Swapped the 17"s Back on and instant drop in MPG.

What are the revolutions per mile for the two sets of tires?

Thanks,
Bob Wilson

 

Do you have pics of the stock vs 17's? Would like to see them please post.

 

Do big wheels make the Prius look smaller or is it just me?

 

You'll definitely need to try the Volk Racing CE28 Eco Drive Wheels for Prius Only! and let us know if they really are ECO ??



p.s. Btw I hope your times in your sig were on the stock wheels. If not, you'll probably be able to go down another second

 

Very Nice! I like the look. Thanks for the pics

 

Thank you!!

One more question: tread width?

We've been discussing tire drag over in "Prius Technical Stuff" and the data begins to suggest that tread width is a significant determinant of tire drag.

I tried to find the "Kumho Escta" in "Tirerack.com" and found multiple styles: MX, SPT, SPT XRP, ASX, AST, V710. But their specifications do not give the tread width.

To measure the tread width, lay a piece of paper over a 'damp' cloth and drive over it. The liquid should permeate the paper and you can quickly measure the tread width.

I have been looking at getting some tires for my NHW11 to make a few changes:

• largest diameter, fewest rotations per mile
• no wider than current, same rolling drag

If this experiment works, I may be able to shift the 42 mph transition point up to 45 mph, true speed, and the MPG fall-off that shows up at 65 mph up to 70 mph. More importantly, it may lead to everyone being able to pick performance tires that give all of our Prius an honest 5%, MPG performance boost.

Thanks,
Bob Wilson

 

First I want to point out that my 2003 Prius has 14" wheels, not the 15" of the 2004-current Prius. As such, the attached image of a spreadsheet would need considerable rework for the equivalent 15" tires. Also, I do not have experimental data to back up the projected performance so this remains 'informed speculation.'

I've been reading "Tires, Technology and Energy Consumption" by K.G. Duleep, who is part of the California effort on tire rolling resistance. You can find a copy Googling the title. There is a fine write up on the effect of tire rolling resistance on mileage in the first part of the report. But my interest is in the significant tire characteristics and their impact on rolling resistance.

"Rotating tire drag can be 20 to 25% of total vehicle aerodynamic drag. Increasing the width of the tire, and changing the tread, or the rim and wheel can change tire drag by 3 to 6%. . . ." (pp. 13) There are two aspects, the profile drag and the air entrapment. Julian Edgar in March 19, 2005 (Browser Warning) wrote an article about forming extended air shields to smooth the air around the front tires, an excellent approach. But you can also see the air entrapment on a rainy day observing the water-air mix spilling out of the wheel wells. The Honda Insight solves the rear wheel entrapment by a wheel well shield and air-tight wheel covers. I've seen modified pizza pan, wheel spoke covers that would also reduce air entrapment.

"RR varies linearly with load and inversely with square root of inflation pressure." (pp. 14) The load effects are well known but this is the first relational formula showing the effects of tire inflation. So using English units:

PSI formula = relative drag effect
1/SQRT(35 psi) / ref. = 120.7% (Toyota psi)
1/SQRT(44 psi) / ref. = 107.7% (OEM)
1/SQRT(51 psi) / ref. = 100% (my current tires)

Because the drag reduction occurs by the square root of the pressure change, it is not as dramatic as a linear increase in pressure would suggest. It is important but not the only effect.

"For a given application and a constant tire technology, tire RR is inversely proportional to radius and aspect ratio but increases with increasing width (about 5% per 20mm). Effect of diameter is quite dominant." (pp. 14) Now here are two very important aspect of which we have some control. Let's start with inversely proportional to radius:

Rev/mile = relative drag effect
867 / ref = 94.3% (best case)
906 / ref = 98.6% (OEM)
919 / ref = 100% (my current tires)

Tire widths = relative drag effect
4.9 = 90.7% (best case)
5.2 = 96.3% (OEM)
5.4 = 100% (my current tires)

"toe-in creates additional side force, increasing RR by 1% per 0.15deg per wheel" (pp. 15) This is why I'm such a stickler for minimizing the real wheel drag by using a shim kit. But it also has an impact on front wheel drag. Unfortunately, the front wheels have a thrust force and this would tend to shift the wheel rolling toe. I have no easy answer for how to manage this beyond using whatever the current wheel alignment services use.

"Both ambient and tire temperature have a significant effect on RR. For example, from -20C to +20C, RR can be reduced by 50%." (pp. 15) Ok, so we must all move to warmer climates during the cold season. However, it also means it may make more sense to shift your morning commute to as late as possible to gain any possible warmth.

"RR increases with speed in a non-linear fashion. Current J2452 procedure fits a V+ V**2 curve, but dependence is small at speeds < 60mph." (pp. 17) Again, 65 mph (104 km/hr) is probably the highest speed one might want to cruise at for fuel efficiency but 60 mph is better. This matches my field data too.

What is important about this paper is using these relationships I can model expected tire performance as a function of diameter, tread width, and pressure. I can even do a 'what if" looking at finding an optimum solution. Shades of linear programming, this becomes a solvable problem:

drag-index = F1(tire pressure) + F2(tire diameter) + F3(tire width)

However, there is a simpler approach that uses the product of the relative efficiencies:

drag-merit = %F1(psi) * %F2(diameter) * %F3(width)

It turns out that even though the relative drag as a function of psi varies as the inverse square root, the higher pressure of the Sumitomo (IF YOU USE IT), predominates the other effects. Based upon this analysis, it looks like I might get lower rolling resistance performance with:

92.8% - Sumitomo 175/70TR14
96.1% - Sumitomo 185/70TR14
99.2% - Sumitomo 195/70TR14
​

The next question is how to test the relative fuel efficiency versus my current Sumitomo 175/65TR14. I plan to buy two of the 175/70TR14s and swap them between the front and rear wheels. Most of the weight and rolling drag comes from the front wheels. I can use both mileage records as well as tire temperatures to determine how these two models of tires perform.

One other option is to split the front tires between the front wheels. I can then look for asymmetrical drag and tire temperatures to determine relative performance. This has the advantage of needing only a single sample, a much more affordable approach.

Bob Wilson

ps. I have not found any reports of barometric or humidity effects on tire rolling resistance with the exception of precipitation, 100%+ humidity. Otherwise, the tires perform as in dry conditions.

 

A TAG weather station would be extremely helpful in reducing the amount of weather related variables. MPG where I live can vary by 4-6 mpg on a day to day basis. BArometric pressure and humidity have as much to do with it as temperature, in my opinion...

 

Well, I would need to go back to school to fully understand the information Bob graciously provided. In my experience, no matter what technologies are involved, the sidewall characteristics, carcass compound, and tread pattern are the three major factors in determining traction. The more traction there is, the greater the rolling resistance in a general sense. Less rolling resistance is primarily achieved through a smaller contact patch area. Other factors such as wind resistance and hardness of compound also have a bearing on the efficiency of the tire.

Note that dragsters typically have very narrow front wheels and tires. Even my Camaro has efficient front tires on it (28" by 7.5" x 15").

I "assume" the tires aredifferent between the two sets of rims. Please correct me if I am wrong. I think the rim differences and tire differences combined make it all but impossible for an average fellow like me to accurately compare just the tires or just the rims.

The other factor that i would highly recommend considering is less rolling resistance, for the most part, involves increased braking distances. The grip of the tire to the road surface which we all want to be minimal when rolling along needs to be maximized when under heavy braking conditions or slick roadways. It is always a tough call to find the right balance of traction and economy and what works fine in one set of conditions may not in another.