Final results on MPG vs Tire Pressure on G3

Cool! So what PSI are you running now?

 

I think your going to find there is a point of diminishing returns. I think it's going to be similar to this, 30 to 40 psi = 1.5 MPG, 40 to 50 psi = additional .5 MPG, 50 to 60 = no appreciable difference.
Don't quote those numbers, I just made them up, but I think the curve will be something similar.
In other words, I think you have achieved almost all of the additional mileage your going to get by tire pressure. I think really light weight wheels and light weight, low rolling resistance, narrow tires would be the next step. Probably take 1000 years to recoup the investment though. I appreciate your work, I knew there was an increase in mileage, I thank you for quantifying it.

 

Great well thought out post!

Does anyone know the highest pressure allowed?

On the big plus side is tire wear - I bet at 50 you'll increase the life by 50%.....

 

I finally looked into my tire pressure after getting consistent 48 mpg tanks while employing various hyper-miling techniques to maximize MPG.

My pressure coming from the dealer was around 33 psi for each wheel. I bumped it up to 46/44 and wow what a difference.

I am now getting a consistent 52 mpg and I actually like the stiffer feel.

I drive in LA with a lot of stop and go which makes me wonder if a higher PSI helps more in a stop and go scenario vrs long runs. I'm no expert, but maybe the force it takes to turn the wheel off the line is what was dragging my mpg down so much?

 

Very good, thank you, but the hit on MPG was much less than I suspected. I'm amazed that the difference between 30 and 50 PSI is less than 2 MPG on average.

 

Based on what i"ve read, there is roughly 1mpg difference for every 10 extra lbs of air in the tire.

Realizing that tires are the very first level of suspension, I prefer about 40 - 44 psi.... any more than that the ride becomes stiff and less luxury feel as if I have a cheap suspension and can feel every rock and bump on the road... plus there is always the risk of rupture if you hit something of significance.

I hit the edge of the blacktop once where it had broken off pieces an it caused a blowout... that was running 60psi. At that point I realized how dangerous that was and stopped it for my families sake.

A cheaper and-or softer tire seems to tolerate higher pressures better, but the more expensive higher mileage tires that have harder rubber dont fare so well without ruining the ride.

Some tires just wont run the higher pressures without wandering all over the road... especially if they are new.. it feels like your running on tread instead of tires.

I agree with the curve of diminishing returns...... the risks, are not worth the benefit.

But the biggest thing that needs to be accounted for in a thread like this is whether they are running the 17" vs. 15" wheels or not.

Unless all the contestants are running the same size wheels within a few mm, its tough to rule out benefits of air pressure differences as its fairly common knowledge the 17" wheels deliver a near 3mpg hit vs the 15".

 

I prob ran 30K miles before that happened. I expect I would have never had an issue if I hadn't hit something that caused an unusually high air pressure all at once.
The other thing that scares me is the hot temps... I believe you gain about 1psi for every 10 degrees..... on a very hot day over 100 degrees, can you imagine how hot the tires must get on that pavement?

Blowouts freak me out because if they happen, it could be anytime any place, and when you least expect it..... Lord help you if it happens on the front.... I just don't think its worth it.
I was fearless until something happened totally out of my control without warning.

 

We sold our 2001 Echo with 30k miles in the spring. It had the original tires with deep treads in part because I kept the pressure up to max sidewall ... and no higher. This made the handling more precise but bumpier. But the new owner, a co-worker, is very happy with the car and sure enough, the Firestone folks said, 'Those tires sure are old ...' and he said, 'Yes, we like them ... they've been in the family for a long time. I'm going to pass them on to my son when he is old enough to drive.'

Seriously, folks now have information they can use to do price-performance tradeoffs. I too have seen a defective tire with a 'bulge' from an impact or internal delamination. Stuff happens and I don't fault folks for being cautious ... as long as we're all working from similar sets of facts and data. <grins>

Excellent study and great comments! I've really enjoyed this thread.

Bob Wilson

 

Blowouts on the front are easier to deal with than the ones on the back. The reason is you can correct for the pull on the front because the front axle steers. Have one on the back and all you can do is counter steer to keep it under control. But I agree with you, I can do without the excitement. I've only had a couple of blowouts in my life and they were really non events. maybe I was just lucky.
I have never seen a blowout from an overinflated tire, all of them I have knowledge of come from under inflation and the excess heat that comes from that. Ever been around mobile homes when they are being moved? That's one blowout after another.

 

I"ve never had a front blowout, but I would think the ability to "steer" out of it is negated by the blowout itself, I would rather be in a position of correcting.... especially in a front wheel drive car.

 

The change in MPG was immediate. I had been really pushing at 33 psi to get 48, but the 52 came easily right after I bumped them up. The 48mpg was derived off 2 450 mile tanks, and the 52 has stayed consistant so far in my current fill up at 100 miles in. I drive the same stop and go traffic everyday so nothing besides PSI and 1 tank fill, nothing has changed.

The large (~4mpg) increase seems to be well above average which is why I was thinking that a higher PSI would do more for MPG in a stop and go scenario.

 

No problem:

• Sumitomo T4s - 52 psi, maximum sidewall 52 psi
• OEM tires on 2010 - 44 psi, maximum sidewall 44 psi

I'm using 50 psi pressure indicator caps on the Sumitomo tires to detect any loss of pressure. The 2010 Prius, tire pressure system has been reset to reflect the new pressure.

I don't see any problem with keeping the tire pressure at the maximum sidewall. This preserves the tire warranty and avoids going into operational regions where I don't have instrumentation to see what is going on. But once the tires begin to reach end-of-life, I have no problem with trying higher pressures.

Bob Wilson

 

That report is surprising.. especially since most of the weight is up front on most vehicles with the engine.

Well One things for sure.. I hope I never have another blowout.... they are not real fun... .fortunately, I was only going about 40 when I had mine.
You basically have an instantaneous flat tire.

 

It's not as bad as it seems. The higher your pressure at the start, the less the pressure increases from driving.

Also, feel your tires after along freeway drive on a hot day, they will be warm but not shockingly so.

Back in the bad old days of bias ply tires, a long highway run on a hot day would get a tire too hot to hold your hand on it. Properly inflated radials don't heat up anythng like that and they will be much cooler than the road surface on a hot sunny day because of the air blowing over them.

 

Rolling resistance is a higher percentage of total drag at low speeds than it is at high speeds, so the percentage effect on MPG will be higher at low speeds.