Tire size expectations?

[SentraSE-R]

My xB is shod with P185/60-R15 tires, standard. The OEM tires are 23.74" tall, and turn 876.1 revolutions/mile.

I'm thinking of buying 17" wheels with P205/45-R17 or P215/45-R17 tires. The former are 2.15% larger. 24.26" tall, and turn 857.3 revolutions/mile. The latter are 3.45% larger. 24.61" tall, and turn 843.1 revolutions/mile.

I am hoping the taller tires and taller effective gearing will result in better fuel economy, but the Internet is rife with claims that larger tires result in worse fuel economy for users.

Adverse factors are potential increased wheel/tire weight with the larger wheels and tires, and worse aerodynamics from the new alloy wheels v. the stock steel wheel/hubcap combo.

Is this wheel/tire swap likely to be a win, or a big fail? I didn't find any good ABA testing. If anyone can point me to testing that's been done, let me know.

Unfortunately, there are no LRR tires small enough in 17" size to fit my car. I could possibly go with 205/50-17 tires, but they're >5% taller, and may start rubbing.

[Frank Lee]

I think it's one of those YMMV things... My experience with oversized tires is mixed. The F150 with a/t was already geared tall and had a propensity to downshift at the slightest provocation; I went taller with the tires and of course the downshifting became even more frequent and I noticed a slight fe drop. But then I've had older vehicles- stick and auto- that lacked OD really benefit from taller tires. Then too, I suspect if you do lots of city driving the weight increase if there is one AND the "gyro" increase- even if there isn't a weight increase- might be detrimental; whereas if you mainly highway cruise I'd say it isn't a factor.

Great answer huh.

It would be nice if you knew someone (friend, neighbor, car club member, friend/relative that works at a tire store, fellow ecomodder, etc., or maybe even a business that has an ironclad trial/satisfaction guaranteed return period) that had rims/tires like what you are considering that would loan them to you for a bit of testing, so that you didn't have to make such a huge financial commitment for such an unknown.

[euromodder]

Originally Posted by SentraSE-R

I am hoping the taller tires and taller effective gearing will result in better fuel economy, but the Internet is rife with claims that larger tires result in worse fuel economy for users.

From reading on EM, there are varied reports here as well.
The engine must be able to make as much power as efficiently, at the lower rpm.
They don't always do that.

Can you still gain some MPGs by slowing down as much as the gearing would slow down the engine ?
You also reduce aero drag when testing it this way, so the drop in fuel use must be significant.

With the V50, I'm at the point where slowing down no longer helps the MPG as the engine can't produce the power as efficiently anymore.


What about 185/65 R15 ?
A moderate 2.6% bigger.
Fits same rims.
Not wider than the current tyres.

Adverse factors are potential increased wheel/tire weight with the larger wheels and tires, and worse aerodynamics from the new alloy wheels v. the stock steel wheel/hubcap combo.

You're losing on 2 fronts here.

Unfortunately, there are no LRR tires small enough in 17" size to fit my car.

That's the 3rd battle being lost.

I could possibly go with 205/50-17 tires, but they're >5% taller, and may start rubbing.

I've had that tyre size before, and requiring a LI of at least 91, the rubber got pretty expensive.
On average 150% of the price of the 205/55/16 I'm running now.
The situation in the US might be different, but my feeling is it will be the 4th battle being lost.

Do you still think you can win the war against excessive fuel use ?

[Bill in Houston]

I think you might find a lot more choices for 16" wheels, with some actual good lrr tires. I'd go more than 3.5% larger, too. But that's just me.

[RobertSmalls]

Originally Posted by SentraSE-R

Unfortunately, there are no LRR tires small enough in 17" size to fit my car. I could possibly go with 205/50-17 tires, but they're >5% taller, and may start rubbing.

I believe that will be a deal-breaker. Considering that your driving style includes low speeds to minimize aero losses, and very little braking, rolling resistance is probably a strong determinant of your fuel economy.

Additionally, considering the traditional customer for such products, there's a poor selection of narrow, aerodynamic 17" wheels. Although your car may never reach 60mph, the tops of your wheels will reach 90mph, so wheel aerodynamics is important.

Do you P&G on the open road? If so, you've already cut back to the minimum number of revs per mile. Regearing would just increase the duration of your pulses.

You've identified many ways for this mod to cost you plenty of fuel, and I don't see many ways for you to pick up mpg savings.

[Vekke]

If your only consern will be fuel economy then I would say no way on the points euromodder pointed out. You will spend money on more expensive rims and wheels and get worse mileage.

I would buy either that 185/65R15 or even more popular 195/65R15. If you want to even maximize your bang vs buck buy them used. Used tires roll better than new ones...

If you want new looks to your rims go with lighter 15" ones on the market, cant go wrong there. After that you attach some cool clear pizza pans to them and you got the winning setup in any conditions .

[arcosine]

Going from 175/70R14(stock) to 185/65R15 to 165/80R15 to 205/70R15 all positive for MPG. SC1 RPM now 2200 at 70 mph. Bought used 205/70R15 tires for $5 each.

[mcrews]

You have to figure weight into the equation.
When you go 2 inches up on the rin alone you are pushing the addition weight to the edges. that is NOT good. Each pound is multipled by 4 as moving mass.
not only is that bad for fuel economy, it is bad for all the suspension parts.

And to make any change pratical, you do need to increase the diameter at least 3.5%.

[arcosine]

The mass moment of inertia of a disk (I) is equal to 1/4 mass times the radius squared. The stored energy in a rotating disk is equal to the 1/2 moment of inertia times the rotation speed squared, which is equal to the vehicle speed divided by the wheel radius. Thus for a given speed, E = 1/8 m * v^2 or in English the energy stored in the wheels is independent of radius for a constant mass. If the bigger tires are heavier, then the stored energy is proportional to this increase. For stop and go driving this is not good, but for me experience with bigger tires saved gas due to the lower rolling resistance, lower manifold vacuum, lower engine friction, lower drive train friction and lower brake drag. Of course you will never break even with new 17 inch rimes. MY experience is your MPG gain will be approximately half the tire diameter ratios. If you put 5% bigger diameter tires, MPG will increase by 2.5%

[SentraSE-R]

Ouch. The truth hurts. I'm now reduced to thinking that if I go through with this, the best I can expect is no net loss in mpg - if I'm lucky.

I'm in a deal for the wheels. It looks like my best option is to sell them & stick with the stockers.