Some off-the-cuff numbers. Let's say it costs $3,000 to have things converted. This is about the cost for an LPG conversion in the UK, though that might be an

Some off-the-cuff numbers. Let's say it costs $3,000 to have things converted. This is about the cost for an LPG conversion in the UK, though that might be an underestimate, as while LPG requires a bunch of stuff installing the engine still works in fundamentally the same way. How many gallons (x) will you have to buy before you've spent the same in both cases, and what will the cost have been (y)? Assume $4/gal petrol and $2.50/"gal" gas. 4.00x = y (petrol) 3000 + 2.50x = y (gas) So, 4x+-y=0, 2.5x+-y=-3000; 4x+-y=0, 4x+-1.6y=-4800; -0.6y=-4800; y=8000. So x=2000. At 20mpg, 2000*20=40,000 miles. That's quite a lot of driving. Don't most people change car every 3 years? And by that point you've only just broken even.