NHW11 tire size study

The best I could do were some high-speed passes over speed bumps. I just didn't have an extra 500 lbs of people. <grins>

I am only running the larger tires on the front and there is plenty of clearance. The rear are something more standard but I don't have their specs handy. It might be in the thread.

Bob Wilson

 

I use Sumitomo T4, 195/70R14 on the front but I don't see them at Tire Rack. There is a Sumitomo Touring LS T that looks similar.

As for rise, it depends upon your current tires. If you have something like the stock tires:

• 24.9" - 23.3" = 1.6" diameter change -> 0.8" radius is the rise

I have not read of any effort to raise the car but this is not an area I've had an interest. I would expect there might be challenges in the steering linkage. Now I have looked at air-bags that are inflated in the rear springs but just to handle heavy towing loads.

GOOD LUCK!
Bob Wilson

 

Buy one and mount on a full size rim. Test it by mounting on a front and later a rear wheel. Then drive in the worst conditions to check for interference.

Remember that my goal was improved, highway, straight-line stability. My worst case condition were speed bumps. So my testing did not worry about the reduced motive forces.

The torque at the front, drive wheels is fixed by the Prius design. The longer radius of a larger tire means the forward thrust and rear braking forces are proportionally reduced. This may become a problem if you have to negotiate a steep grade.

Bob Wilson

 

Ending status: 2014/09/05
Thread started: 2010/07/03

So here are the tread depths, no swapping, just running:

	position		driver						passenger		Sumitomo T4
1	front	.053	.097	.110	.079		.071	.104	.104	.079	P195/70R14
2	rear	.077	.158	.148	.122		.064	.076	.068	.058	P175/70R14

2/32 = 0.0625 - legally worn out
3/32 = 0.09375 - close

Lessons learned:

• Because of the different sizes front and rear, left-right swaps would be the only practical option
  • Left-right rotation could probably have gotten another year
• Higher tire pressures than normal 51 psi (max sidewall) needed
• Check camber on driver rear as the inner and out wear ratio is too high
• Data suggests driver rear wheel is relatively unloaded compared to passenger side rear wheel

Background:

Earlier this week, I had an unexplained, tire slippage while on a rain-slick street. The car handled it fine but I knew it was time to look at the next tire. So I ordered a pair of Yokohama, P195/70R14, which are slightly wider tread than the Sumitomo T4, P195/70R14. But testing revealed they have interference with the rear wheel well on a dip. I tried to map out the problem but it looks like major surgery would be required. Fortunately, they work just fine in the front.

A quick test this morning found, 22.8/21.1 ~= 108.0% +/- 0.5% over size. It is amazing to see 60 mph indicated and GPS reporting 65 mph true. But they have interference with the rear wheel well.

So I've ordered a pair of Yokohama, P185/70R14, that are slightly smaller than the Sumitomos scheduled to arrive Tuesday. These will go in the rear and the old driver-side tire and rim will become the new spare.

We'll have Firestone mount them and do an alignment. I'll check their numbers against the wear pattern of the last set and adjust the shims as needed before fully inflating them. Then I'll restart the driving stunt.

Bob Wilson

 

I ran a preliminary test, not even fully inflating all tires, just to see how the P195/70r14 Yokohamas might work out:

• 55.8 / 52.0 = 107.3% +/-0.2 %
• 52 / 74.7 = 0.696 gallons
• 55.8 miles (GPS)
• 55.8 / 0.696 = 80.17 MPG (true)

Correction, a 954 mile tank. We won't really know until the new rear tires are mounted, wheels aligned, and fully inflated tires.

Bob Wilson

 

Adding the results of tire alignment:

	position		driver						passenger		Sumitomo T4
1	front	.053	.097	.110	.079		.071	.104	.104	.079	P195/70R14
2	camber			0.1					-0.1
3	toe			-0.02					0.06
4	rear	.077	.158	.148	.122		.064	.076	.068	.058	P175/70R14
5	camber			-1.3					-1.0
6	toe			-0.12					0.39

2/32 = 0.0625 - legally worn out
3/32 = 0.09375 - close

Excessive toe on the right rear would have driven the rear to the left or clockwise looking down at the car. This probably induced the outer wear on that tire and likely led to the excessive wear on the front left tire and others.

Bob Wilson