I'm surprised the LS doesn't have a factory MPG gauge!
I thought most luxo-barges had them back then, meanwhile the MPG-focused economy cars didn't. (Oh, Irony!)
Regardless, since it's pre-OBD-II and the usual options (eg. ScanGauge) won't work, you'll need a DIY MPGuino. You tap into 12v power, vehicle speed sensor, and one fuel injector signal and you'll have a very accurate gauge with useful features.
FYI: I put one in my '90 Miata this summer:
http://ecomodder.com/forum/showthrea...vss-35550.html
Look for more info in the MPGuino sub-forum.