Larger Tires to Increase Gear Ratio

[comptiger5000]

My Jeep didn't really do much mpg wise (within measuring error) when I stepped up from 225/70R16 to 245/70R16 tires (yes, I corrected the speedo). It dropped me from about 1825 rpm at 60mph to about 1760 rpm at 60mph. It definitely didn't hurt though.

However, keep in mind, with a 360 in the Jeep, it's still very over-geared, and will climb 7% grades (with 2 people and a couple hundred pounds of crap in it) at 60 with the cruise set without a downshift.

[abell75]

Originally Posted by comptiger5000

My Jeep didn't really do much mpg wise (within measuring error) when I stepped up from 225/70R16 to 245/70R16 tires (yes, I corrected the speedo). It dropped me from about 1825 rpm at 60mph to about 1760 rpm at 60mph. It definitely didn't hurt though.

However, keep in mind, with a 360 in the Jeep, it's still very over-geared, and will climb 7% grades (with 2 people and a couple hundred pounds of crap in it) at 60 with the cruise set without a downshift.

Unless Wikipedia is wrong, the first number is the tire width and the second number is the height, expressed as a % of the width. So in the example you gave comptiger, your tires got 20 mm wider and 2*20*.7 taller. Wouldn't the wider tire cancel out some of the gains?

I planned on going from 225/60 (stock) to either 225/65 (3% increase) or 225/70 (6% increase). Width remains the same, but the height is increased.

[darcane]

Originally Posted by Big Dave

Pickup truck guys try this all the time and invariably it backfires and they get WORSE MPG, even after speedo error is corrected for.

.....

Empirical data shows thar gearing works and bigger tires are counterproductive.

Invariably?

I saw noticeable gains (~3%) by switching to taller wheels/tires on my truck. Steel 16" wheels to Aluminum 17" wheels with 5% taller tires. Overall weight was the same, but taller, heavier tires means the moment of inertia still increased.

I can see it go either way, but it certainly helped on my truck.

[comptiger5000]

Abell - Yes, I went both wider and taller (.75" wider, 1.1" taller). That may have negated some of the benefit on the highway, but with a tall riding vehicle with a pair of big solid axles underneath, aero drag from wider tires isn't exactly significant, relative to the rest of the undercarriage, etc.

[abell75]

I wasn't thinking aero, i was thinking RR/contact patch geometry. Maybe my assumption that narrow tires have better RR is incorrect....I'm fairly new around these parts

[roosterk0031]

On my car I felt the 215's it came with were too wide sacrificing snow traction, I went from 215/60/15(129mm high sidewall) to 205/65/15 (133 sidewall) 1% taller tire, I did it a tire or 2 at a time so never saw any FE change. But snow traction is better using the same make model tires. I may go to 195/70 next depending on what tires I find.

Some shops won't mount anything but factory size, I carry mine home and mount them myself. Other option is to carry in wheels, they might mount anysize that way.

[SuzukiSteve]

I went from 155/80R13 tires to 155/80R15 on my suzuki swift. So taller but just as wide. I went from ~42 to ~47 highway mpg. But as others have said it depends heavily on your cars gearing. My car still revs higher than I would like on the highway.

[taco]

On my se camry stock is 215/55r17' but mine came with Trd wheels so it got 225/45r18. Which are smaller than stock. 1.3% too small. So when they wore out I got 225/50r18. 2% larger than stock.

Mpg didn't chage a whole lot. I have noticed about 1 mpg increase on trips but if my wife drives it,'it spends more time idling then driving But this car turns 2000rpm @63 mph.(stock)

No on my work truck it has 4.88'gears and with 31's it turns 3k at 65'mph. I plan on upping to 33x10.5r15'and this should drop a few hundred rpms.

[abell75]

I checked on the way home tonight - I'm at 1950 rpm @ 60mph. 2250@70mph. So gearing down further may not help much...

[d0sitmatr]

Originally Posted by cfg83

Big Dave -



Can you tell me the typical before/after tire size change you are talking about? Can you point to a non-ecomodder thread on this?

CarloSW2

I can do this, as I experienced exactly what BD was talking about.

my stock tire size on my Ranger was 225/65/15 on steel rims.
I changed my rims and put on a set of aluminum 15x10 sport rims, with tires sized 255/70/15 a difference of about 8.75% increase in diameter, from 26.5" to 29.0"
initially saw a slight increase in mpg on the hwy, but a decrease in any sort of driving I had to do in town. but also on the hwy, my truck would need to downshift from OD into drive, and sometimes even down to 3rd, just to go over a simple overpass.
then I dropped to 255/60/15 tires, which brought the diameter of my tires within 1% of stock diameter, and both my hwy and my in town mpg increase by a fraction (this was all going by tank fill ups)
the truck also would no longer need to drop out of OD to go over an overpass.

average tank with stock: 240-280(best) miles mixed
average tank with the 29"D was about 240-300(best) mixed
average tank with 27"D was 260-340(best) mixed.

that was with no real hypermiling techniques used (other than timed lights and drafting, which Ive always done)
once I sarted really paying attention to my driving habits, I was able to get close to 400 miles from my tank.

so I would say your best bet overall would be to only go up 1 size (if running 50's, go to 55's) if possible, go a little skinnier (if running 225 try and go 215)
and look for LRR tires (Low Rolling Resistance)