The mpg difference 15s vs. 17s

Is rolling resistance necessarily in direct proportion to width though? If the inflation pressures are the same then the wider tire should deflect less, so I'm not sure that the relationship is necessarily linear.

Comparing bicycle tires (mtn vs road bike) is a little bit different IMHO. Because mountain bike tires are not just wider, they're also much more knobbly and typically inflated to lower pressures.

I think that air resistance and rolling resistance play a part, but I also think there may be some differences relating to construction as well (like the bigger tires might be more optimized for performance or load carrying and less for LRR, or something like that). Maybe it's a small contribution from all of these things?

 

If the tyres weighed the same yes, but I guess the wider tyres weight at least as much per area, and maybe more.

 

honestly.. i think it's F8L's details i'm regurgitating... if i remember correctly, he did those test at highway speeds over many periods of times. (i just realized i didn't post which speed)

what i get from it is that at highway speed since the engine is usually at a somewhat constant rpm.. it's predictable in a sense.. while at lower speeds it really depends on how your foot goes...

as for tire width.. keep in mind that toyota put little "fins" (air damn?) in front of the tires to break the air... they were designed around the stock width (lexus has bigger fins..) so.... that may have an impact as well... along with how the wind was designed to roll around the rim of the tire.. and so much more... LRR means it's usually a more solid compound.. less gummy... cornering drops but straight line is easier to maintain. i put heavier duty tires on my car because the normal material for a stock 215 (think mustang tire) wears down in 20 to 30k miles (so i've been told and i've seen)... these tires are stickier... so you get the deformation in front of the tire.. and a velcro like stick off the back of the tire... the bigger the tire, the worse these effects get... it almost has nothing to do with the car except that toyota has put little things in effect (trim rings, etc) to lower the effects of wind and drag on the tires. in a higher end car that uses these type of tires standard... they require better brake cooling... some of these rims may be designed in a way where they vaccum air through the wheel to keep things cool. (where stock trim rims make it a little bubble of "dead air" in a sense on a stock prius)

in the end... there's probably 20 or 30 little key details we could bicker over that caused it to drop.. the point being.. it drops.. it always drops with a bigger tire.. if it didn't.. my brother's old bronco with 36 super swampers at a high psi should be somewhat near his stock huge tires.. but it's not.. because there is always a penalty going bigger. (weight, aerodynamics, compounds, etc)

 

He may be including the difference in weight of rims or not but just the tires are pounds a piece heavier.

Lets take the General Altimax HP for example to compare 15" to 17".

The correct 15" Altimax HP for a Gen II Prius would be 195/60R15 at 850 RPM 24.2" diam section width of 8" and weight of 17 pounds per tire.

The correct 17" Altimax HP for a Gen II Prius would be choice between the 215/40R17 at 862 RPM and the 215/45/R17 at 839 RPM. They both way 19 pounds per tire. The width is 8.6" or 8.5" and the diam is 23.8" or 24.6". Most would go for the 215/45/R17 with the 8.5" width, 24.6 diam, and 839 RPM and 19 pounds per tire.


OK how about a tire that has a rev per mile closer to OEM? I'll use the Yokohama Avid ENVigor.

The correct 15" Avid ENVigor for a Gen II Prius would be 185/65R15 at 850 RPM 24.4" diam, section width of 7.5" and a weight of 19 pounds per tire.

The correct 17" Avid ENVigor for a Gen II Prius would be 205/45R17 at 855 RPM 24.3" diam, section width of 8.2" and a weight of 22 pounds per tire.


Speaking of OEM what are the specs for the OEM Integrity?

The OEM Integrity is 185/65/15 at 855 RPM 24.4" diam, section width of 7.4" and a weight of 17 pounds per tire.

Goodyear doens't make the integrity in a 17 version close enough to the desired RPM / Diameter for a Prius.

I'd be curious to know the weight differences between OEM 15" rims, OEM 17" rims, and some of the common aftermarket choices (including swaps from other car models)

 

Can't we just cut to the chase here with absolute rolling resistance values ?
Seems to me that a nice close estimate for fuel economy drop (as a percentage) will be:

50*(RR(1)/RR(2) - 1)

Where RR1 is the higher value, testing at about 60 mph.

 

True, there are 'fins' in front of the tires, but I don't know for sure they exist for aerodynamic reasons. Maybe they help keep water from getting under the tires in the rain?

 

Beware what you ask for ..

California Energy Commission Website Search Results

 

They are there to push the airflow away from the tires and wheel wells. The Gen III Prius also features hard chines on the corners of the front bumpers to enhance this effect.

Tom

 

Justin,
Are you familiar with the Yokohama super e-spec ?
dB Super E-spec Green Performance Tires | Yokohama Tire Corp.
http://www.yokohamatire.com/assets/docs/yokohamatire_db_super_e_spec.pdf

They report 11% decreased LRR compared to the Michelin Hydroedges I run now, which I calculate out to a 3 mpg difference at 60 mph when both tyres are equally worn. One very interesting aspect is the warranty: 5 years, and no mileage limit!

I think it makes a lot of sense for me to have two set of tyres: an LLR for 9 months of the year, and better traction for the other 3 months. Any opinion how tyres wear sitting in an unconditioned garage ?

 

I didn't see any comprehensive list there but I did find some interesting reading material.

Page 11 of Rating System Presentation 4-8-09 has an interesting comparison that shows at the same load (1000 pounds per tire) the RRF is lower on a corolla with Firestone Affinity Touring 195/65R15 than with Turanza 185/65/15 with both at 35 PSI.

Unfortunately tirerack doesn't show a diamater or RPM for the Firestone Affinity Touring and the name Turanza isn't specific enough to pick from the numerous tires with that name (assuming that tire is still for sale).

Proposed 5-Star RRC Rating System from page 17 is:

5 stars no min to 7.5 max
4 stars >7.5 to 9.0
3 stars >9.0 to 10.5
2 stars >10.5 to 12.0
1 star >12.0 to no max

Notes: (1)units are kg/tonne
(2) RRC values and ranges based on SAE J1269 single point test data.

going up from each star level in RCC will give a hybrid 6 gallons a year savings over 15,000 miles assuming OE recommended tire size, PSI, and following speed limits.

Going from 2 star to 4 star is twice the benefit of going 2 star to 3 star.

seems some have tried suggesting ratings in RRF instead of RRC but RRC is probably the better way to compare.

 

hyo, i'm pretty sure those little air dams are used to break up wind... they altered them a few times and they said on the webpage that they are used for that purpose. they really showed it off when they released the rx400h because it was technology carryover from the prius. lexus mentioned something about using them before (LS?) but it wasn't standard practice... or so they made it seem.

just looked it up.. some pages call them fins, and wiki said it was called a flairing. most of the time, it's listed as a new vehicle aerodynamic improvement item (new camry, lfa, lexus non hybrids, etc)

 

LOL!
I don't see nothin' in your sig, but yeah

 

Air resistance on tyres isnt responsible for the difference in drag that could be accounted for. Even as flat plate drag its percentage over the rest of the wet area of the car is tiny.

Also anyone with wide 15" wheels would be in the same situation
Q1. Are they?

Alternately anyone with skinny 17" tyres (LLR be damned) would be in a better situation
Q2. Are they?

F8 lowered his can 1", based on the 'air' theory this should have seen hime some material gain in MPG
Q3. Did it?

If wheel weight makes such a difference then LRR be damned, why not just find the lightest rubber and rim combo available

F8's test at such a speed over so short a sample time would need to account for wind resistance. A 10mph tail wind one day, and a 10mph head wind the next makes a 20mph differential, add to that 10mph is a common airspeed over ground

Also his tyres differ in diameter, which is really corrupting things. Mostly because we cant figure out the effects if any on the CVT.

If theres a difference it has to be rolling resistance. You can figure rolling resistance of your car by letting it roll from the same point on a hill into a flatish road at the bottom, the least rolling resistance will travel furthest.

But erm, can a Prius roll like that?

Im not saying you are wrong F8, I cant because I dont know any better, but Im continually troubled by the integrity of these sneak peak tests, and stuff that just doesnt seem to add up.

Finally, if big rim tyre combo's are so bad, then explain this

 

When changing alternate final drive ratios, they would either swap a new differential for a Prius in the field or fit different diameter tires. Guess which is quicker?

What I wish we had was the weight of those rims and tires. [Edit: this shows steel wheels so they do not seem to have worried about weight. http://www.youtube.com/watch?v=fjKntmE4jlA]

"He also installed a taller final-drive gear and reshaped the inner fenders to accommodate tires up to 29 inches in diameter. A larger tire equates to a taller top gear, so the bigger you go, the faster you go until your engine can't make enough power to overcome the increasing workload." http://www.caranddriver.com/features/04q4/got_hybrid_-sport/modifying_the_prius_page_4

"I had already done one pass down the course on 25-inch tires, proving the system worked and recording a speed of 129.443 mph."

[With 26 inch tires] "The Prius hit the measured mile at 130 mph, crested 131 a quarter of the way through, and exited the back door around 129. The official speed: 130.794 mph. Thunderstorms rolled in before even larger tires could be tested"

It also occurs to me, unless they got software that defeats the speed limiter, the only way to get to 130 MPH is to fool the Prius into thinking it is under 112 MPH.

 

seems like a good point but
only 2 wheels do the driving