Larger Tires to Increase Gear Ratio
 

I'm considering larger diameter tires for my Grand Prix next time I need to buy them. Reason would be to reduce my engine RPM at cruising speed (I have plenty of power and want to get closer to optimum BSFC on the freeway when I need to maintain a constant speed instead of P&G.

My speedometer will not be accurate. I'm ok with that.

But if my diameter goes up 2" then my ride height goes up 1". Will the height increase cancel out some/most/all/more than all of the ratio improvement? Maybe it just means I need that underpan mod as well! ;)

Thanks,
Aaron

It depends on the vehicle. If the gearing is way too low, then yes, it will typically help. However, if it's not geared all that badly to start with, then the gearing change may not overcome the aero penalty.

What do you turn as far as RPM at 60mph?

You must also take into consideration a few other things...one is increased mass of the tires, suspension travel clearance, alignment, steering radius.

You also need to consider how much room is in the fenderwells. Rememeber the worst case is with the suspension fully compressed and the sterring fully turned. Any rubbing could cause a very bad situation.

Plus, you need to be aware that tires can vary widely in rolling resistance, so any change you might make could be completely overwhelmed by the effect a different tire has.

you should have no problem with fitting one or two sizes larger, unless you have lowered the car and then how much.mine is lowered about one inch and i still run one size larger with no problems.

I think it's hard to know in what cases it'll work and in what cases it won't. For example, my Tempos like the gear-up a slightly taller tire gives, while my F150 won't stand for it.

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

Wheel/tire assemblies are flywheels. The rotational moment of inertia goes up with the square of radius.

Everytime you accelerate away from a stop (even if you accelerate very gently) you have to pour energy into those flywheels in order to spin them up. Guess where that energy comes from?

This is why vehicles with bigger wheel/tire diameter tend to eat brakes.

If you could drive without ever starting or stopping, accelerating or decelerating bigger tires would help, but that ain't the real world.

Maybe we need a sticky to address this evergreen fallacy.


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

Ooo, I wish I could do a good test on that right now.

I do believe my Tempos, '59 Bel Air, Corvair, Bug, and Bus all had improved fe because of taller tires, but I have no fuel logs or decent testing for that.

Thanks for the posts everyone; it seems this one is not as clear cut as I'd assumed.

My reason for changing was to get closer to BFSC at highway speeds, but maybe that slight improvement would not compensate for all the other considerations. The GP runs about 2500 rpm at 70 mph

Big Dave, I understand the flywheel concept and agree that getting to speed would take more energy; conservation of momentum means that, unless I'm braking, I'll also coast further due to the additional energy in my wheels/tires.

It sounds like the verdict is out on whether or not this approach improves MPG....

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

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.

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.

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.

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 ;)

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.

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.

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.

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

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)

d0sitmatr -

Thanks. This is egg-zactly the kind of feedback I was looking for. I guess it's the law of (quickly) diminishing returns. It makes me feel I was lucky to stick to one-size-up from stock, which added about 1 lb per tire. Two-size-up from stock always added at about 3+ lbs per tire.

I would guess that if you bumped-up the tires and decreased the weight of the wheels by the same amount, then you would be able to negate the flywheel-effect.

CarloSW2

Wouldn't it depend on what type of driving is done most? For speeds less than say 40 mph a larger tire hurts mpg d/t increased rr. For averaged speeds over 40 mph a larger tire should help r/t the lower engine rpm.
See you're in St. Louis and have an '02 GP. Is it the 3.8L with or without the supercharger? Perhaps 'aero' rims might be a better way to go?
VT247

Been wanting to put on taller tires on my '83 RX-7. It's nearly impossible to find rims that fit the odd bolt pattern. Even better would be better gears, but same problem about locating them. Has the original rotary with 83k miles. Runs 4k rpm (7k redline) on the interstate on 13' tires.
VT247

According to the Smithers Report:

Barry's Tire Tech

Bigger is better for RR - and that also includes wider.

CapriRacer, thanks for getting my head screwed on straight. Now that I think about it more, since the contact patch needs to be the same size for a given load & pressure, a wider tire will have less sidewall deformation. The patch area is WxL; if W is more then L must be less. And I suppose less deformation results in lower RR.

Well written article BTW ;)

no problem, but I had to go back and edit some of my numbers, I went through some of my logs and my numbers were off some (that dam memory just not as good as it was 5 yrs ago...)

I think there may be data to refute this.

But I think the issue of less deflection due to the larger load carrying capacity is true, but I think the size of the fottprint is not connected in a way that can be used in any quanitative way.

Thanks