"Testing the performance effects of upsizing wheels and tires"

We ran across an interesting article run in a past Car and Driver magazine. While the testing was not done with a Prius, or with LRR tires, it nevertheless provides meaningful information regarding the
effects of tire amd wheel upsizing. The test was performed by the Car and Driver Tech Dept.

"The aesthtic appeal of larger wheels and tires is undeniable. but what about the alleged performance benefits? In general, larger wheels and tires are heavier, and additional weight hinders performance. To examine the effects of installing larger wheels and tires, also known as "plus-sizing," we tested five wheel and tire combinations ranging from 15 to 19 inches on a 2010 Volkswagen Golf and got a good
sense of what is gained and lost in the process."

"The plus-size concept works like this: As the wheel gets larger, the size of the tire's sidewall must shrink to maintain the overall tire diameter. Keeping the overall tire diameter righly the same is crucial to keeping the gearing the same and the speedometer accurate, as well as for making sure the wheels and tires fit within the fenders. But larger wheels and tires are often available only in enlarged
widths as well."

The test employed nearly identical Goodyear Eagle GT ultra-high performance all season tires inflated to mfr recommended pressures in all five sizes. " The tires were nearly the same because the 15, 16. and 17 inchers have a lower speed rating ( V , or a top speed up to 149 mph, than the 18 and 19 inch tires W-rating ( up to 168 mph. ) That was as close as we could get to testing the same tire in every size. We used the stock steel wheels for the 15 inch test and went to the aftermarket for the larger wheels. The aftermarket wheel we chose was a cast aluminum ASA GT1, which is available in 16-19 inch sizes on the Golf. It's obvious from the test that as the wheels and tires grow in size, they also grow in weight. Here's how the test shook out:"

Wheel size 19x8.5 18x8.0 17x8.0 16x7.5 15x6.0
Tire size 235/35R-19 225/40R-18 225/45R-17 205/55R-16 195/65R-15
$/wheel 199 166 145 130 N/A
$/wheel 160 125 118 97 69
C/D Test Results
0-60 7.9 sec 7.9 sec 7.8 sec 7.7 sec 7.6 sec
0-100 23.4 sec 23.3 sec 22.7 sec 22.6 sec 22.3 sec
Standing 1/4 mile 16.1@86 16.1@86 16.0@87 16.0@87 15.9@888
( sec @88)
Braking 60-0 126 ft 126 ft 128 ft 133 ft 130 ft
Roadholding
300 ft dia skidpad 0.88g 0.89g 0.85g 0.85g 0.83g
Soundlevel
70 mph cruise 71dBA 70dBA 70dBA 70dBA 70dBA
Fuel Economy
C/D Loop (mpg) 21.1 21.9 22.8 22.9 23.3
Speedometer Error -3 -1 -1 none -1
Weight
(Wheel+tire) lbs 54 51 48 46 40

"What's immediately apparent from the results is that as the wheel and tire packages get larger and heavier, acceleration and fuel economy suffer, Neither is a huge surprise, but we measured a 10%
drop in fuel economy and 4% degredation in the 0-60 mph acceleration from the 15's to the 19's, which is worth considering should you be thinking about "going big."

"We asked the folks at Goodyear about why the 19 inch set-up showed less grip on the skid pad
than the 18's and they postulated that the added width that comes with the tire may have given the
tire more grip, which would increase body roll and could therefore decrease the load on the
inside tire enough to lose 0.01 on the skidpad."

"The heavy, 19 inch suffered with the most impact harshness and taxed the suspension the most.
In contrast, the smallest wheel and tire combination showed a propensity for more understeer on the skid pad, but provided a much more controlled and supple ride. And although it didn't register on
the dBA-meter, the 15 and 16 inch arrangement had a more pleasant sound quality than the larger
wheels and tires.

"Larger wheels and tires may look the coolest, but the smaller set-ups offer a better compromise of grip, acceleration, fuel economy, price, and ride ride quality. Unless you believe it's better to look good, rather than feel good, take our advice and stay away from extremely low-profile sidewalls
and massively heavy wheels."

( A table was created for the C/D results, but the "system" compressed them tightly....sorry about that )

 

Thanks ! We tried that after we "previewed" the thread......and gave up after several attempts. You're right...
it doesn't work very well with the software !

 

Yes, you're correct that the higher skid pad number the better. However, in our use we don't spend any time at all on a skid pad with the Prius or teasing the limits of higher lateral G's before losing grip.....(.except when tracking the ZR1, and thats a different situation altogether. ) The larger set ups as reported provided 0.06 more lateral G's on the skid pad and Car and Driver did recognize that, but in everyday street and traffic driving, it's doubtful the average Prius
driver would get anywhere close to those limits. And as FBL has already stated, the very few feet braking difference
doesn't make for a very substantial difference.

As Car and Driver stated in the begining of the article, larger wheels and tires can and do provide more aesthetic appeal. And they can contribute to somewhat improved braking and handling, but considering the design purpose
of the Prius, which is to provide comfortable, reliable, highly economical transportation, we would also agree with the
findings of the Car and Driver test.

Thaat being said, our daughter HATED the appearance of the 15 inch wheel and hubcaps, and within 2000 miles
took a trip to Tire Rack Headquarters in South Bend, Indiana and replaced the "hideous" looking wheels and hub caps
with a set of Sport Edition A7 15 inch wheels and remounted the Bridgestone Ecopia EP 20's on them. ( All attemps
to persuade her towards a set of 16" set of wheels and tires failed......very strong willed gal. )

The handling of the Prius with the 15" wheels is more than adequate ( especially after SHE installed the TRD rear sway bar ) and she consistently achieves 52-56 calculated mpg's. While she occasionally drives in a "spirited" manner, no track time or testing the lateral g limits are planned. She shares track time with me in the ZR1 for that ! ( And posts quite respectable lap times. )