Hi,
I have a '98 MKIV LS400 with 214k on the clock.
On a recent run to Gatwick, I averaged 35.2mpg door to door (inc. return).
Around town, I get 17-19mpg, which I have recently checked with the litres vs. miles calculation as opposed to relying on the on board computer.
I am fairly new to Lexus myself, having only owned mine for 11 months, but I must say, it's much better than any of the 7 series BMW's I owned up until now, and I never thought I'd say that :)
Go for it. I will, hopefully, someday upgrade to a 430.
Thanks
Mike