[bwilson4web]

INTRODUCTION

In testing tires, we like to hold as many variable constant as possible and then vary one to measure the effect. Mounting another brand changes the rubber compound, different construction, air pressure, tread design and dimensions. But using a single vendor, Sumitomo, and tire style, T4, holds as many variables constant as possible.

Five years ago, I replaced my NHW11 tires with the Toyota service center provided, Sumitomo T4, 175/65R14. This tire was also Consumer Reports rated as one of five, lower rolling resistance tires. Since the Sears Mastercraft tires on the used NHW11 showed evidence of abnormal wear, edge wear, I also began mastering four-wheel alignment. With a combination of shims and caster bolts, my NHW11 now has nearly perfect alignment.

TEST TIRES - SUMITOMO T4

I have owned and am testing these tires:

Tire (T4)	Revs/mile	tread inches	diameter inches	max lbs	weight lbs	% dif
175/65R14(*)	919	5.4	23.0	1019	15	100.0%
175/65R14(OEM)	902	5.3	23.1	1102	16	98.2%
175/70R14(*)	886	5.2	23.7	1102	15	96.4%
195/70R14(*)	849	5.8	24.8	1321	19	92.4%

Tire Rack specifications for Sumitomo T4 tires that run 51 psi max sidewall pressure. The stock wheels are 5.5" wide. I've included the OEM Bridgestone Potenza RE92 which was not tested as a reference data point.

Although the "<section width>/<aspect ratio>R<inner diameter>" is the common tire size identification, as Tire Rack points out, these numbers do not tell us the critical wheel well dimensions: tire diameter, tread width and for safety, the load rating (see bold text above.) Testing has shown the 195/70R14 is about the largest size tire that fits in the smaller, rear wheel wells. Here is a photo showing the three, Sumitomo T4 tire sizes:

The rear tire is 175/65R14, middle is the 195/70R14 and front is 175/70R14.

Thanks to "john1701a.com," "Toyota Prius User-Guide for the CLASSIC (2001-2003) model" the stock aluminum rims are 5.5" and the tires should be inflated to carry 1,102 lbs. The original tires required 35 psi minimum to meet this load rating. However, these tires are no longer available. Unfortunately, most shops go by the door posted psi instead of calculating the true weight pressure. Just top off the tires after visiting a tire shop.

Test Protocol

The 175/65R14 is treated as the baseline tire since this is what Toyota mounts when replacing the OEM tire. However, this tire is undersized so the speed is under reported while the MPG is over reported. This was discovered after running calibration tests with highway mile markers. Regardless, it represents a standard that any other NHW11 owner should be able to reproduce (click on signature link to see mileage history.)

A pair of 175/70R14s were bought prior to Hybrid Fest 2008 and mounted on the rear wheels on the drive to Madison WI. Before the return trip, the 175/65R14s were swapped front-to-back and calibration checked:

• 3% measured difference - since the calculated difference is 3.6%, this is well within the margin of error. All subsequent mileage records had the distance adjusted by 3%. No significant change in average mileage has been found.
• handling - the neutral handling on the trip to Madison was replaced by a slightly improved (and welcomed,) tracking on the return trip. The neutral stability was improved with a slight tracking improvement.
• 175/65R14 spare - the best of the 175/65R14s became my full sized spare and all four tires became 175/70R14s
• improved indicated speed - compared to the earlier tires, the GPS indicated speed now reads just under the true speed including highway speeds. The smaller 175/65R14 always led to smaller true speeds than indicated.

A 195/70R14 was bought when a nail puncture was declared "too close to sidewall to repair:"

Hindsight, I should have kept the case, pulled the nail and used an aftermarket 'plug kit' to repair the tire. The nail actually was in the middle of the outer tread and since I have perfect, 4-wheel alignment, and run 51 psi, I no longer suffer edge wear.

The major problem is finding out if the larger diameter 195/70R14 will fit, the critical dimension:

It appears the rear well is the limit. Here you can see there is only an inch between the rear 'mud guard' and the tire. Any diameter larger than 24.8" and the tire risks rubbing against the rear well mud catcher. Any tread width larger than 5.8" risk impacting the upper wheel well edge on bumps or under heavy loads. Regardless of brand, these are the critical dimensions.

If someone lived in snow and icy areas, this small clearance could be a problem. But any larger diameter tire is not going to fit:

Muddy and gravel covered roads might also pose a problem. However, this may also serve as a 'nail remover' for those familar with the bicycle tire device.

The other dimension is vertcal clearance to deal with bumps and heavy loads:

There is at least the width of the tire tool between the inner well and the tire. I estimate it would take at least a 4" compression before the sidewall could make contact. However, testing continues and this photo more clearly shows the wheel well shape:


In contrast, here is the 175/70R14 on the other side of the car:

Clearly the 175/70R14 fits well within the wheel well. In contrast, the 195/70R14 seems to extend from the side of the car. This is likely to increase high speed drag but this is something yet to be measured.

I've ordered a second 195/70R14 and will replace the 175/65R14 on the spare. Then I'll run both 195/70R14s to baseline and compare performance:

• 175/70R14 front - 195/70R14 rear
• 195/70R14 front - 175/70R14 rear

Once the second 195/70R14 arrives, I plan to test:

• handling - going from 175/65R14 to 175/70R14 slightly improved straight-line stability. The first sets of tires have the same weight but the larger diameter would have increased the rotational momentum. The 195/70R14 increases both diameter and weight and should give a distinct improvement in straight-line stability.
• drag effects - rolling and aerodynamic. For this, I may do left-right-left tests.

Tire Rack has a reasonable summary of tire rolling resistance. But one repeated claim is misleading:

. . . larger tires generate higher Rolling Resistance Forces than smaller tires [at the higher test load. rjw] . . .
. . . (larger tires will often have a lower Rolling Resistance Coefficient than smaller tires), it still does not change the fact that a larger tire actually generates more Rolling Resistance Force that the vehicle’s engine has to overcome. . . .

The article points out that the Rolling Resistance Coefficient is the ratio of the rolling drag over the load, the weight of the vehicle. With the 195/70R14 on the NHW11, I'm holding the vehicle weight, the tire load, constant except for the 4 lbs additional weight of the tires, an insignificant load increase.

One idea is putting a pair of the smallest tires on the front and the largest ones on the rear. This will tilt the car down at the front and possibly change the aerodynamics. But I won't if it returns the neutral stability ... looks don't trump handling.

Bob Wilson

* - Tire Rack wants confirmation that you are buying the tire sized for the vehicle. The 195/70R14 is used by the "2003, Chevrolet, Cavaller." The 175/70R14 is used on the "2000, Saturn SW1". The 175/65R14 fits the NHW11 Prius, 2001-03.

Eeferences found at Tire Rack:
Tire Tech Information - Tire Rolling Resistance Part 2: Defining Rolling Resistance
Tire Tech Information - Tire Rolling Resistance Part 3: Changes to Expect When Switching from Worn-Out to New Tires

[bwilson4web]

INTRODUCTION

These first tests have the 195/70R14s on the rear and the 175/70R14s on the front. These establish a baseline before swapping the larger tires to the front.

RESULTS

This is what I'm getting:

mph (GPS)	MPG (GPS)	miles (GPS)	miles (trip)	MPG (veh)	temp F
18.0	85.7	10.3	9.7	80.7	81
18.0	85.7
25.0	79.5	10.2	9.6	74.8	75
25.0	79.5
39.8	85.6	5.2	4.9	80.7	73
39.1	63.4	5.1	4.8	59.7	73
39.5	73.0
40.0	68.7	2.5	2.3	63.2	73
40.0	71.8	2.6	2.4	66.3	73
40.0	72.7	2.4	2.2	66.6	73
40.0	71.1
50.3	64.7	14.7	13.7	60.3	77
50.3	60.8	14.7	13.7	56.7	77
50.3	62.7
65.0	49.7	14.5	13.7	47.0	77
65.0	47.9	14.6	13.9	45.6	77
65.0	48.8
65.0	52.0	14.4	13.6	49.1	77
65.0	49.4	14.8	14.0	46.7	77
65.0	50.6
74.5	41.0	14.4	13.5	38.4	79
74.7	40.4	13.8	12.8	37.5	79
74.6	40.7

Benchmarks where handled by a combination: 1.1 mile loop for 18-25 mph; five 2.5 mile samples and keeping middle three segments at 40 mph; the rest were linear tests in no wind conditions. All test routes had ~20m or lower change in altitudes for at least 10 miles (16 km.)

Using the GPS to adjust for the tire diameter has given similar mileage as seen in the past:

I've combined my latest measurements with the calculated mph vs MPG based upon NHW11 drag and assumed engine-to-wheel efficiency of 31%. This is the first time I've used GPS technology as my speedometer and distance measurement with these tire tests. The gray line is the calculated MPG based upon vehicle drag and a 31% efficient, engine-to-wheels (aka., incorporates transaxle, bearings and rolling drag.)

Using just the indicated MPG, new tires with a larger diameter would give a false indication of lower mileage. It also leads to faster true speeds and this increases aerodynamic drag which lowers MPG.

This chart also shows higher efficiency over the hybrid mode, speed limit, 42 mph. There is an energy loss charging the traction battery and later discharging it to sustain speed . . . normal hybrid mode. So when I swap the larger diameter tires to the front, to reduce transaxle rpm for a given indicated speed, we may see the inefficiency of traction battery charge/discharge extend to 45 mph. But we may also see higher efficiency at all speeds from lower transaxle stirring and gear losses.

Bob Wilson

[bwilson4web]

195/70R14 front, 175/70R14 rear

Due to the heat, these numbers are going to be too high. However, they give an indication that there will be no loss of MPG:

mph (gps)	MPG (gps)	miles (gps)	miles (trip)	MPG (veh)	temp F	%
0.0	0.0
25.0	81.4	13.7	12.9	76.6	95.0	106.20%
40.0	76.8	2.8	2.6	71.3	82.0	107.69%
40.0	74.5	2.6	2.5	71.6	82.0	104.00%
40.0	75.6	2.7	2.6	72.8	82.0	103.85%
40.0	72.6	2.7	2.6	69.9	82.0	103.85%
40.0	71.8	2.6	2.5	69.0	82.0	104.00%
40.0	74.2
50.0	61.8	2.5	2.4	59.3	95.0	104.17%
50.0	62.5	2.6	2.4	57.7	95.0	108.33%
50.0	68.1	2.7	2.5	63.1	95.0	108.00%
50.0	61.8	2.5	2.4	59.3	95.0	104.17%
50.0	68.4	2.7	2.5	63.3	95.0	108.00%
50.0	64.1
65.0	52.1	13.8	12.9	48.7	95	106.98%
65.0	54.8	14.2	13.3	51.3	95	106.77%
65.0	53.4
75.0	42.7	14.4	13.5	40.0	88	106.67%
75.0	40.1	14.0	13.2	37.8	88	106.06%
75.0	41.4

These temperatures are high enough to distort the standard day values from hotter lubricants and lower air density. The 18 and 40 mph data points are missing and needed.

Without using a GPS to independently record the speed and distance, it would be very difficult to measure tire mileage effects with any degree of certainty. The tendency of a new tire would be to depress the indicated mileage.

I was hoping the larger diameter tire would have a measurable reduction in rolling drag. I don't see that in this data because if I adjust for the air density, the numbers will fall in line with the earlier results. However, it may be improving my commuting mileage by extending the hybrid mode speed.

Normally, 42 mph is the boundary between hybrid and gas-always-on mode. Increasing it by 6% brings that up to 45 mph so the car is able to turn off the ICE in hybrid mode. For Huntsville AL, 40 mph is a common speed found on many roads or can be maintained without undue impact on other traffic. But 35 mph or even 38 mph is often too slow.

One maximum acceleration seemed depressed but I don't have metrics, yet. I would also expect the built-in, accelerator off, drag to have less effect due to the higher torque. Whether these lead to other vehicle problems awaits gathering metrics.

Bob Wilson