Well, you'll see somewhat better highway cruising mileage but beyond that it depends largely on your driving style. Your city and aggressive driving mileage will most likely take a hit as your acceleration will be worse and you'll have to apply more throttle or you'll have to stand on the throttle longer to reach any given speed. If you're a laid back, 'I'll get there when I get there' sort of driver and you want a mild improvement in mileage then go for it. The mileage improvement isn't huge. The LSD is definitely worth having over the open diff in most cases but you might want to see if you can find a 4.30:1 LSD from another N/A first.
Your speedometer and odometer will be off unless you swap in the speedo drive gear from an auto equipped MKIII. The speedo drive gears from the W58, the R154 and the A340E are not interchangeable.
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