What fuel economy are you currently getting from the TQ? This will help you work out if you're going to make a saving.

You've probably done this already: if you're doing 50 miles a day, then you're working week commute is 250 miles. Suppose you're getting 30mpg from your TQ; that means you're using 8.33 gallons per week. Multiply that by 4.54 to get litres (37.83) and then multiply again by the cost of unleaded (say £1.12p) which gives you a weekly total of £42.37 commuting costs.

Now, in an A3 8P 2.0TDI 140 over the same motorway-based distance, I think you're likely to see almost 50mpg but lets say you only get 40mpg (as a total worst case estimate). Same calculation: over 250 miles, that's 6.25 gallons or 28.38 litres, and assuming diesel is around £1.22p a litre, that's a weekly cost of £34.62. So it is almost a £7 saving per week. 45mpg in the diesel would be £30.77, and if you were really light-footed and got 50mpg, that's gonna be £27.69 or a saving of nearly £15 per week. Pretty good.

You get the idea - adjust the mpg and fuel prices to those that are relevant to your car and area and you'll get a decent estimate of what savings you might make. Diesel is more pricey than petrol but the difference has really got to climb before you'll ever save money with unleaded, although my Prius does ... but that's another story!