does BACK-PRESSURE increase MPG vs straight thru...

[07WYarisRS]

The driver affects the mileage way more then any muffler will

Its the ECu's job to control the fuel system , using the MAFS and O2 sensor readings it will comtrol the amount of fuel needed to maitain the same fuel air mixture...
If one did modify the exhaust so that the engine combustion scavenging is greatly effected and fuel is being waisted... the o2 sensor will pick up the overly rich combustion gasses leaving the cylinders and the ECU will alter the fuel and timing to help control it to maintain the factory set air fuel ratio. Sure this may cost you a MPG or two at the most but, I have found in most cases everything kept the same just freeer flowing muffler actually improved MPG. its not until you get into different intake designs, and complete exhaust systems that you see major mileage changes

Changing the exhaust will slightly alter how efficiently the engine will combust the fuel mixture but on a car with an exhaust system like the yaris (these things have what 3 cat convertors?) VVT-i, the elaborate ECU system the effects from a free flowing exhaust will be very small.

The biggest issue with an open exhaust is the driver...
As a motorcycle mechanic I'm asked this daily and deal with this everyday.
Why after changing to louder pipes and re-jetting does it use so much more fuel...
The answer... the operator.
With a quiet exhaust people tend to ride it or drove it much easier.. why because there is nothing impressive there... however with a loud exhaust people like to HEAR that exhaust and 90% of the time accelerate much harder and to much higher RPMs...
I have explained to many customers that before thier exhaust they would putt putt through town, now you can hear them honkin on it 4 blocks away...
You don't hear the guys on the Stock Hondas and Harleys flogging on it WOT through the gears like the guys with open drag pipes or fart can exhaust....

I challange anyone to drive /ride the same way with both quiet and open exhaust and you will usually find a very small change in MPG.. I understand there is no fun in that though.

I rode my GSX750F for a year with no problems, got 2 speeding tickets within a week after my new Hindle race pipe.... My mileage on the hwy actually went up slightly if I maintain a steady cruise speed, however now that i find myself shifting at red line or higher rpms a lot more fuel mileage suffers a lot when i'm accelerating like a tool...but man o man the pipe sounds sweet.