What are the cons of doing this?. Thinking of either 3.70's or 3.90's. I know that the 3.70's won't change much.

I am concerned with the added increase in engine wear. This car is planned to be a long term keeper.

I usually cruise at only 5 to 8 mph over the posted speed limit on wide open freeways.

I do like to get to speed quickly.

No track, just a weekend car.

I would like to hear from those that have made the swap.

 

I don't need one in my car to tell you what to expect.

You will have more torque to the wheels in every gear, with the drawback of less MPH out of each gear. Your cruising RPM in 6th gear will increase (meaning poorer fuel economy and yes, a little more engine wear). It won't drop your top speed since you will still have enough gear to reach the limiter, you'll just reach it faster. You might drop a few tenths off your ET if you can still hook up with the increased torque in first. Traps should remain pretty much the same. Personally... I'd rather have more power than shorter gears, but that's just me.

 

I just picked up a set since I'll be doing my diff build soon. From what I've read you can expect about 10% quicker acceleration with the 3.9 and for me that's worth it, especially in the lower gears.

 

Shorter FD gears actually reduce stress on the engine. Think of it like pedaling a bicycle. In first gear, you could pedal your legs really fast and easily, and your tires wouldn't rotate to a high speed, but it'd be effortless to get you moving. Whereas if you tried in a higher gear, you had to push a lot harder to move, but it took less pedal rotations to get to a higher speed.

Same concept applies to the engine. Your engine will spin at a higher engine revolution, but it'll spin more easily and freely, reducing the stress to the internal components such as the rods and crank. Yes, you'll have the higher wear of the increased engine speed, but it won't have the same stress of fighting against the drivetrain to get the wheels moving.