'11 Kia Soul 1.6L 32-36mpg
 

2011 Kia Soul base model with 5-speed and 1.6L engine.
EPA stats: 26/31mpg.

I could only best about 30-31mpg with stock tires 195/65-15 and my aftermarket ones of 205/60-16 (+2.8% diameter). This was driving on the highway at speeds of 60-63mph and over the course of several years, just keeping track with onboard mpg and calculated mpg when refueling.

Modification: Took the old rims/tires from the Mazda CX-5 and put them on the Kia. New tire size is 225/70-16 (+13.6% diameter).

New best: 32-36mpg at 60-63mph. 32 is what the car says. 36 is what I calculate when adjusting for the 13.6% difference.

Reasons for going bigger:
The 5th gear ratio sucks. You can always skip 4th and go straight to 5th. On the highway I am at least 500rpm higher (3500+rpm) than the Mazda when driving 70mph.

Issues:
Tires are nearly bald, so this is a summer-only experiment.
Warmer months skew the data towards better.
Big potholes and square-edge bumps cause tire rub on the fenders.
It is noticeably slower to start off, not as peppy, and requires more clutch on hills. I suppose a good compromise between mpgs and pep would be a 215/65-16 tire.

As an aside, I found the gear ratios on a Kia forum, and the 6-speed Kia used on later models didn't solve the high rpm issue because they also changed the final drive ratio. It would appear that if I replaced my 5th gear with the ratio of the later model 6th gear (it would remain a 5-speed gearbox), then all would be well and I wouldn't have to use larger tires and 4th gear would be useable again.

I would go with taller tires first, before a gear ratio change, but stick to narrower tires like 65 series. Wider = more drag. Then run just below max. tire pressure, then compare mpg.

Check your mileage on a 30 mile trip against google maps and compare. I run a non factory tire size but I found my odometer matches perfectly, but my speedometer is off by 5%. Not sure how that happens. So my mpg is correct when I calculate it.