On or around Sun, 17 Oct 2004 18:14:06 +0100, "Paul S. Brown" enlightened us thusly:
The payback on a typical car is something in the region of 15-20K miles. If you do 15K per year, for example, and keep it for 3 years, you'll achieve savings in years 2 and 3. ASSUMING the price doesn't go up much WRT petrol. Costs for fuel are about the same as diesel, like for like, (e.g. 3.5 disco vs. TDi) with the proviso that the diesel doesn't quite have the power of the 3.5 unless you tune it up a bit, which will make it slightly less economical.
you can add in that like-for-like, a V8 disco (say) in the 1-3K bracket costs quite a bit less than a TDi, in terms of vehicle per buck.