Wheel size and MPG

[Evil D]

My logic tells me that a taller tire should equal better MPG...since it cuts down your gear ratio (sort of), correct? Well my first tank of gas netted me 32mpg, and right at the start of the 2nd tank i put on the 13s and have gotten a solid 2-4mpg less every tank since. Is this just my driving habits or could it be related to the tires? Would it be worth putting the 12s back on for a tank for comparison sake? I know there's something to be said about wheel weight and rolling resistance but i would think a taller tire (even being wider and heavier) should still yield better mpg.

[JPT]

Do you do a lot of stop and go driving?

[sasquatch]

the bigger tires only help when you do alot of highway driving... in the town it takes more HP to get them rolling where the 12s took less...

[festiva351w]

well did you calculate that with bigger tires ur odometer will record less than what u actualy went, meaning that the bigger tires go around less times than a smaller tire in the same distance, also it will make ur speedometer off, if you have a gps it is easy to calculate, i put bigger tires on a truck and it changed the speedometer reading about 5 mph at 60mph, and in stead of going only a 100 miles i actually went 110 miles,

[kartracer46]

I switched back to my 12's from 14's and noticed a huge difference. Sasquatch is right, it definitely adds more strain to get the 13's rolling. Once the mass is in motion the 13's would prevail in overall circumfrence, but you have to be able to keep it in motion. Or you can do a BP swap and add the HP in to overcome the extra energy it takes to make them roll.

[blkfordsedan]

I recently switched from the 155/80R12's (21.7" OD) on stock steel wheels to 175/50R13's (20.0" OD) on Mazda 323 alloys. For some reason, my speedo was about 5MPH slow with the stock 12", so when I went to the shorter 13" my speedo is now dead-on accurate.

My average MPG increased as well - from 42-44 to 46-48. The reason is because the engine is now running just into the power band (2500 to 2700) at highway speed vs. just below the power band (2300 to 2500) with the 155/80R12's. I don't have to press down on the gas pedal as much to maintain speed when climbing hills. It is also a lot zippier around town in stop-n-go traffic.

It is true that it takes more HP to get a larger tire rolling, based on the mass they have more inertia. But on the flip side, taller tires actually have more potential energy when rolling because the center of mass is higher. IMO, the real advantage of the shorter tire is the change in the final drive ratio. The result is better matching to the engine's optimum operating RPM range. Plus, the lever advantage (force X distance) that the ground has to resist the rolling wheel is decreased due to the shorter radius. That results in a more efficient transfer of power (or less of a loss of applied torque) to the ground for a better "seat-of-the-pants" feel. There is a reason the Aspire trans has a lower final drive for use with the taller tires!

[Evil D]

Do you do a lot of stop and go driving?

About 50/50, with a couple 30 mile trips now and then to the junkyard per tank. A tank usually lasts me about 20 days though because i only live 4 miles from work.

well did you calculate that with bigger tires ur odometer will record less than what u actualy went, meaning that the bigger tires go around less times than a smaller tire in the same distance, also it will make ur speedometer off, if you have a gps it is easy to calculate, i put bigger tires on a truck and it changed the speedometer reading about 5 mph at 60mph, and in stead of going only a 100 miles i actually went 110 miles,

I think you nailed it...although it may also be the weight/rotating mass issue as well...for some reason i completely forgot about the effects of the odometer.

[WmWatt]

Switching from steel to alloy rims should improve fuel efficiency. I work on bicycles where small weight savings in the wheels are the best way to improve peddling efficiency. Switching from 27" bike wheels to 28" isn't as noticable. Another big change comes from cleaning and repacking the wheel bearings. Tread and tire diameter are also noticable. People who ride mountain bikes with knobbly tires on pavement expend a lot more energy than people who ride road bikes or hybrids with little or no tread on thin tirer.

[Evil D]

Yeah rolling resistance and rotating mass.....i guess if i put my bald 12s back on, being smaller/lighter/bald they should get me better mpg unless i hit the highway..lol. I think i'd rather be safe in the rain and give up a few mpg's.

[Evil D]

Just FYI, my last tank i ran on the 12s and ended up doing a lot more highway driving than i normally do...so i don't know if 12s are hurting or helping because this tank turned out to be 36.07mpg which is my best to date. That's also after doing a full tune up so i'm sure that has a lot to do with it also. After talking about shift points, i'm gonna run this tank out shifting at a lower mph than usual and see what that gets me.