A larger diameter exhaust can make fuel economy worse... You run a serious risk of reducing the effectiveness of the cylinder scavenging that factory exhausts are usually tuned for.
You can get better economy from a well tuned aftermarket engine management system... even better if you tune it to control a modified EGR system which is modified (reduced cylinder displacement
).. EGR FTW!!!
Over inflating your tires, not to the point of distortion, but a few extra psi. Narrower tires would help in a number of ways: rolling resistance, wind resistance, weight.
A 3.73:1 differential (only in '89 and newer 7M-GTE equipped MKIIIs).
Lightweight drive shaft... I'd recommend still getting a two piece given the amount of mileage you'll be putting it through but you can get a lighter two piece fabricated and one that accepts a more common center carrier bearing.
The factory intake is cold air... just leave it alone.
Run a W58 (manual) transmission from a N/A... much lighter than the R154 and shifts smoother too.
Reduce rotating mass as much as possible; lighter flywheel, lighter wheels, driveshaft, smaller alternator, lightweight aluminum flex fan (don't forget that you need to use the correct fan clutch with it too).
Run the LOWEST octane gas you can without any pinging. Lower octane fuel has more kinetic energy. It has more power per unit than higher octane... BUT it's a balance; If you go too low you'll get pinging and the ECU with pull spark advance costing you power in a different way.
Lay off the boost.
Remove any spoiler from the car, including the factory one. They all increase the drag coefficient of the car. smaller whale tails may actually serve to reduce it but unless you can find actual wind tunnel test results, the odds are that it was just made for looks and isn't going to help anything else... it's either just more weight or it's more weight AND more drag.
Driver at speeds under 60MPH. Resistance to motion increases almost exponentially with the object's speed; It doesn't matter if you're not gunning it, 65MPH is going to cost you a lot more than 60MPH.
The aftermarket EMS (and running the right octane) is going to net you the biggest gains; If you keep a functional EGR system (stock or beefed up) you may run as lean as 16.1:1 during smooth cruise conditions and the engine will still idle smoothly at about 15.1:1.
The best engine to do all of this on is a 1JZ-GTE, IMO... Although, I've seen about 28MPG (IIRC) on a 7M-GE with most of these mods. I was stupefied by the gains. It was on a VERY long, two way road trip... two tanks of gas to compare results with... no city driving at all though.
Back to the exhaust thing; Replacing the cat, resonator and muffler with more free flowing / high flow components is GOOD. The stock resonator and muffler are baffled and HEAVY!