NHW11 tire size study

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	tire equiv. mass
1	175/65R14(*)	919	5.4	23.0	1019	15	100.0%	12.874 kg/tire
2	175/65R14(OEM)	902	5.3	23.1	1102	16	98.2%	13.732 kg/tire
3	175/70R14(*)	886	5.2	23.7	1102	15	96.4%	12.817 kg/tire
4	195/70R14(*)	849	5.8	24.8	1321	19	92.4%	16.200 kg/tire

Source: 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.

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:

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.

 

The first goal, determination of maximum tire size, has been accomplished, Sumitomo 195/70P14. 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 I remember correctly, it was a problem with getting the right load rating. Toyota put the 1019 lb rated, 175/65R14, as replacement tires for my NHW11. So this sets a lower limit.

This makes a lot of sense. One thing I found from the Tire Rack specifications are the limits of:

<section width>/<aspect ratio>R<inner diameter>

• tread width - limits the upper wheelwell clearance and is independent of the above
• wheel outer diameter - limits the rear wheelwell clearance and varies due to tread depth

​

So when someone proposes a different tire, we've got hard numbers to evaluate the potential success. At $70+ per tire, expensive mistakes can be avoided.

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

• handling - going from 175/65R14 to 175/70R14 slightly improved straight-line stability. These first 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.

One intriging 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

 

Thank you for the comments. I'm going to incorporate them in the first post as well as add these references I 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

Not yet but I'll be following your earlier postings when I do.

Bob Wilson

 

The second 195/70R14 arrived and now both are mounted on the rear and the 175/70R14s on the front. The first impression, the seat position is a little higher. It is as if there were a rear adjustment that raised the rear of the seat.

Informal tests showed a 3 mph difference between the indicated and GPS speed at 50 mph, nearly a 6% difference. However, I had not properly inflated the tires and I found the rear tires were at 30 and 32 psi and the front pair were at 45. They are now at 50-51 psi.

After I calibrate the new tires, I'll use a Graham scanner, accurate data, to map mph vs MPG and do a hill climb test. Then I'll swap the tires front-back, and repeat the process. I still don't have a good metric figured out for testing straight-line stability. Regardless, the initial data appears promising.

Bob Wilson

 

FRONT: 175/70R14 and REAR: 195/70R14

	mph (GPS)	MPG (GPS)	miles (GPS)	miles (trip)	MPG (veh)	temp F	Alt. m
1	39.8	85.6	5.2	4.9	80.7	73	25 m -
2	39.1	63.4	5.1	4.8	59.7	73	25 m +
3	39.5	73.0
4	50.3	64.7	14.7	13.7	60.3	77	21 m -
5	50.3	60.8	14.7	13.7	56.7	77	21 m +
6	50.3	62.7
7	65.0	52.0	14.4	13.6	49.1	77	21 m -
8	65.0	49.4	14.8	14.0	46.7	77	21 m +
9	65.0	50.6
10	74.5	41.0	14.4	13.5	38.4	79	21 m -
11	74.7	40.4	13.8	12.8	37.5	79	21 m +
12	74.6	40.7

GPS MPG calculated using indicated fuel burn (i.e., miles/MPG) divided by GPS distance. Using a Garmin nuvi 265wt for GPS speed and distance. The benchmark routes:

• 40 mph - Bailey Cove
• 50 & 65 mph - I-565

I plan to add a 25 mph data point and use 0 MPG for 0 mph. These points should create a usable, polynomial trend line. However, I'm worried about the 40 mph point and will try to repeat this pair.

After running a test, four point, 4-degree polynomial, it looks like the highest velocity point distorts the data values just under that point. I'll add at least one high speed, 80-85 mph point to terminate the high-speed range and possibly one at 75 mph. These should give usable high-speed range values for those who like to cruise at or just above the usual, Interstate speed limits.

FYI, to hold the speed at 40 mph (GPS) the car showed 38 mph (indicated.) Past experience has shown this to be on the knee of the high efficiency curve. Only now the car is traveling at a posted speed limit of 40 mph. This is a critical effect I was after, faster cruise with hybrid MPG. But for data purposes, it would only take one or two 'timed' EV segments with the ICE off to account for the 20 MPG difference in the two runs. So I'll use a Graham scanner/Auto Enginuity so I can equalize the ratio EV and ICE powered segments.

Bob Wilson

 

I may add a 10 mph data point but this is what I'm getting:

	mph (GPS)	MPG (GPS)	miles (GPS)	miles (trip)	MPG (veh)	temp F
1	18.0	85.7	10.3	9.7	80.7	81
2	18.0	85.7
3
4	25.0	79.5	10.2	9.6	74.8	75
5	25.0	79.5
6
7	39.8	85.6	5.2	4.9	80.7	73
8	39.1	63.4	5.1	4.8	59.7	73
9	39.5	73.0
10
11	40.0	68.7	2.5	2.3	63.2	73
12	40.0	71.8	2.6	2.4	66.3	73
13	40.0	72.7	2.4	2.2	66.6	73
14	40.0	71.1
15
16	50.3	64.7	14.7	13.7	60.3	77
17	50.3	60.8	14.7	13.7	56.7	77
18	50.3	62.7
19
20	65.0	49.7	14.5	13.7	47.0	77
21	65.0	47.9	14.6	13.9	45.6	77
22	65.0	48.8
23
24	65.0	52.0	14.4	13.6	49.1	77
25	65.0	49.4	14.8	14.0	46.7	77
26	65.0	50.6
27	7
28
29	4.5	41.0	14.4	13.5	38.4	79
30	74.7	40.4	13.8	12.8	37.5	79
31	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 highway speeds.

Bob Wilson

 

This morning I swapped the 195/70R14s to the front the 175/70R14s are on the rear. Initial impressions:

• front steering seems a little more steady or true - could be tire balance but this is a fleeting impression
• informal MPG - huh? that high??
• GPS vs indicated speed - no change. It is as if the smaller rotating tire(s) are used for the indicated speed. However, the GPS is showing we're about 6% faster than indicated

Bob Wilson

 

Actually the slowest turning wheel or the 'donut' spare would badly distort the speed.

I found doing a reply to a posting with a good table followed by 'advanced' let me see what was being done. In the upper right hand is an "A/A" icon that reveals the tags. Here is the short version:

• "table" within "[" and "]" is the begining
• first row is titles each separated by "|" character
• all subsequent rows separate data columns by "|"
• "/table" withing "[" and "]" finishes the table

What I often do is copy from an excel spreadsheet into a plain text editor. Then I replace the <TAB> characters with "|", add the leading and trailing "table" text and; cut-and-paste into the PriusChat editor.

GOOD LUCK!
Bob Wilson

 

Interesting but I'm not having much luck with Google finding the specifications. In particular, what are the revs per mile?

I'm finding the first order effect is too many revs per mile gives a false sense of MPG but also by reducing the true speed. Too few revs per mile gives a false indication of poor mileage but also causes higher true speeds. Fortunately, it has the benefit of shifting the hybrid threshold speed up so the ICE will shutdown at higher true speeds.

Bob Wilson

 

Interesting datasheet, especially the 185R14 . . . 817 revs/mile.

As for rolling resistance, beats me. California was hot-to-trot and then the Governator came in. Maybe Jerry Brown will breath some life into it. We'll see.

Bob Wilson

 

I've been pretty happy with the Sumitomo T4s that can run 51 psi, maximum sidewall, and are low-rolling resistance. I understand they are crap in snow and ice but living in North Alabama, my limited experience suggests this is not a problem in Dixie. Sad to say, it looks like the Sumitomo T4s are going away.

You really need to calibrate your tripmeter/odometer versus a surveyed distance, say using mile markers or GPS over a flat, straight line. Once you know the calibration offset, you can start to calculate the true MPG using the pump values.

Bob Wilson