to hold, even up the (mild) hills and the engine noise improvement is dramatic. Now an indicated 59 mph is a gps 70 mph so you get the idea of the experience. (A speedo change is in the future) I actually think the 0-50 time is improved noticeably but the 0-60 time is about the same (which translates to an abysmal 50-60 time). I'll report on any significant fuel economy change.
Would I do this in a hilly environment or at higher altitude? Probably not. So far I don't regret it though and my car is about due for a valve adjustment and I have a dragging front brake so it may work out even better once those are corrected. I dread summer with the additional a/c compressor drag on the highway though.
Time will tell......
