hi
Remember the us gall. is smaller.
My WJ V8 uses twice as much fuel local as compared to open road. In fact on open road it is nearly the most economical car I have owned. And it is the 4sp as I wont use my spare computer to upgrade the auto too a 5 speed.
My SJ used to do on the open road about 18mpg AUS. on lpg but the lighter ltd 351 could not match it on lpg . LTD was a ton lighter.
My 4cyl. Mazda work ute on LPG was the worst car on fuel I have owned.
Gave up trying to tune it and Iam a Gas fitter. Mate had a customer with the same problem.
But you are moving a brick around, and aerodynamics don't work when your idling in traffic.
Best of luck some vehicles luv more air in tyres.
My WJ on a trip will actually do as well as my Mum's Patriot for the same trip.
But in town it kills the V8.
Holden once did a fuel economy run from Adelaide to Melbourne I've been told and could not publish the results. The V8 statesman out performed the 6 cylinder back in the blue motor days.
Peter