I've seen that mFactory makes a higher final drive ratio gear for the mk6 ST (4.571) for approx $600 USD.

I'm wondering if anyone with knowledge of ratios knows how it would effect the car? Would it make it faster? Is it practical for a daily driver? Would it harm my gearbox?

If I do it, I would install an LSD and new diff bearings at the same time, which requires a fair bit of money so these kinds of upgrades are a while away for me, I'm asking now so I can make a strong informed decision before I make a modification that can't be easily reversed.

It will shorten the gears a lot which will make the car slower if you don't raise the rev limiter and motorway cruising will be horrible.

P.S. The final drive ratio is a multiplier of torque, the higher the number the more times torque is multiplied and vice versa that's what makes the car feel nippier.

Thanks for your help. Looks like it will make the car accelerate quicker but at the cost of terrible daily driving. I think I'll put the final drive on hold until I get a second car (st225 :D) and have the st150 as track only

You could, fit a Puma 1.7 gearbox which has better 1-st and second gear. The Fiesta 1-2 gears have a massive hole between them and that hurts acceleration.

Does the racing puma have the same bellhousing as our cars? So it can be a straight fit or does it need some hacking?