Guys unless you do alot of miles, the cost differential between petrol and diesel isn't that great.
If you calculate for argument's sake an average of 10k miles per annum, with petrol at say 119p/ltr and ave 30mpg, then diesel at say 129p/ltr and ave 40mpg:
Petrol
10,000m/30mpg = 333 gallons
119p x 4.544 (ltr:gallons) = 540.7p/gallon
333 x 540.7 = £1800 pa = £150 pcm
Diesel
10,000m/40mpg = 250 gallons
129p x 4.544 = 586.18p/gallon
250 x 586.18 = £1465.44 = £122.12 pcm
A difference of £28 per month. Only £6.50 per week or less than £1 per day.
LPG Petrol = £2000 + 30mpg + 60p/ltr (?)
= £900pa = £75pcm
= £2900 for 1 years driving
A difference of £1100pa (£92pcm) more for the petrol and £1435pa (£120pcm) more for the diesel.
Compare a S4/RS4 with only 20mpg, and then that's 'only' a difference of £75pcm with the petrol, or indeed £103pcm with a diesel, and £15pcm LESS compared with the converted petrol car.
Obviously the differences become perhaps alot more noticeable keeping the converted car for a number of years...
Having had 2 1.8T's (a remapped 163bhp and a remapped 190bhp), an S4 and a 1.9tdi (remapped 130bhp), over recent years fuel consumption varies a bit with the 1.8T and 1.9tdi depending how much you use the 'GO' pedal, but doesn't vary massively with the S4 (as it's a massively potent unit not needing to be heavy with the accelerator). Is £75pcm a deal-breaker to have an S4 vs a 1.8T ? Also then consider the 2.4 or 3.0V6 models, with fuel consumption not actually much greater than the S4 (at c24mpg). Makes me wonder why people by the V6 models at all !
Go out and have some fun. Buy an S4 instead !!
PS
Apologies in advance if indeed I have made any arithmetic errors in the above...!!