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Old 12-11-2013, 02:55 PM
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Recommended Tire Pressures

Tire pressure has such a huge effect on MPG.

I'd really love to understand the intent of a manufacturers recommended tire pressure. I know load and handling are a big part. But I think there is more to it. Mainly because the tires vary in pressure so much and it will vary differently depending on environmental conditions. So I think they have to guesstimate.

For example my VW has a single low recommended pressure. A similar Audi has a whole range of pressures depending on the conditions (so did my prior VW's) and so I went by that. MPG went way up. Pressure made sense based on prior similar VW's. 45K on the tires and they are wearing better than average. My guess is they dumbed down the recommendation.

On the Jeep I've been putting in 4 lbs over and it helps quite a bit on MPG. I rarely get less than 22 in the city, often 24. Highway as high as 28MPG on a longer trip. I have been chasing down the lower and lower temps.

But with the nice TPMS you can see the pressure go up a few pounds fast.

So do they have to guesstimate how hot you'll get the tires and target keeping a hot tire under the Max Tire pressure?

The reason I ask is I do 99.9% driving in the city during the winter. So my tires never get that hot. So if I put more air in and stay off the highway (i.e. target my Max pressure to be 44 lbs). Isn't that the same as putting in less air and going 80 MPH on the highway (letting them get hotter) and letting that get to 44 lbs?

Basically what I'm saying is if I monitor TPMS and keep the Max pressure below the max on the tire I should be fine? I don't expect tires to explode. I know so many people that put in what the tire says (Max). I'm more concerned with safe handling and proper tire wear while keeping pressure high but safe and optimal for the primary conditions I use it.

I'm actually a little surprised they have not come up with a way of maintaining the ideal tire pressure at all times. I would think this would save quite a bit of fuel, add safety and increase tire life.

I did ask the Tire guy that mounted my snows if 4 lbs would hurt and said I'd be fine. But recommended not going over 4lbs over. I've been doing it for the last 5 years or so on both cars and I have not seen any bad side effects. But significant MPG increase. On good day my wifes VW CC gets into the low 40's (gas fuel) 50 miles door to door commute. It's rated 31 MPG highway.

Neither car seems to ride harder. Wheel diameter seems to influence ride much more than a few lbs of air.
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Old 12-11-2013, 03:02 PM
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Re: Recommended Tire Pressures

My normal pressure is set to 38 psi cold. On the highway, it goes up to about 40-41 when the tires get hot. It's floated down a little right now with this extra cold weather and with the little bit of snow/ice we have on the ground, I'm not fretting about it, but if it stays cold I'll put some more air in since I don't prefer it to be under 35 psi. I generally get excellent fuel economy, although as expected, it goes down at this colder time of the year.
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Old 12-11-2013, 03:42 PM
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Re: Recommended Tire Pressures

From track days you can probably get out to about 45psi maybe 50psi depending on the tire. Watch out for potholes though

You could always chalk your tires and go play in parking lot.

At least in my 2012 GC with the OEM TPMS the TPMS were within a psi or less of my tire pressure gauge. Gauge is accurate to 1/2 of 1%. Damn TPMS only reads whole numbers.

The pencil type gauges and the gauge on the air at your gas station are inaccurate big time.

I am running 33psi cold in my winter tires. I run 35psi cold in my no seasons.

My tires just have good ole air in them.
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Old 12-11-2013, 04:45 PM
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Re: Recommended Tire Pressures

I run 35psi cold and avg 20.6 combined that includes 10 min idle before n after work due to remote start use
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Old 12-11-2013, 06:17 PM
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Re: Recommended Tire Pressures

I run 42-43 psi cold.
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Old 12-11-2013, 06:30 PM
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Re: Recommended Tire Pressures

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Originally Posted by Jim_in_PA View Post
My normal pressure is set to 38 psi cold. On the highway, it goes up to about 40-41 when the tires get hot. It's floated down a little right now with this extra cold weather and with the little bit of snow/ice we have on the ground, I'm not fretting about it, but if it stays cold I'll put some more air in since I don't prefer it to be under 35 psi. I generally get excellent fuel economy, although as expected, it goes down at this colder time of the year.
That's close to what I was shooting for and doing.

I was shooting for 41-42 when hot but around town.
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Old 12-11-2013, 06:47 PM
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Re: Recommended Tire Pressures

The recommended setting by every car manufacture also takes into account the suspension effect of the tire.
I am not referring to it in a road holding manner, more of a comfort level manner.
I big part of a cars small bump compliance comes from the undamped suspension a tire offers. So when calculating the comfort ride level the tire pressure is a major factor.
This will result in a lower tire pressure than if maximum grip and lower rolling resistance is desired.
A manufacturer can supply a range as a guide, but the amount of variables they have to take into account makes it impossible to truly give you the best setting for your desired outcome.
The amount of weight being carried and its position, ambient temperature, road surface temperate (this can vary greatly independent of ambient temp), desire comfort, control, mileage etc..
Their recommendation will also only cover the size of the tire, not the make and model. The different durometer, tread pattern, tread depth and use can all have major factors in how the tire heats up and maintains temp.

While running at max or above may seem ideal for control and mileage, it may cause the tire to be over inflated for the weight being carried and thus causing the tire to balloon more and give a smaller contact patch, wearing the centre prematurely.

One thing you did miss, was the size of your tires.
So what one recommends here may be wrong as no one has mentioned their sizes.

So, lots of possible factors, you need to experiment yourself.
You actually pay attention to your tire pressures, so that puts you in front of 99% of the population that never even think about it.
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Old 12-11-2013, 07:01 PM
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Re: Recommended Tire Pressures

I found that increasing tire pressures improved both the handling and the fuel economy. Traction may suffer slightly but only in extreme cir instances.

aa I said, on the jeep I run 42-43 psi cold. Last summer during my 4000 mile trip out west, I've seen as high as 48 psi after long stretches at high speed (70-75 mph) for several hours in the high desert areas with outside temps in the 80-90s F.

now that we have snow on the ground, I find the electronic nannies to do a good job and I will not lower the pressures unless we get 1 foot of snow overnight.

on the Subaru there is a manufacturer mandated bias, with lower psi pressures on the rear, to help with stability in low traction conditions. This was critical, on the very poor OEM tires. After upgrading the transmission and now with 3 LSDs, I am far more confident in not spinning on ice.
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Old 12-11-2013, 07:07 PM
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i was running 38-39 on 20" all season. my snow 18" came back from discount tire at the same cold, but I'm gonna lower to 35 I guess
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Old 12-11-2013, 09:24 PM
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Re: Recommended Tire Pressures

I tried running higher pressures than what the owners manual stated and found a very minor increase in mpgs.
I then read a few articles like this one
Debunking a Mileage Myth: Can You Really "Pump Up" Your Fuel Economy? - Popular Mechanics
and came to the conclusion that running a few pounds over doesn't hurt anything.
If I want the best pressure for even tire wear I use the chalk as well and see where the tire wears.
I was surprised that increasing the tire pressure didn't improve fuel mileage that much. However in my case most of my driving is highway so those of you in the city driving might well have more significant increases...
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Old 12-11-2013, 10:28 PM
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Re: Recommended Tire Pressures

Quote:
Originally Posted by fun2drive View Post
I tried running higher pressures than what the owners manual stated and found a very minor increase in mpgs.
I then read a few articles like this one
Debunking a Mileage Myth: Can You Really "Pump Up" Your Fuel Economy? - Popular Mechanics
and came to the conclusion that running a few pounds over doesn't hurt anything.
That article is pretty idiotic. It makes the highly inaccurate assumption that all other factors involving fuel consumption remained the same and that only tire pressures were the variable.
As they have had no control or testing of the wind resistance variable, this test isn’t worth the one second of thought.
The effect on fuel consumption does exist (as they also say), but the bigger change you will notice in increasing you pressures is a slight drop in comfort, and an increase in grip. A trade off I will take every day of the week.
There is a point at which the increase in grip tips due to smaller contact surface with silly high pressures based on the same mass.
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Old 12-11-2013, 10:58 PM
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Re: Recommended Tire Pressures

I'm running OEM 18's which recommends 36 lbs and I'm running 40 lbs.

I think it adds about 4 MPG on average. With a very light foot. If you are heavy footed you might notice less.

What do the 20's recommend for pressure?

I wonder if the 20's vary pressure with temp changes a lot less because they have a lot less air volume. Or should it be the same in theory? I'm not sure

I figure an 18" wheel could be inflated quite bit before it reach the harder feel of the 20".
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