Tire sizes to improve MPG?

[dumbcomputers]

Has anyone thought about a slightly skinner and/or taller tire to improve MPG?

 

215/55-17 would be 0.4" taller (10mm) and 0.4" narrower

205/55-17 would be the same diameter but 0.8" narrower

Edited January 17, 2014 by dumbcomputers

[obob]

I would have gladly exchanged performance for lower tire replacement costs and extra mileage.  205's or even 195's would have been fine with me. 

 

As far as replacing the current tires, I am not sure what the stock rims will take.

[dumbcomputers]

I would have gladly exchanged performance for lower tire replacement costs and extra mileage.  205's or even 195's would have been fine with me. 

 

As far as replacing the current tires, I am not sure what the stock rims will take.

 

Minimum tire width on a 7" wide wheel is 185

 

Prius V's come with 205 standard and 215 as an option.

Edited January 17, 2014 by dumbcomputers

[mtb9153]

It is my opinion that the tires the factory chose are optimum for the car and it's design.  A taller tire would mess with the settings the electronics are designed to work in tandem with.  Even worse is a wider tire, which would degrade the MPG numbers.  Just my thoughts.

[RaPieR]

A tire with the same rolling resistance as the stock tire but narrower might give a very slight increase in MPG but it would increase braking distance because the C-Max is so heavy.

[dumbcomputers]

A tire with the same rolling resistance as the stock tire but narrower might give a very slight increase in MPG but it would increase braking distance because the C-Max is so heavy.

 

That's the whole reasoning for a narrower tire. The same tire model of the same diameter but narrower will have LESS rolling resistance.

 

Less braking performance is also true for running your tires at high PSI.

[fbov]

Rapier, I'd like to hear your logic, as there's a strong argument that a narrower contact patch improves braking. Pascal's Law says the contact area is equal, so a narrower contact patch must also be longer, which favors braking.

 

And same for you, dc. How does higher tire pressure hurt traction? What's your logic, as there's again a strong argument to the contrary. Pascal's Law says that the higher PSI has a smaller contact patch, but it's carrying the same load, so the contact pressure is also increased. This gives the tread better bite into the road surface, so the net tractive force available is the same. Where you really see this is in winter driving, as modern snow tires need contact pressure to distort the tread blocks, raising edges within the tread block.

 

Now, reducing tire section will reduce drag, and changing tire diameter will change effective gearing, but the available effect is small unless there's a lot of room for really big tires. Plus, you now need to apply correction factors, as your mileage will be a wash unless you calculate actual miles driven.

 

But then, the only way to improve an optimized system is with a buch of little improvements that add up.

 

HAve fun,

Frank

[dumbcomputers]

Rapier, I'd like to hear your logic, as there's a strong argument that a narrower contact patch improves braking. Pascal's Law says the contact area is equal, so a narrower contact patch must also be longer, which favors braking.

 

And same for you, dc. How does higher tire pressure hurt traction? What's your logic, as there's again a strong argument to the contrary. Pascal's Law says that the higher PSI has a smaller contact patch, but it's carrying the same load, so the contact pressure is also increased. This gives the tread better bite into the road surface, so the net tractive force available is the same. Where you really see this is in winter driving, as modern snow tires need contact pressure to distort the tread blocks, raising edges within the tread block.

 

Now, reducing tire section will reduce drag, and changing tire diameter will change effective gearing, but the available effect is small unless there's a lot of room for really big tires. Plus, you now need to apply correction factors, as your mileage will be a wash unless you calculate actual miles driven.

 

But then, the only way to improve an optimized system is with a buch of little improvements that add up.

 

HAve fun,

Frank

 

If you believe the Internet:

 

http://www.wheels.ca/over-inflated-tires-are-a-bad-idea/

 

But I'm sure you can find just as much information that says the opposite.

 

I never believed the improvement of going to a skinner tire would be dramatic...but I do think it would be about the same as the improvement going from 44lbs to 51lbs PSI in the stock tires, maybe a bit more.

Edited January 20, 2014 by dumbcomputers

[fbov]

I believe what makes sense. That's why I asked for rationale - what are you thinking? I'm more interested in that discussion than finding what's recommended, as all recommendations are the same... thanks to our legal system.

Frank

[RaPieR]

Rapier, I'd like to hear your logic, as there's a strong argument that a narrower contact patch improves braking. Pascal's Law says the contact area is equal, so a narrower contact patch must also be longer, which favors braking.

 

And same for you, dc. How does higher tire pressure hurt traction? What's your logic, as there's again a strong argument to the contrary. Pascal's Law says that the higher PSI has a smaller contact patch, but it's carrying the same load, so the contact pressure is also increased. This gives the tread better bite into the road surface, so the net tractive force available is the same. Where you really see this is in winter driving, as modern snow tires need contact pressure to distort the tread blocks, raising edges within the tread block.

 

Now, reducing tire section will reduce drag, and changing tire diameter will change effective gearing, but the available effect is small unless there's a lot of room for really big tires. Plus, you now need to apply correction factors, as your mileage will be a wash unless you calculate actual miles driven.

 

But then, the only way to improve an optimized system is with a buch of little improvements that add up.

 

HAve fun,

Frank

 

My logic is based on autocross/road racing experience.  With the same tire type/compound, changing to a wider tire provides more grip.  This is of course on clean dry asphalt.  The wider tire would have worse performance if the road is wet/gravel/snow etc.  This is evident in rally racing where their snow/gravel setups use narrow tires and for dry asphalt their setups use wider tires.  I found a short article that I've linked below that seems to make sense.  

 

http://www.datsuns.com/Tech/tech_tires-2.htm

 

Also have another link from an experienced autocross that discusses tire pressure.

 

http://www.awdriven.net/autoX-tire-pressures.htm

 

 

Hmm, just saw this article a short while later.  And maybe the same tire model/brand with different widths don't use the same compound.  Which is why a wider tire has more grip because it is a softer compound and has less stress/heat during braking/cornering.

 

http://www.autospeed.com/cms/A_108915/article.html

Edited January 21, 2014 by RaPieR

[fbov]

comments below...

My logic is based on autocross/road racing experience.  With the same tire type/compound, changing to a wider tire provides more grip.  This is of course on clean dry asphalt.  The wider tire would have worse performance if the road is wet/gravel/snow etc.  This is evident in rally racing where their snow/gravel setups use narrow tires and for dry asphalt their setups use wider tires.  I found a short article that I've linked below that seems to make sense.  

 

http://www.datsuns.com/Tech/tech_tires-2.htm

 

This makes sense, in terms of varying tire frictional characteristics, but he's got a mistake in the 3rd paragraph. While it's true that both tires support the same total normal force, one does so over a larger area, and so the force/area (what you really care about) is lower, and so the rest of his argument is rendered questionable. The wider tire should not heat up as much because there's not as much normal force on each tread block (because there are more of them).

 

Also have another link from an experienced autocross that discusses tire pressure.

 

http://www.awdriven.net/autoX-tire-pressures.htm

 

This is very familiar, and I don't have the time to go into it in depth, but as a process, this will lead you toward high performance. His ationale, however, is suspect (see Goodrich tires, below)

 

Hmm, just saw this article a short while later.  And maybe the same tire model/brand with different widths don't use the same compound.  Which is why a wider tire has more grip because it is a softer compound and has less stress/heat during braking/cornering.

 

http://www.autospeed.com/cms/A_108915/article.html

 

Yet another well-written article from autospeed.

Most of my performance tire experience also comes from autocrossing. By way of credentials, I have the complete runner-up set - 2nd, 3rd, and 4th - in H-Stock at Nationals in the 1980's to early 1990's. I ran a BMW 320iS, 1.8L 5-speed. 13x5.5" wheels and no suspenstion adjustability, so positive camber up front and negative camber, semi-trailing arm in back. I ran everything from 175/50 P7's to Yoko 008R/001R and the Goodrich autocrossers, leaving the pursuit about the time Goodrich came out with their asymetric carcasses, and dropped everyone's tire pressures 10-15 lb. That's where I see link #2 failing in the rationale side, even as he presents a functional process for optimizing pressures.

 

Gotta run,

Frank

[ptjones]

If you believe the Internet:

 

http://www.wheels.ca/over-inflated-tires-are-a-bad-idea/

 

But I'm sure you can find just as much information that says the opposite.

 

I never believed the improvement of going to a skinner tire would be dramatic...but I do think it would be about the same as the improvement going from 44lbs to 51lbs PSI in the stock tires, maybe a bit more.

I believe the tire companies have decided to go with a legal answer to put the responsibility back on the car manufactures. I have been run around 49-50psi for about 10k miles and have 6/32" outside , 5.5/32" center and 5/32" inside front and back. I have 43K miles on MADMAX.  I increased the size of tire on my 2007 Focus which did increase MPG's, but I had to calculate actual mileage and speed was off by 2mph at 70mph. I'm guessing 1mpg improvement + 2mpg for GasMiser tires. The odometer is already off by 1.5% to the low side on the CMAX. :) 

 

Paul 

[RaPieR]

comments below...

Most of my performance tire experience also comes from autocrossing. By way of credentials, I have the complete runner-up set - 2nd, 3rd, and 4th - in H-Stock at Nationals in the 1980's to early 1990's. I ran a BMW 320iS, 1.8L 5-speed. 13x5.5" wheels and no suspenstion adjustability, so positive camber up front and negative camber, semi-trailing arm in back. I ran everything from 175/50 P7's to Yoko 008R/001R and the Goodrich autocrossers, leaving the pursuit about the time Goodrich came out with their asymetric carcasses, and dropped everyone's tire pressures 10-15 lb. That's where I see link #2 failing in the rationale side, even as he presents a functional process for optimizing pressures.

 

Gotta run,

Frank

Cool, I haven't been autocrossing for very long but based further reading the increased grip on wider tires may be attributed to softer compounds vs narrower tires.  I always assumed that tires of the same models would have the same compound make-up but increasing width would provide more grip.

 

On another note, the 225 wide tire was picked by vehicle engineers for the C-Max.  If they could put a smaller or narrower tire for increase mileage without negative impacts to driving dynamics they would have done so.  The C-Max is pretty heavy so how narrow could you go before you experience tire deformation that would negatively affect driving dynamics or mileage.  

[fbov]

What autocrossing taught me was that it was more important to mount the optimum size tire for your wheels than it was to mount a wider tire on it's narrowest recommended rim. Running Yoko 001R in 185/70 and Yoko 008Rs in 205/60 on a 13x5.5" wheel, the 185s cornered better, but the 205's were smaller diameter, resulting in better acceleration. I used the 205's at track events, and the 185's for autocrossing.

 

Now, I'll admit the 001 and 008 were distinctly different tires, but the trend applies - best cornering comes when wheel and tire are selected so they work well together.

 

As to why P225/50-17s, one big source of rolling resistance is sidewall deflection. The shorter the contact patch, the less deflection occurs.  If you look at speed-record tires, they're tall and thin, for less drag, but run very high pressures so the contact patch is still short.  Passenger cars can't make that large a tradeoff, especially when they carry a substantial load, so we get wider tire, but still with a short contact patch.

 

Have fun,

Frank

[HannahWCU]

I would agree that the Tire Manufactures, from a legal standpoint, will ONLY say "use what the OEM recommends".  If they were to say ANYTHING else would subject them to a lawsuit. 

 

As to why the OEM recommends the pressure they do; the article dumbcomputers linked to says it best "The pressures have to take into account passenger comfort, vehicle grip, vehicle load, tire life, rolling resistance and fuel economy." (emphasis added) Passenger comfort (or how rough the car rides) is affected by the tire pressure. for the most part, lower pressures increase comfort while higher pressures decrease comfort. My opinion is that the tire pressure can be increased without major detriment to the vehicle dynamics.  How much can you increase it?  I don't know.  Personally, I run 44psi in my tires.  That is about midway between what Ford recommends (38psi) and the max allowed (51psi).

 

I believe the tire companies have decided to go with a legal answer to put the responsibility back on the car manufactures. I have been run around 49-50psi for about 10k miles and have 6/32" outside , 5.5/32" center and 5/32" inside front and back. I have 43K miles on MADMAX.  I increased the size of tire on my 2007 Focus which did increase MPG's, but I had to calculate actual mileage and speed was off by 2mph at 70mph. I'm guessing 1mpg improvement + 2mpg for GasMiser tires. The odometer is already off by 1.5% to the low side on the CMAX. :)

 

Paul 

[ptjones]

I would agree that the Tire Manufactures, from a legal standpoint, will ONLY say "use what the OEM recommends".  If they were to say ANYTHING else would subject them to a lawsuit. 

 

As to why the OEM recommends the pressure they do; the article dumbcomputers linked to says it best "The pressures have to take into account passenger comfort, vehicle grip, vehicle load, tire life, rolling resistance and fuel economy." (emphasis added) Passenger comfort (or how rough the car rides) is affected by the tire pressure. for the most part, lower pressures increase comfort while higher pressures decrease comfort. My opinion is that the tire pressure can be increased without major detriment to the vehicle dynamics.  How much can you increase it?  I don't know.  Personally, I run 44psi in my tires.  That is about midway between what Ford recommends (38psi) and the max allowed (51psi).

I run 49-50psi now and was surprised that I didn't notice any significant difference in ride quality. Someone had a formula for figuring out MPG's improvement vs psi. :) 

 

Paul

[obob]

Where I live I lot of the major roads are bumpy, especially in the right lane.  My sense is that a say 215-60-16 tire would take the bumps better.  ( 215-65-15 would take them even better but that is not as popular of a size so tires may be more expensive.)

Edited January 23, 2014 by obob

[fbov]

My snows are 215/60-16s, but they're not a tire optimized for comfort...

[ptjones]

Where I live I lot of the major roads are bumpy, especially in the right lane.  My sense is that a say 215-60-16 tire would take the bumps better.  ( 215-65-16 would take them even better but that is not as popular of a size so tires may be more expensive.)

TireRack has 215-60-16 in stock and they are .3" taller so I'm guessing your odometer is going to be off by 2% and speedo by 1-2mph. seems doable. Then to find some light weight wheels that you can put wheel covers on. :)

 

Paul

[fbov]

Compare rev/mile to find what your speedo will do. Per Tire rack:

215/60-16 - 796 rev/mi.

225/50-17 - 803 rev/mi.

 

It's more like 1% because the narrower tire has a longer contact patch, and it isn't as big around as the static diameter indicates. I didn't notice this difference with my snows; the magnitude of the change is comparable to tire pressure or tread wear effects.  If you want to change gearing, try 215/65-17 (742 rev/mi) or 225/65-17 (730 rev/mi)... if they fit.

 

Frank

[ptjones]

Compare rev/mile to find what your speedo will do. Per Tire rack:

215/60-16 - 796 rev/mi.

225/50-17 - 803 rev/mi.

 

It's more like 1% because the narrower tire has a longer contact patch, and it isn't as big around as the static diameter indicates. I didn't notice this difference with my snows; the magnitude of the change is comparable to tire pressure or tread wear effects.  If you want to change gearing, try 215/65-17 (742 rev/mi) or 225/65-17 (730 rev/mi)... if they fit.

 

Frank

I do think we are limited in going much bigger, just not much room there turning and snow considerations. That would mean the odometer would be off by 2.5% or 2.5mi/100mi and 72mph at 70mph indicated. Michelin Energi tires are limited in tire sizes. It might take more than 1% bigger diameter to see a change in MPG's. Having difficulty finding 16" wheels with the 55mm offset at TireRack. :) 

 

Paul

[fbov]

My snows came on 16" Ford OEM steel wheels from tire rack. Could be stocked seasonally, though...

 

Frank

[ptjones]

My snows came on 16" Ford OEM steel wheels from tire rack. Could be stocked seasonally, though...

 

Frank

I wonder what the weight difference would be? Rolling weight can be a issue, might be able to save 20lbs. What do you think? :)

 

Paul

[fbov]

I didn't weigh the tires when I did the Fall swap... but I'll get another chance in the Spring!

[GaryM]

I am at 19,000 miles on my C-Max SE Hybrid and the tires still have a lot of tread left, so I am not in a hurry to replace them just yet, but I have also been thinking about this. I really liked the road feel of the new Pirelli P7 LRR EcoImpact all season. I put a set of them on my Camry and they handled very well. Did not see a mileage improvement over the BFG G-Force all season tires, but the car did have 150,000 miles on it. They have thistire in a run flat version in the right size for the C-Max.

 

The stock size Michelin 225 50 17 is 25.9" diameter. Rated 93V for load and 1433 pounds. 

 

Going to the Pirelli 205 55 17 comes in at 25.9 inches tall, close enough to be tread wear, but drops load ratings to 91V and 1366 pounds. 

 

The Max gross axle weight is the front at 2414 pounds, this is fully loaded. Assuming reasonable balance side to side, that is 1207 pounds per tire, carrying a full tank of gas, 5 passengers, and cargo load. The max cargo load does seem a bit low though, rated at just 852 pounds for people and cargo total. In any case, I certainly do not think it is unsafe to run the 205 tires. There is no 215 at the correct diameter. Going up or down 1/2 inch is not too big of a deal and seems like it would clear. 

 

As for rim fit, the 225 50 stock size tire is rated for 6 to 8 inch rims, but trust me, jamming a 225 onto a 6 inch is an serious pain. I used to run 225 50 14 on my 14 x 6 wheels when I autocross raced. The 205 is rated for rim sizes 5.5 to 7.5 inch. Putting them on a 7 inch is a very good fit. I also ran 205 60 14 on 14 x 7 inch rims, the side walls were near dead straight and the grip was excellent. The tread on the Pirelli is 7.1 inches wide with a max section width of 8.3 inches. 

 

I just may try the Pirelli in the 205 55 17 size, when I wear out the stock tires.