One thing I find funny about mileage and Lucas UCL, is that some people get gains, and some get losses in mileage.
Personally, and I haven't figured out 'exactly' why, but I get worse gas mileage with the Lucas UCL in my two vehicles I've noted using it in. A 1994 4.6 V8 HO Thunderbird and a 1986 F-150 5.0 EFI. In the car, I could always tell the Lucas made it run a little better. Felt better in the lower RPM range, in the truck, I can't really tell much difference at all. Maybe when the truck is fully warmed up, it seems to have more power than normal, but that's about all I can tell. The Thunderbird, though, definitely used more gas. I still like the product and the idea behind it, though.