I might end up doing so, would be great to get some proper help and inspect the car thoroughly. I'll even ask if he can set up another tech day maybe

And yeah I get what you mean - they are Toyotas after all! I'm not so much worried that something is broken, I'm just annoyed that I haven't fixed the issue since I purchased it. I've been putting up with the sub-optimal fuel economy for a while now, I can just imagine how much I could have saved for mods if each tank got me that extra 25% or so

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).

Everyone seems to have mixed opinions on whether an N/A should be getting 7-8L/100km or 10/100km. But thanks for your input Adrian. In saying that, please remember that my 10L/100km is best case scenario - in most situations its worse (such as half tank recently only getting me 150km-ish, which is 50-50 mixed highway/street driving)

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.

Yeah it's weird, I went to the gas station today after work to top up as I said I would last night. 154 km since I reset it last when the needle was dead on, or maybe even the slightest bit above half a tank. Driving to the gas station and bam, fuel light comes on.

Surely everyone can't be agreeing "Oh yeah that's normal". Like I said, the 10L/100km was best case for me, and was the only ever happened once. It seems more like 13L/100km at the moment..

Anywho, I'm gonna drive with this full tank for now and when I top up next use the formula Lumix suggested

Another 2 cents:
 
http://www.aa1car.com/library/poor_fuel_economy.htm
 
"If the thermostat does not close tightly or does not close at all, coolant will be circulating while the engine is trying to warm up. This will prevent the engine from warming up quickly, and it may never reach normal operating temperature. This can delay the powertrain control module from going into closed loop operation, causing a rich fuel mixture and poor fuel economy."

mr2y
Another 2 cents:
 
http://www.aa1car.com/library/poor_fuel_economy.htm
 
"If the thermostat does not close tightly or does not close at all, coolant will be circulating while the engine is trying to warm up. This will prevent the engine from warming up quickly, and it may never reach normal operating temperature. This can delay the powertrain control module from going into closed loop operation, causing a rich fuel mixture and poor fuel economy."

Thats actually mighty useful and I feel like you hit the nail on the head.
We were talking privately about how the coolant thermo never reads properly (you were saying it was the same for you when you owned it). But yeah, that sounds almost spot on - my car according to the thermo gauge takes 1.5 years to warm up, fuel economy is crap and there's always that overwhelming fuel smell.

If only I could find a place with a reader for it..

Edit: Apparently you can do a resistance test on it as per the BGB. I've got a pdf copy of the 3S-GE Repair Manual but wouldn't know where to look for the numbers I'd need.. Also, after reading a few threads on MR2OC, it seems like replacing the green coolant sensor has helped heaps of people in terms of fixing their overfuelling issues. Does anyone know the part no.? I figure I'll have to order a replacement from Amayama

May have found a replacement from Autobarn. Price isn't listed, but it looks right - 2 pin, green..

https://www.autobarn.com....rature-sensor-sp112730

Yes you should be able to check the voltage at the ECU temp sensor to see if it's within spec of a properly warmed up engine. A digital multimeter will have an internal resistance high enough not to foul with the ECU.
 
Also, not sure if you're aware, the coolant thermostat is a mechanical device. It opens when it's too hot, and closes when it's too cold. It would seem to me that it's jammed open (a common problem). So with the excessive flow of coolant it's forever going to be too cold, and the ECU will read this as an engine that is still needing to warm up and run rich. It's possible that it's purely a mechanical problem that is throwing off the ECU. Thermostats are cheap as, the big cost is the coolant. Best to collect it first if you aren't up for $60.

This is the thing that's jammed:
 
http://www.ebay.com.au/itm/Genuine-OEM-Toyota-Thermostat-MR2-SW20-3SGTE-GEN-III-Engine-/141833901003?hash=item2105f593cb:g:DLEAAOSwMmBV4vQL
 
It's just a little valve that opens when it's hot, and sits somewhere just above the water pump usually where there's a bend in the pipe. I have no idea if genuine is better, but you can get cheaper ones.
 
Edit: they are a bit more complex than that actually, they are more of a heat valve that varies the flow of coolant, it's not like an on/off switch. Also when they are cold they allow a tiny amount of coolant through a little hole, I think that's to reduce wear on the water pump when it's cold.
 
Edit2: it's kind of an important piece of the engine, it has to be designed exactly right to keep the engine at the right temp. So I would assume genuine is always better. It's one of those things that COULD be made electronically but that would probably be a disadvantage with reliability and cost.