Over Inflated Tires, Does it Save More in Fuel Cost than Tire Wear?

If there is any take-away, it is don't let 'perfect be the enemy of good enough.' Sir Issac Newton was one of the inventors of calculus and defined the laws of motion that sent astronauts to the moon, used his pulse as a stop-watch. We are not helpless to take: (1) usable measurements; (2) make a change, and; (3) repeat the measurements.

In 2005, our 2003 Prius had ~55,000 miles and I wanted to test AMSOIL as a replacement for the existing transaxle oil. So I decided to use what I had at hand:

Before changing the transaxle oil, I waited for freezing weather to 'cold soak' the car. Then I drove 30-40 meters to the crest of a gentle hill. The car would accelerate by itself once I let the brake loose. The hill was there on the street I live on.

The bottom graph shows the altitude in meters on the left axis; the distance from the base in meters, and; the slope, 2.5% just below the crest. The peak of the hill is on the right and ends on the left going from 2.5% grade to 1.0% grade. The car rolls to a stop when it is cold enough.

So I ran the tests, one each day to see when the car would stop rolling. On the old oil, white bubbles with the temperature in F. After the change to AMSOIL, the new stopping distances and temperatures in F. The oil let the car roll significantly further. Also, when warm from driving home, the car would reach the stop sign at the bottom with considerable speed, 13 mph with old oil and 16 mph with AMSOIL. But the results of this 10 year old study are not important.​

What I am suggesting is we have our own Prius and it only takes curiosity and commitment, a little time, to do a study. Then we can tell with certain knowledge if it works and how much.

Bob Wilson

 

It's been a long while since I posted since I no longer own a Prius, but here's what I've done over the years with tire PSI. I've noted that at the standard recommended psi auto makers recommend, the front tires will wear a little more on the outer edges. Tire rotation will help balance tire wear out, but not completely. So I bump up the psi by 3-5 lbs psi, just enough to take out a little sag in the sidewall, and help even out tire wear. I do this all around, keep the same differential recommended for the car and doesn't affect the ride much, while increasing responsiveness and feel. My current VW recommends the same psi in all 4 tires since their tire pressure monitor works off the abs sensors comparing tire rotation against one another versus tire psi monitors at each wheel. At the standard 32 lbs psi, the ride is smooth and soft, so I bumped them up to 36. Any higher and the ride degrades too much for me.

 

Just replaced the last pair of the Yokohamas. Odometer reading, 71,000 miles. Here are the tread-depths of the front, worn tires:

	Driv_Out	Driv_#2	Driv_#3	Driv_In		Pass_In	Pass_#3	Pass_#2	Pass_Out
1	.141"	.218"	.194"	.150"		.130"	.168"	.147"	.078"

So the tires probably had another summer of service except for the early wear on the passenger side, outer tread. In a perfect world, the left and right front tires would be swapped without changing direction of rotation. This would even out the outer tread wear to make it somewhat symmetrical. Legally, the minimum tread depth is 3/36 or 0.094". The average of .141" and .078" is 0.109, easily more miles.

The other, simpler option, is to swap the left and right wheels which reverses the tire rotation direction. However, this may change the tire dynamics in wet conditions in a 'bad way.' Something that bears investigation but much easier.

The last tire change suggests there was marginal, front camber. I'd already bought the front bolt needed to adjust, a unique Toyota part, but I had not installed it in part because it requires:

1. Drive to Firestone for a baseline alignment, 1-2 hours
2. Install bolt, take home; jack-up; remove tire; change bolt; install tire, 1-2 hours
3. Drive to Firestone for change alignment, 1-20 hours
4. If OK, done, otherwise repeat steps 2, 3, and 4 until done

Instead, I added 'install camber adjustment bolt on passenger side' to the installation of the two new tires. The new tires go on the rear and the rear tires go on the front. Since Firestone has the tools and reasonable rates they did the camber bolt correctly and here is their report:

If they'd followed my original instructions, 'install on passenger side', there would have been at least two trips to Firestone. But they took an initial measurement and correctly identified the problem as driver side camber.

This makes sense because I primarily use this car for commuting so that side is more often heavily weighted. In contrast the passenger side is lightly loaded so it is more easily able to spin and thus wear. I have had intermittent 'traction control' warning because I like to take my turns fast enough to strip-off the d*mn tailgaters.

Now I'm beginning to wonder IF our left-hand drive, Prius may have more weight on that side, asymmetrical loading than the right-hand drive. On the left, we have the transmission with two dense motors versus the passenger side that has an engine with four mostly gas filled cylinders and crank case. The steering wheel shaft is on the driver side too.

I think it makes sense to measure the driver and passenger wheel loads of the North American Prius. If there is a significant weight difference, heavier on the driver side, then a single driver would make that difference even worse. What this means is we may have to rethink our front-to-rear rotation versus left-and-right. This would even out the tread wear leading to getting more miles from the tires.

Bob Wilson