Experimenting with Tire Pressure?

I inflate all my tires to max sidewall pressure. With
our Michelin Energy Saver tires that is 44 psi.

We have 58,000 miles on them now and they are wearing
evenly and should get another 10,000 miles out of them.

They only have a treadware rating of 480.

MPG's increased 2-3 mpg over the Yoko Avid tires that came
with our Prius from the factory. (we had them inflated to
44 psi also max sidewall pressure).

I would inflate your tires to max sidewall pressure
on all 4 (51 PSI) and give it a try. I bet you will
get 2-4 mpg more than you were getting with 32 psi.

 

Most Prius owners (all versions) tend to find 40-44psi acceptable in terms of ride, noise, and fuel economy. On the other versions we use Toyota's recommended split pressure so that the front is 2psi higher than the rear which is why you see owners stating they run 44/42psi or 40/38psi. Check your door placard to see what Toyota recommends for the c.

We do not recommend going over the tire's rated max pressure which is usually 44psi but can be 51psi in the case of the Fuel Max and AVID Ascend tires. Rolling resistance decreases the most when going from 35psi to 40psi but beyond that the gains in reduced rolling resistance run up against the losses in traction, noise, and discomfort. So it's totally up to you. I ran 50psi in my GenII tires for years. I even went as high as 60psi for quite some time. I'm starting to enjoy the comfort of lower pressures so I'm somewhere between 44/42 and 40/38 now. The MPG difference is not going to be huge unless you do some extreme hypermiling. My suggestion, air them up but stat safe and watch your tire wear closely and adjust pressure as needed.

 

^^ as F8L said. Going past 40-44 does not improves MPG much, but the grip and ride suffers.

I run 42 front in Gen3 and usually keep 4psi less in rear, car is empty most of the time and it helps with cross wind stability.

 

on my new prius c model 4, the dealer knew that I kept my tires set to 44/42 on my 2004 prius and set the C to the same. Seems fine.

 

I'm running 40f/36r

 

I have 16" alloys on mine but find the ride a little stiff. Haven't checked what psi they're at but could someone recommend settings for a smoother ride without sacrificing mpg?

 

Going to buy a digital tire gauge tonight and start with 40/38, after checking the door placard.

Might try 42/40 after. Thanks for all the tips guys.

 

Ok do I just checked the door placard for my 16" alloys and they say 32/29. I confirmed with a digital gauge that my tires are actually inflated to that. Is it safe for me to increase them up to 42/40? That sounds like an awfully big jump to me. I already find the ride a little harsh as it is, so I imagine it might be unbearable at those levels.

 

The maximum cold (that is, hasn't been driven more than one mile in the past hour) pressure is given on the side of every tire. Low rolling resistance (LRR) tires can easily be rated at 50 PSI or more. Up to the stated maximum you should feel free to use whatever pressure gives the best compromise for you between fuel economy, handling, and ride. Have fun!

 

If you checked the tires cold then yes you can air them up to the max tire pressure listed on the sidewall of the tire. Some people go above this number but most recommend against this.

I generally try it at placard levels then increase to max pressure and if the ride is too harsh I lower the pressure in 2psi increments until I find a level I am happy with. Just make sure you adjust the air pressure when the tires are cold each time.

Cold = car has been sitting in a shaded spot like a garage or under a tree/carport and ambient temps are about as low as they will ever get for the current season. e.g. In the summer I try and adjust my tire pressure early in the morning when temps are at the lowest they will be. This ensures I will never go below placard recommended levels because the day will only get warmer not cooler. If you were to adjust the pressure when the tires are hot then when temperatures drop so will your tire pressure and you could end up below manufacture recommended levels which is unsafe and could cause the tires to wear prematurely.

It is important to note that if your car is sitting in the sun then the tires on one side of the car may be hotter than the shades side and thus the tire pressure will not be the same. This is another reason to adjust air pressure in a shaded area or early in the morning. If you have to drive to a refilling station that is more than 1 mile away then you may want to add an extra 1 psi to compensate for a semi warmed up tire.

 

I had mine at 40/38. I'm thinking the ride is a bit jarring. I dropped down 2 psi to see how it rides for a while. Then down a couple more. Then we'll takestock and make a decision.

The absolute most MPG possible regardless of the discomfort or annoyance is not really my cup of tea.

 

In my new C4, with the Bridgestone tires, I currently have 42/40. But I am finding the ride a little harsh, so I think I will go to 40/38 and see how it feels.

 

I upped mine to 39/37 and noticed a 4mpg just up from the previous fill up (very very similar driving conditions)... but this was only my third fill up, and I know the mileage has been increasing each time anyway. Unfortunately it's going to be hard for us to tell how much it helps until we've put several thousands of miles on the engine.

On a positive note the harshness of the ride doesn't seem to me to have changed much. I'm happy.

 

I have my C3 on steel 15s set at 40/38. A little rougher ride and road noise, but higher MPG. It's OK with me.

 

Just brought the Ecopias up tp 40F/38R, we will see how the ride is. I will post thoughts later.

Edit: I have not noticed any decrease in the ride quaility bumping up the air pressure on the Ecopias.

 

Wow I had never heard of running your tires that far over recommended. I may give it a try, but not anywhere near the max on the side of the tire. That's just asking for trouble.

I usually inflate all my cars and bikes at 2psi over what is listed in the manual (in the doorwell). On a motorcycle it's very important for stability, etc, but on a car I guess I can see the benefit... but ride quality is going to plummet past a certain point. Ride is rough enough on the c...

Main thing for wear is to keep the pressure at or above recommended (not at max on tire!) and rotate regularly.

 

I just took delivery of my C2 two days ago. I checked the tires yesterday and all four were at 50psi! I lett out some air so they are 42/40. It seems not as harsh now on the bumps

 

Over the years, I've collected graphs from various sources:


These charts pretty well show the effect of tire inflation on rolling drag. But I had read about race car teams using tire temperature to tune for the track.

Several years ago, I bought an IR thermometer and decided to measure tire tread temperature as a function of tire inflation. I fully inflated the tires and drove on the interstate for 10 miles to heat-soak the tires at different temperatures. I also ran different speeds to understand speed effects on tire temperature. The tire drag shows up as heat or a temperature increase:

Some observations:

• 53 psi@65 mph - lowest tire temperature, significantly lower than any others
• 44 psi@53 mph - lowest tire temperature at 44 psi compared to 62 and 65 mph
• 47 psi@65 mph - local peak temperature, not good, no explaination

The tire literature discusses various resonance modes in tires that can result in higher drag at higher speeds:

It is very likely that these effects are not linear and vary by tire brand, temperature, and speed. What this suggests is tire rolling resistance is a function of pressure and speed and what works for one tire is death to another. Sad to say, the only way I know to find out is to measure the tire temperature in operation, not the easiest thing in the world. However, all data suggests the highest inflation pressure and lowest speeds work best.

Bob Wilson