Tire Pressure Poll - Where is Yours?

If that curve were representative of the Prius and my tires, and given the usual 44 PSI max pressure limit, I'd probably never my tires higher than 40/38.

Right now I'm running 37/35 and that curve suggests I'm probably already within a single digit % of lowest rolling resistance, without compromising either safety or comfort.

 

I'm not entirely sure what to make of this graph.

The data on the graph coincides nicely with what we've been
told, but "Coincidence is not causality."

How can I tell that this graph applies to the tires on a Prius?

At first glance it would appear to be meaningful, but how can I
determine it's intended applicability...

Or is it universal? Can that be? Wouldn't it have to be calibrated
or adjusted to max side wall pressure, or maybe max load, or what?

Would it also apply to the tires on a large SUV, on a Miata... on a
motorcycle?

Edit: Here is the site where the graph originated. It is just a general
discussion of the factors contributing to rolling resistance.

So unless I'm mistaken, I don't see the numbers on the graph being
the uncontrovertible source for the "best" tire pressures on our Prii.

MSantos, thanks for posting this thought provoking information.

 

50/48 4.3L/100km

 

Here is what he suggests.

I recommend 3 to 5 psi above the placard specification. Anything more than that starts to compromise traction - with decreasing effects on fuel economy - and in my opinion this compromises safety too much.

But under no circumtances am I advocating the use of inflation pressures higher than what is written on the sidewall (with the exception of tires with 35 psi max. on the sidewall - and there's a long story there!)

Again, that's a general recommendation and not one specific to the Prius or the tires, but it does jive with my 40/38 max number above.

 

However, it doesn't jive with anecdotal evidence gathered by the extreme hypermilers. There is a disconnect somewhere and it is either in that graph or the experiences of extreme hypermilers is wrong. Even non-extreme hypermilers can feel the difference in ease of gliding when going from 40psi to 60psi. *shrug* Until I set up a test for such hypothesis I will not stand strong on the idea but I do lean towards the higher psi improving mileage.

 

I'm sure higher PSI does improve mileage to a certain extent. However, the improvement in mileage after 40 PSI is likely comparatively low, and going to 60 PSI compromises traction. 60 PSI is actually 36% higher than the usual maximum recommended specification too.

Remember, this is not a hypermiler competition. This is driving on city roads and highways under a variety of conditions with all sorts of yahoos on the road.

 

Are we sure it decreases traction? Could you point me to the most current tests on this? I know with older style tires this was definately the case because of tread squirm and such but I would like to see how the newer radial tires fare. This would be helpful in determining where a good happy medium between mpg and safety is.

I ask because in nearly all auto racing situations, other than drag racing, that I have participated in we tend to overinflate our tires to improve handling, reduce tread squirm and sidewall flex as well as. The daily grind is not a racing competition though so I'd like to learn more about the safety aspect if you can point me somewhere.

 

Since when do racers recommend increasing pressures to way beyond specifications? Where do people recommend racing 44 PSI street tires at 60 PSI?

People do raise their pressures, but generally not to those levels.

 

When the chalk on your tires tells you that the sidewall is rolling over when you corner hard. I never stated they run 60psi in a 44psi tire. I just said they overinflate them. Racers also overinflate their front tires to reduce rolling resistance to decrease your "roll out time" from a trip light (drag racing) and to reduce rolling resistance and when the skinny front tire is not rated for the extra weight of a full size drag car. I.E. using VW bug sized tires on the front of a Camaro.

 

I don't believe the graph represents the empirical truth since there are still so many variables that would have to be factored in.

Still, despite its shortcomings it still helps most of us visualize the relationship between higher inflation levels and rolling resistance.
We do notice that while it is fraught with a trend of diminishing returns, the benefits still continue as we inflate even further... except the effects will be far less dramatic for most people and only meaningful to a smaller group of folks (hypermilers).

Most people will agree that there is a big difference in FE when going from pressures in the mid 30's to mid 40's. Once you try bumping the pressures to max sidewall (or close to it), anything less just seems to produce poorer results and for many going back to the mid 30's is just unthinkable and mediocre.

Hypermilers in the other hand will be quick to point out that there's a big (still relevant) difference between pressures in the 40's vs 50's vs 60s and beyond. As you climb to higher pressures it becomes a game of minimums where a fractional improvement can mean a great deal in terms of final results. It is very addictive.


Cheers;

Msantos

 

Perhaps not, but you're the one that originally brought up the 60 PSI number.

Not really. Lots of people end up lowering their pressures back down to the high 30s, because the ride above 40 often becomes much harsher.

That's true even on this forum, where many people are obsessed about gas mileage.

 

Again, I said that "many" (not all) people would not consider returning to pressures below 40 psi and I believe I remain correct... at least on the basis of my personal experience in different forums... and the very results of the poll at the head of this thread.

Cheers;

MSantos

 

Relating to another topic, yes I did.

Now will you stop arguing with me and present the data source where you got your information regarding traction loss with higher pressures so I can see where the critical pressure is?

 

This happened to me today going 50mph on the [dry] highway. I wasn't pleased.

42/40 here, 54-56 mpg in spring and summer weather.

 

44 front 42 rear

 

I guess you haven't been reading this very thread. People have traction issues once the pressures increase too much. That "too much" of course varies depending on the tire, but it's commonly reported.

If you want to do this, that's fine, but it seems some here almost feel that there are no consequences to extremely high tire pressure aside from ride harshness. That IMO is foolish, esp. since the tire pressures some advocate are far beyond what the tire manufacturers themselves set for the maximum limit.

For the record, that's not what you actually wrote.

"Most people will agree that there is a big difference in FE when going from pressures in the mid 30's to mid 40's. Once you try bumping the pressures to max sidewall (or close to it), anything less just seems to produce poorer results and for many going back to the mid 30's is just unthinkable and mediocre."

 

Can you please point them out?

I've not had any traction issues. In fact my cornering ability has increased. I was specifically talking about traction during hard braking situations though and I have not found any data concerning this issue but it sounded like you had givin your conviction on the subject so that is why I asked you for more details. I truely do want to learn more but I am not interested in opinions not backed up by data.

 

just changed my pressure to 42 front / 40 back and filled up today. We'll see what happens!

 

Even on this very page is a quote of reduced traction with higher pressures. Scroll up a few posts. It links this post.

Remember, cornering ability is not the only measure of traction.

 

side issue/ I have a dodge caravan with tires that say 40PSI max cold on the sidewall. I did not realize this and have been running 44PSI. First tire i have seen that is not 36/44/51/60/120 etc