Never ended up buying any, I kinda assumed they weren't the issue / don't wanna fix what's not broken

I almost always run it till like 1 tick left (but not fuel light) then fill up with 98 and that'll get me 400-415km.  Always reset the trip meter as well. So I'm guessing if it let it run completely dry, maybe I'll get 430-445km max?

I feel like mr2y is on to something regarding the cat however so I'll pop into an exhaust shop.  I've kinda done everything I can really.. apart from replacing the O2 sensor and spark leads. Even if the O2 was dead, I should be getting a CEL

mr2y
Did some more research because I'm interested myself, yes a blocked cat causes more back pressure and causes some engines to run richer to compensate (not sure why exactly). Also, good exhaust shops should be able to test it. If you can get a readout of the O2 sensor you'd be able to see if it's running rich 100% of the time, but I'm not sure how you would do that, sadly Mr2s dont have ODB.

Yeah my mechanic said good luck finding a reader for it haha

I asked for some info on the MCM Brisbane page regarding inspecting a suspected blocked cat, how much a replacement should be, etc. and all the feedback I've gotten is "smash the honeycomb out". I swear I'm one of the few people my age that actually cares about car integrity

I'll fill the car up tomorrow and run it over the next few weeks or so till the fuel light comes on next. I haven't used that actual formula to figure it out, I've just kinda guessed it. I run the trip meter, it gets to roughly 400km give or take 10km every time before I top back up (which is usually 40-ish litres, depends how low I let it go). 

Just by looking at the numbers and guessing, I can say that my best is usually 10L/100km, but I'm usually getting worse than that (previously filled up to half tank on the spot, been driving to work the past few days. Trip meter says 150km and it's already used almost all the fuel put in, hence I have to top up tomorrow)

Hoonsy,
you need to take your car to a mechanic, experienced with MR2's, that can check over your car, replace what's worn out and tune it correctly. We have two experienced mechanics and MR2 experts in Brisbane Club (Scope & Nate) who you could contact.
You should be getting 165 - 175Kms per quarter tank ie approx 550 to 600 before fuel warning light comes on. On refill once light comes on you should need 40 - 44 litres. These figures are for suburban driving. That's around 7 litres per 100Kms. On an open road trip you should be able get <6 litres/100Kms using 98 Octane. Falcon from Qld Club has been able to get down into low 5's!!
cheers
Steve

Hoonsy
Lumix: Oh fair point. I'll give it a go then and see how I go? I'll keep that formula in mind, thanks buddy

Groper: Trust me, I'd honestly love to get one of the boys to get a proper look at it. I remember seeing that "tech day" but couldn't go - would love for another opportunity like that to swing by. Just not sure if I could afford proper professional help either unfortunately.. I mean, I can afford the check up/inspection at my mechanic, and he's good, but he doesn't seem to do a thorough enough job as it's more of a "car in, car out" kind of workshop (your average workshop I guess). 

You should send Scope a PM and see if he can help you out Hoonsy.

I think because you do all short trips in city traffic, that is why your fuel economy is the way it is. 

I know my car had a worked head and an exhaust and intake, but it was still averaging around 400km in city driving. And lets be honest, we do like to put our boot into it every now and then. 

On my trip down to Sydney, I got nearly 600-700km out of a tank (cant remember exact figure).
It may be more conditions based. It's easy to worry about things with these cars, but in my 4 years of ownership I have learnt that they will run for a looooong time as long as you keep changing the oil haha.

I flogged my old engine every day of its life and it still made 130RWHP.

For city driving your fuel usage sounds perfectly fine. 40L for 400km = 10l/100km which is normal. Nothing majorly wrong and you probably won't find one magic "fix" to save you heaps of fuel.
 
Usage is also highly condition dependent. For straight long distance highway driving you could expect the 600-700 km Eric was getting from 50L (or around 7l/100k). On the track I can polish off a tank in 150km (+30l/100k).

In my experience newer generation engines (late nineties on) get far better economy city driving. They figured out all sorts of tricks with cam timing, EGR and closed loop stuff. I had a 2008 Corolla that would get 8L/100k with stop start driving only, sometimes I'd see 500km on a tank doing city only which is extremely cool IMO. I wouldn't dream of getting better than 10L/100k city driving in an SW20. But still, it does sound slightly high by the figures you are giving Hoonsy.