Tire Size & Weight Question

[Thai]

Hey all,

I am thinking about upsizing my tires. I currently have LT265/70/16 BFG AT KO (51 lbs/tire). Load Range D. This translate to a 31" tire.

I want to go to 265/75/16 (32") tire. I was thinking of going with Goodyear Wrangler SilentArmor. The size LT265/75/16 has a load range C...weighs in at 48 lbs/tire. It is taller than what i have now (32" vs. 31").

As for load range differences (C vs D), i don't worry too much. First, i don't tow anything or carry anything real heavy. Secondly, GY SA has an extra Kevlar layer (5 x stronger than steel belt) that should provide further puncture resistance and possibly close the gap between the two load ranges. Benefit is that GY SA weighs less.

So, what do you guys think?? Will my fuel economy be affected that much?? Yes, i am going to a taller tire, but each new tire weighs 3 lbs less than what i have now.

BTW, i want to try something new and this is why i am not going with BFG AT KO 32". In addition, BFG AT KO 32" weighs 51 lbs/tire.

 

[OUTRNNR]

My educated guess would be that since you are changing to a taller tire, the difference in weight either won't matter, or may even give you worse gas mileage. The taller tire has more of its mass located further from the center of rotation, which essetially means its a longer lever arm, not much, but it is longer. Being that tires naturally have the bulk of their mass around the perimeter of the tire means that with a taller tire you are increasing the rotational inertia of the tire, which is what affects acceleration and deceleration of the tire.

The rotational inertia for a ring or hoop is I=m*r^2 where m is mass and r is radius. As you can see rotational inertia is proportional to the mass times the radius squared. Obviously this means that the increase in radii is alot more prevalent than the decrease in mass. Plug in your numbers and see if the rotational inertia is more or less with the new tires. More will probably mean decreased gas mileage.

 

[Casper]

for a difference in unsprung weight of 3lbs per tire, but an increase in overall tire diameter, you will see a change in gas mileage. Im with OUT on this one, the energy needed to either start or keep a ring in motion, especially with weight placed more near the edges is going to offset any weight savings that you may have.

Not only that but your final drive ratio will also climb by swapping out to a taller tire. Thus you will be running more gear (more rpm) at a given speed.

 

[Kmvreter]

Not only that but your final drive ratio will also climb by swapping out to a taller tire. Thus you will be running more gear (more rpm) at a given speed.

I thought you would have a lower rpm at a given (actual) speed because you are travelling over a larger circumference/radius per revolution?

 

[Bluto]

I think the Corolla will get bad gas mileage with any 32".

 

[Bluto]

Joking aside, I have the same question at Thai. But I'm looking at 3 different tires:

The 265/70/17 KOs are 44 lbs, Load range C (6 ply).

The 265/70/17 LT Revos. 46 lbs, Load range E (10 ply) tire.

Goodyear has a Wrangler SilentArmor 265/70/17 range C at 48 lbs.


I tow some and will load up when traveling. But nothing requiring a C or E. Stock dunflops are 36 lbs. So all 3 are a big jump in weight, but not much taller.

 

[Thai]

Thanks for the quick answers.

Bluto, i think that you should stick with Load Range C...E is overkill. For me, there are only two AT tire that i would consider for my need (on-road and moderate off-roading over rocks): Goodyear Wrangler SA and BFG AT KO.

 

[Bluto]

Thanks for the quick answers.

Bluto, i think that you should stick with Load Range C...E is overkill. For me, there are only two AT tire that i would consider for my need (on-road and moderate off-roading over rocks): Goodyear Wrangler SA and BFG AT KO.

I like the GY SA - great reviews on tirerack from previous Revo and KO owners. I'm not sure about spinning and stopping 33% more tire than stock.

 

[DLSWyo]

I picked up my new 4Runner (at least to me '05) SR5 V6 about 5 weeks ago, traded in the old reliable '97 4R.
I have had the opportunity to run 3 sets of different tires since I picked her up.
Started with the 16" rims with Michelin Cross Terrains (CT) 265/70/16, a good tire, wimpy tread, the ride was good but looked like soccer mom tires. I then purchased a set of Sport Rims 17", came with Generals Ameritrac 245/65/17, about the same diameter as the Michelins. Rims looked good but the tires were even wimpier than the CT's. The ride was actually a little worse (shorter profile) as well. I then traded them in on a set of Goodyear Wrangler Silent Armor 265/70/17.
The new tires look good, I have had absolutely no issue with rubbing, the ride is excellent, and I have noticed no difference in road noise in relation to the other two brands. I was a little disappointed at the performance hit; I could defiantly tell I had bigger heavier tire on, and what I would guess to be about .5 to 1 MPG on fuel economy.
After driving the new 4Runner for about 1200 miles with the new tires I can recommend GY SA tires without hesitation.

Hope that helps.

 

[Her4Runner]

I thought you would have a lower rpm at a given (actual) speed because you are travelling over a larger circumference/radius per revolution?

Ding, ding ding - we have a winner!

Taller tires serve to lower final drive ratio. A final drive ratio is the number of times the engine revs relative to one rotation of the rear drive wheel. So, if the ratio is lower, truck will be a better highway cruiser, but slower off the line. If it is high, acceleration from a standing stop will be good, but motor will rev higher at highway speeds.

*Granted, this does get confusing when you start thinking about rear end gear ratios, b/c a higher gear ratio # is ref'd. to as being a lower gear etc.*

So back to the effect of taller tires on gas mileage... Certainly, the weight savings will make a difference as it is said that reducing 1# of rotational mass is roughly = to removing 6-7#'s of static weight. So if this theory holds true, shaving 3#'s of rotational mass off each wheel is about the same as removing 72 lbs. of static weight from your vehicle (3# rotational mass/tire x 4 tires x 6). Will removing the equivalent of ~72 #'s of weight produce a noticeable effect on gas mileage. Dunno.

But, if you do alot of highway driving, where the taller tire is allowing your motor to turn at lower RPM's, AND you in effect shave ~72 #'s off of your vehicle weight, theoretically you could realize a slight improvement in gas mileage. But it will probably only be noticeable/tangible on long trips. (Sorry for the novel, but I like thinking questions like this!)

Her4Runner

 

[Thai]

Woohoo...thanks for the responses! Now, i have a very good explanation (a big long winded) for my future misses.

 

[OUTRNNR]

So back to the effect of taller tires on gas mileage... Certainly, the weight savings will make a difference as it is said that reducing 1# of rotational mass is roughly = to removing 6-7#'s of static weight.

Not sure where this relationship came from, but it sounds plausible. However, remember that any "static" weight saved by using lighter tires will be cancelled out by the required additional torque to spin up a tire with a higher rotational inertia...think driving uphill

All of this is nearly a moot point however, because we are talking about such miniscule differences (3 lbs/tire and 1" diameter per tire). It really depends on what kind of driving you'll do: If you drive in stop & go alot and/or accelerate hard often I'd say mileage will suffer (with comparable driving on the smaller tires). If you tend to accelerate slowly and cruise alot (don't deal with alot of stop & go traffic or stoplights, if you are then you're lucky!) then I'd say it would be a wash between the tires.

In all honesty, we could sit here all day and night quoting the science behind it, but science is ideal and the real world is not, so the only way to know is to test both sets of tires and see which ones provide the best fuel economy/acceleration/whatever...

 

[Koz]

Gents,

The differences between these tire sizes will not make that big of a difference in overall performance. You will not need any kind of gear change. If you do any towing or off-roading the "E" rated REVOs will make the biggest difference. The stiffer sidewalls will be much more compliant, especially when aired down. I have these tires (LT 31"X10.5 "E" rated) on my Taco and there is a noticeable difference in all aspects between them and a "P" rated tire. As soon as the H/Ts need to be replaced I will be getting the LT REVOs on the 4Runner. I would not even think twice!

Koz

 

[Bluto]

Another factor - tread compound, tread pattern, and contact area will change rolling resistance and affect MPG. There is not a tanigible way to measure numbers. But the amount of noise from a tire is a good indication of resistance.

 

[Thai]

Ok...update. I went to Discount Tire Co. today to check out the GY SA 32". It looks great...definitely more on-road friendly than BFG AT KO. However, the tread design is bit milder...off-road traction may not be as good.

Next, i checked out it's sidewall thickness. There is a big difference in stiffness between my current BFG AT KO 31" (load range D) and GY SA 32" (load range E model). The BFG's sidewall is definitely thicker and a bit stiffer.

According to DiscoutTire, the GY SA 32" Load Range C weighs 2 lbs less than my current BFG. Nice. For comparison, BFG AT KO 32" weighs about 7-8 lbs more than GY SA 32".

The final test...GY SA 32" did not fit in my spare tire cover. So, with this and the above points, i decided to keep my BFG.