42/38mpg in the two TFSI Audi's, compared to 22mpg out of the Mondeo is significant. As is are the emissions of 158/178gkm compared to 242gkm from her mondeo.
If you are getting 22mpg from a 2.5 litre Mondeo there is absolutely no way on this earth you are going to get 38mpg from the Audi 2.0TFSI on the same usage profile. It simply wont happen. It will be considerably less economical than that by quite some margin. 22mpg from a 2.5 litre Mondeo suggests most of the usage is around town - where you wont get 40mpg from a 2.0 Turbo Petrol Audi.
You really need to make sure you are calculating this sort of thing properly because you are going to be bitterly dissapointed with the fuel economy if your new car if you are not careful.
How much I spend on a car bares no relationship to how much I wish to give the taxman for the priviledge of driving it.
Well it does, because it simply doesnt matter compared to other costs involved, does it? It's 200 quid a year, its nothing, especially when you consider a car at the upper end of your budget is going to cost you that £200 every *WEEK* in depreciation. Forget about the road thing, it doesn't matter and most newer cars of the sort you are considering are going to be about the same or less to tax anyway. Just stop thinking about it.
You'll be paying £6000 to the taxman in VAT anyway if you spend £35k on the car
Besides which, if she wanted a Mondeo, she wouldn't have said she definitely didn't want one. She doesn't like it, neither do I. Why would anyone pay 25-28k for a Ford when you can buy better for a little more.
Errr I'm not suggesting you buy a Mondeo, I certainly wouldnt with your budget!
I think you both need to sit down and work out what type of car you want and what sort of engine it should be powered by first. How many miles does she do a year? How much of a big deal is fuel economy really - does it actually matter or do you simply think it matters?
I certainly wouldnt be chosing between a 320d, a 520d and an A5 2.0TFSI. If these are cars you are interested in you need to be considering:
325i/330i
530i
A5 2.0TFSI (Or 3.2 FSI if they still do it?)
OR
320d, 520d or A5 2.0 TDI.
Or perhaps a bit less economy and a bit more refinement (But still not as economical as a 2.0 diesel nor as refined and quiet as a petrol
330d
530d
A5 3.0 TDI Quattro
These will be stretching the budget if you are buying new, but I'd advise against this anyway as mid £20k's would get an absolutely loaded 12-18 month old example of these better engined variants.
If you can get away without the noisy diesel then great, the petrol cars will be quieter, smoother, more refined and generally more suited to the sort of car you are looking at. But they wont be doing 40mpg around town. At all.