I agree that it will be a small enough difference that it would take pretty careful measurement to codify.
In principle there are two conflicting conditions in play with tire size and fuel mileage. One is if the tire is taller than original one gains final drive (gear-ish) ratio. The other is an increase in rotational resistance that comes with a taller and/or heavier tire. Where those to lines cross exactly can be left to the smart kids. But when you look at cars designed solely for fuel efficiency they all have the smallest wheels practical. It appears that the tax for resistance is greater than the revenue of ratio.
For folks who give a flip, Avon publishes how many revolutions per mile / km each tire size turns.
The tires on my V2 turn the rear tire 30 more revolutions per mile than does the front tire.
I guess that means that the computer engineers for traction control must be calculating the difference / change of percentage of wheel rotation rather than the the actual "speed" change before activation so as to accommodate non-OEM tires. Like I said - trivia.
I noticed a ~2 mpg change when I went from nearly bald tires to new on my truck. I would assume the more tread increases: rolling resistance - better grip, air resistance - deep tread, rotational resistance - heavier around the outside edge. But a newer tire with new tread should deform less (improve gas mileage) at same air pressure and have a smaller contact patch than a thin / pliable worn tire.
It will definitely decrease your mileage, but hard to say by how much. It makes a big difference on car tires. Look at the special low rolling resistance tires they put on the Toyota Prius for example.
The other thing that throws off the calculation is that speed and distance are not measured directly (unless you use GPS). Instead, both speed and distance are calculated backwards from wheel revolutions (on an ABS bike, which has sensors on the wheels), or even countershaft revolutions (non-ABS bikes).
This means that tire circumference changes (e.g. replacing old, worn tires with new ones) also lead to apparent mileage changes. Ditto for changes to front/rear sprocket tooth count on non-ABS bikes.
On a 19" tire, 5mm thread wear leads to a reduction of circumference by a fraction over 1%.
I do think rolling resistance can make a measurable difference. Changed the rear tire on my 150cc scooter, the top speed (power-limited) went up a couple of MPH, and the average mileage seems to be better as well. More tread, not less, so that's not the sole factor. Since the speedo is cable-operated from the front hub, no change in speedo function. Perhaps more noticeable with less power to work with. But still a factor.
After keeping accurate records for nearly 100,000km, and running the same tyres all that time, my fuel consumption has varied from 3.84 to 5.1 ltr/100km. This leads me to conclude that there are far more influencing factors on fuel consumption other than tyre type. I'd guess that even a reduction in tyre pressure would effect consumption more than a change to a different type.
Really it's academic, just stick on the rubber you most like then go have fun riding the damm thing.
A forum community dedicated to all Suzuki Strom owners and enthusiasts. Come join the discussion about performance, modifications, troubleshooting, purchasing, reviews, accessories, maintenance, and more!