Tire Pressure on Fuel Economy

Yeah likely is, and I'd agree with your reasons. Obviously there could be a slight increase, but luck, driving conditions, ambient temps, and your driving style all play a factor.

I went the other way: when we first had our Prius (with 215/45R17) I raised pressures to over 40 psi, like I typically did with our previous car, which had 195/65R15 size. Someone here mentioned keeping the 17" pressures around 34~36 psi, more comfortable, little or no drop in mpg. That was my experience. The 17" spec'd pressure is lower, 33/32 f/r, I go with 34 all around.

 

What you have observed is the benefit of correctly inflated tires. Without a doubt, 34F/34R PSI is under inflated.

From observation of tire wear, it indicates the placarded pressures of 35F/33R PSI leaves the tires under inflated. The sweet spot where you get maximum improvement is around 39F/37R PSI; after that the benefit and safety decreases. (Just in case you were thinking 15% increase - good, therefore 30% increase - better; it won't. It does not extrapolate in a linear fashion.)

I run my tires at 39F/37R and as the tyre wear indicates this the correct inflation pressure, for the tyres on my car, it also gives much better FE (fuel economy).

I can't say exactly as tire pressure is only one of a few variables that can affect FE. But just changing tire pressure (up to the correct pressure) makes an improvement in FE. This makes sense as under inflated tires makes for higher rolling resistance. This is true across all tires, not just those marked LRR (low rolling resistance).

I would never advocate running on over inflated tires. You will wear out the tires faster and it is not safe. Always try to keep the tires inflated at the correct pressure.

With all due respect Mendel, comparing 17" wheels with LP tires and 15" wheels with SP tires is like comparing apples and oranges.

 

i run 40/38, but the mpg increase is not measurable by eyeball. perhaps lab testing would offer better insight. you can always try nitrogen...

 

Don't get too excited, you live in San Francisco. Going down the hills really helps boost your MPGs. Get out of the hills and onto flat roadways elsewhere in California and you'll notice a decrease in economy.

 

I tend to do better on flat. Trouble with hills: they go down and up.

 

How do you go down a hill without first going up? There is always a net loss (up a hill then down the hill), so your statement makes no sense.

 

Plenty of examples. Here, you can constantly go downhill or uphill:

 

well I have a 2014 with 68k miles now. I am up for tires...
When I take my car in to the dealership for an oil change they let all my air out. I then get 53mpg... and i go... CRAP I FORGOT... over to Wawa for air and Viola... back to 63-70mpg.....

I run 43/43 (f/r)

I am going with a totally different tire so hopefully I dont take a mileage hit... Hankook Optimo H727

 

fyi I just see my fuelly now.. LOL.... ummm that was at 34/34 air... then I saw the light.. and i dont have time to track all my fillups...

 

There are a lot of factors at play. If you drive a lot of roads at lower speeds (e.g. 25-55mph) without much braking, then the rolling resistance of the tires will be a large portion of the energy use of the car. However if your drive involves lots of braking or high speeds that incur lots of air resistance, rolling resistance will play a smaller function.

There's a good chart on this page that I feel is pretty accurate: Tire pressure for 2014 Prius | PriusChat

You can see that that increases from 35 to 50 are fairly marginal, going from 48 to 49.5mpg. I agree that the increase in fuel economy you're seeing likely has more to do with a change in driving style instead of increased tire pressures. However the tires pressures didn't hurt

 

The great "PSI experiment" is over, for me. I'm back down to factory specs. And I haven't noticed any decrease in mpg.

This is my first Prius, so I don't know if the Gen4 suspension is more sensitive to changes in PSI... or if it's just my Toyo tires. But I got tired of the stiff and harsh feel every time I went over even a small bump or imperfection in the road. I was only 5 PSI over specs, but the ride quality was more degraded than I was comfortable with. And I also noticed occasional rattles in the dashboard.

Now that I'm back down to down to 36, everything is smooth and comfortable again.

 

Our '09 seems to demonstrate a much harsher ride at increased tire psi than our '12 does

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Higher pressures have got to have some impact on shocks, suspension and wheel bearing longevity. I push pressures up but only slightly.

 

Where is ETP hiding???

"42 PSI is the answer to all things in the Universe."

Yeah, it's good, right up to the point where your teeth rattle out of your head!

 

With 15" tires 42 psi feels not bad, but with low profile 17" harsh ride.

 

I will let you know next week! as I will be getting my new tires tomorrow. Stretched 68k out of the factory 195's and just dont feel comfortable completing my 65 mile commute (1 way) in them...

well I guess I dont mind the stiffer ride as my 1978 chevy 1 ton rolling on 40" bias ply hawgs will shake the snot out of ya....

 

my new michelins came back at 36 all around. very smooth and comfortable. i'll pump them to 40f/38r like i had the oem's, to see if there's much difference in ride quality.

 

Or..., leave 'em where they are, try a tank, see if there's any drop? Or an uptick due to having the LRR champions.

What were the old ones?

 

forgotten already. goodyear assurance i think. the trouble with trying a tank, is that i have nothing to compare it to, unless there's a major change. a few mpg either way can have a lot of different causes. just going by my local ev miles, they are similar, even at the lower psi.