Probably 10-12k
With petrol at an average of £1.35/litre, and LPG at £0.79/litre, plus taking a typical LPG conversion at £1500, and an average fuel consumption of 24MPG, it would take around 18 months for you to effectively pay for the installation costs of the LPG system. That's 18 months before you see any savings at all.
You'll need to bear that in mind, how long were you thinking of keeping the LS400 for once converted?
Hmmm - thanks for that. I'm anticipating at least 12 months. But presumably the car will be worth a little more if it is LPG converted so hopefully I'd get back a reasonable proportion of the cost. I guess the issue is would someone buy an LPG 400 with over 200,000 miles on it.
Sorry, but in my head that makes no sense at all.
In 12 months time, you'll be around £500 worse off if you convert the car to LPG, than if you continued to just use petrol.
Even if you recoup the £500 difference through selling the car, you're still no better off, so I don't see the point in going to all that trouble for zero benefit.