Taller tires = slightly better fuel economy* ... 195/65 R15

[MetroMPG]

So I did a quick 'n' dirty test last week to compare MPG at the stock tire size vs. 12% larger (circumference) tires at the same, steady speed.


Long story short: I saw a 3.4% increase in MPG *at a steady cruising speed*.

I will be the first to admit this testing wasn't comprehensive. (A better test would have had many more runs.) So take this all with a grain of salt, but here are the details and the numbers I saw.


The details:

Last month, I bought a used set of Bridgestone Ecopia EP422 Plus low rolling resistance tires, half worn out for $80. These were OEM tires off a Prius. They're as tall a tire as will fit the Mirage - there's very slight occasional rubbing on the front inner fender liner.

They decrease engine RPM by 12% vs. the stock tire size on the Mirage.

I attached 100% smooth wheel discs on each set of wheels to minimize aerodynamic influences from the different wheel styles.


(Pic actually shows 185/65 R15's on the front wheels only... old tires tested last year.)


Testing conditions & route:

- 26 C / 79 F
- negligible wind
- dry, 2-lane road, 1.6 km = 1 mile test section which is straight as an arrow and almost dead nuts level
- NO other traffic in my lane, and rarely any opposing traffic, so no chance of "drafting effect" or buffeting skewing the numbers


Speed:

- 70 km/h = 44 mph cruising speed (5th gear), monitored by GPS (because the car's speedometer slightly OVER-reports actual speed on the stock tire size and UNDER-reports it on the taller tires.


Measurement:

- ScanGauge II, reset at the beginning of each run.


Results:

- 61.1 MPG (US) ... average of 2 runs in each direction on the oversize tire
- 59.2 MPG (US) ... average of 2 runs in each direction on the stock tire size


* Caveat/Disclaimer/Discussion:

Assuming the numbers are accurate (that's a big assumption), it doesn't mean I should expect ~3% better MPG overall with these taller tires.

For one thing, the EP422 wheel/tire combo are MUCH heavier than the stock combo I used (Miata wheels & Bridgestone RE92). TEN POUNDS per wheel heavier! That's a lot!

I would expect city MPG could even suffer from the extra power needed to accelerate that much more rotating mass. On the other hand, I definitely notice the car coasts MUCH farther than it used to. So if you can make use of that increased coasting distance (eg. approaching turns/stops) it might even out in the end.

But if you're burning up that rotational kinetic energy through harder braking, fuel economy is going to go down.

But since I do ~90% highway driving, I'm not too concerned about the potential city MPG hit.

Aero penalty of a taller tire

The other factor to consider is the oversize tires raise the ride height of the car by 33 mm or just over 1 1/4 inches. That's enough to have a negative aerodynamic effect (generally a higher ride height = increased drag). So if I lower the car back to the stock height, the percent gain should go up even more.

Also, a 30 mm wider tire presents a larger frontal area to the wind, increasing drag vs. the stock 165 tire.


LRR vs. non-LRR

Another point to consider: you probably won't see MPG gains by slapping any old "taller" tire on your car to replace your factory LRR tires.

Rolling resistance is a critical part of efficiency, which is why I specifically sought out a tire known to have good LRR properties.