Tire PSI question

[gotsmack]

I used to drive a Volvo 760 which took 36 psi in each tire. Now I drive a Subaru Forester which recommends 30 in the front and 29 in the back. They both weigh the same, why is there a difference in recomended psi?

 

[brblx]

it's just different between manufacturers. the specs have generally gone up (with most cars rated at 30psi or more), probably for the sake of safety and fuel economy (ford's 26psi rec was part of the firestone debacle), but there's still a variance between car makers, even for cars that are of similar weight with similar tires. volvos have always been rated pretty high, most are still in the 35psi region.

 

[Bignate603]

It can also include differences in the recommended tires. While the cars are similar in weight the size and type of tires can cause a difference. I'd recommend just sticking with the air pressure from the manufacturer.

Also, just an FYI, there was a big debate on tire pressure here a few days ago in a thread that eventually got locked. You might get some overflow into this thread too.

 

[PlasmaBomb]

You mean to say you aren't running your Volvo at 55 PSI?

 

[kalrith]

Originally posted by: Bignate603
It can also include differences in the recommended tires.

That's probably it. If you had a tire that could hold more air and had a higher max weight in the Subaru, then it probably wouldn't take as much tire pressure as a tire that holds less air and has a lower max weight (assuming the same max pressure between the tires, of course). By "hold more air" I just mean the tire with more volume for air space (like an SUV tire), and "hold less air" would refer to a lower-profile tire.

 

[MBrown]

Braking distance, fuel econony, handling. My guess is the volvo spec is that high to increase MPG.

 

[JDub02]

the way i've always understood it is that it's not the pressure of the air holding your vehicle up, it's the volume. the smaller the "container" (tire in this case), the larger the pressure required to contain the same volume. so recommended pressure is determined by the size of the tire vs. the weight of the vehicle and "tuned" from there for ride comfort, tire wear, grip, etc.

for example, i used to own a 95 wrangler running 33x12.50's on 15" rims. the stock 215/75R15's called for ~32 psi. at 32 psi in the 33's, i was only running on the center 2" of the tire tread. i had to drop the pressure to 20 psi before the outer edges of the tread even touched the pavement. i typically ran it at 18 on the street. if i put those same tires on my dodge ram, i'd run 45-50 psi because i would need a much larger volume of air to support the weight of the truck.

 

[BassBomb]

I don't understand tire pressure too well, my tires look low all the time but they are at something like 40PSI right now

 

[kalrith]

You can't tell if your tires are at the correct pressure just by looking at them. One way to test them is the chalk test. You basically put a thick chalk strip across the treads on your tires. Drive a short distance on a smooth road (maybe 100 feet) and then check the chalk line again. If it's worn out more in the middle than on the edges, then your tires are overinflated. If it's worn out more on the edges, then your tires are underinflated. If all the chalk is evenly worn, then your tires are the correct pressure.

I've read that some people use formulas using the max weight allowance, max pressure, and weight of the vehicle to determine the tire pressure, but the information is conflicting. It seems that the support that tires give at different pressures is not linear. i.e. If your tires support a max weight of 2,000 lbs. each at 44 psi and your vehicle weighs 4,000 lbs., then 22 psi is the result of a linear equation but probably not the correct pressure for your vehicle.

I love math, but sometimes real-world tests trump any amount of calculating that can be done. I just bought new tires of a different size for my vehicle and really need to get out the chalk to test them.

 

[compman25]

Originally posted by: gotsmack
I used to drive a Volvo 760 which took 36 psi in each tire. Now I drive a Subaru Forester which recommends 30 in the front and 29 in the back. They both weigh the same, why is there a difference in recomended psi?

That's funny, my Volvo 770 took 110psi. Think mine might have been bigger though.

 

[kalrith]

Originally posted by: compman25

Originally posted by: gotsmack
I used to drive a Volvo 760 which took 36 psi in each tire. Now I drive a Subaru Forester which recommends 30 in the front and 29 in the back. They both weigh the same, why is there a difference in recomended psi?

That's funny, my Volvo 770 took 110psi. Think mine might have been bigger though.

:shocked: Did it run on bicycle tires?

 

[PlasmaBomb]

Originally posted by: kalrith

Originally posted by: compman25

Originally posted by: gotsmack
I used to drive a Volvo 760 which took 36 psi in each tire. Now I drive a Subaru Forester which recommends 30 in the front and 29 in the back. They both weigh the same, why is there a difference in recomended psi?

That's funny, my Volvo 770 took 110psi. Think mine might have been bigger though.

:shocked: Did it run on bicycle tires?

Have you been taking inflation advice from fleabag?

At 110 psi surely your tyres would look worse than this ... RHS.

That is also over double the recommended maximum pressure.

 

[Toastedlightly]

Originally posted by: PlasmaBomb

Originally posted by: kalrith

Originally posted by: compman25

Originally posted by: gotsmack
I used to drive a Volvo 760 which took 36 psi in each tire. Now I drive a Subaru Forester which recommends 30 in the front and 29 in the back. They both weigh the same, why is there a difference in recomended psi?

That's funny, my Volvo 770 took 110psi. Think mine might have been bigger though.

:shocked: Did it run on bicycle tires?

Have you been taking inflation advice from fleabag?

At 110 psi surely your tyres would look worse than this ... RHS.

That is also over double the recommended maximum pressure.

Tis a semi truck. You been trolled!

 

[JDub02]

Originally posted by: compman25

Originally posted by: gotsmack
I used to drive a Volvo 760 which took 36 psi in each tire. Now I drive a Subaru Forester which recommends 30 in the front and 29 in the back. They both weigh the same, why is there a difference in recomended psi?

That's funny, my Volvo 770 took 110psi. Think mine might have been bigger though.

just by a little.

 

[compman25]

Originally posted by: JDub02

Originally posted by: compman25

Originally posted by: gotsmack
I used to drive a Volvo 760 which took 36 psi in each tire. Now I drive a Subaru Forester which recommends 30 in the front and 29 in the back. They both weigh the same, why is there a difference in recomended psi?

That's funny, my Volvo 770 took 110psi. Think mine might have been bigger though.

just by a little.

But his is a 760, mine was only 1 model higher, a 770.

 

[PlasmaBomb]

What?

The 760 is a car, the 780 is a car... the 770 is a truck.

Thanks Volvo

 

[compman25]

Originally posted by: PlasmaBomb
What?

The 760 is a car, the 780 is a car... the 770 is a truck.

Thanks Volvo

This 780 isn't a car.

 

[gotsmack]

Originally posted by: kalrith
You can't tell if your tires are at the correct pressure just by looking at them. One way to test them is the chalk test. You basically put a thick chalk strip across the treads on your tires. Drive a short distance on a smooth road (maybe 100 feet) and then check the chalk line again. If it's worn out more in the middle than on the edges, then your tires are overinflated. If it's worn out more on the edges, then your tires are underinflated. If all the chalk is evenly worn, then your tires are the correct pressure.

I've read that some people use formulas using the max weight allowance, max pressure, and weight of the vehicle to determine the tire pressure, but the information is conflicting. It seems that the support that tires give at different pressures is not linear. i.e. If your tires support a max weight of 2,000 lbs. each at 44 psi and your vehicle weighs 4,000 lbs., then 22 psi is the result of a linear equation but probably not the correct pressure for your vehicle.

I love math, but sometimes real-world tests trump any amount of calculating that can be done. I just bought new tires of a different size for my vehicle and really need to get out the chalk to test them.

Thanks, Will pick up some chalk when I get the chance.

 

[gotsmack]

Originally posted by: compman25

Originally posted by: JDub02

Originally posted by: compman25

Originally posted by: gotsmack
I used to drive a Volvo 760 which took 36 psi in each tire. Now I drive a Subaru Forester which recommends 30 in the front and 29 in the back. They both weigh the same, why is there a difference in recomended psi?

That's funny, my Volvo 770 took 110psi. Think mine might have been bigger though.

just by a little.

But his is a 760, mine was only 1 model higher, a 770.

had me going there!

 

[JulesMaximus]

what determines how much air goes into a tire

fleabag

 

[PlasmaBomb]

Originally posted by: compman25

Originally posted by: PlasmaBomb
What?

The 760 is a car, the 780 is a car... the 770 is a truck.

Thanks Volvo

This 780 isn't a car.

This 780 is a car.

 

[Kadarin]

Originally posted by: brblx
ford's 26psi rec was part of the firestone debacle

That was the most fucking ridiculous decision on Ford's part to make that recommendation for an Explorer.

 

[PhoKingGuy]

Originally posted by: JulesMaximus

what determines how much air goes into a tire

fleabag

:beer:

 

[fleabag]

Originally posted by: PlasmaBomb

Originally posted by: kalrith

Originally posted by: compman25

Originally posted by: gotsmack
I used to drive a Volvo 760 which took 36 psi in each tire. Now I drive a Subaru Forester which recommends 30 in the front and 29 in the back. They both weigh the same, why is there a difference in recomended psi?

That's funny, my Volvo 770 took 110psi. Think mine might have been bigger though.

:shocked: Did it run on bicycle tires?

Have you been taking inflation advice from fleabag?

At 110 psi surely your tyres would look worse than this ... RHS.

That is also over double the recommended maximum pressure.

That diagram is of BIAS-PLYS you retard.

 

[PhoKingGuy]

Originally posted by: fleabag

Originally posted by: PlasmaBomb

Originally posted by: kalrith

Originally posted by: compman25

Originally posted by: gotsmack
I used to drive a Volvo 760 which took 36 psi in each tire. Now I drive a Subaru Forester which recommends 30 in the front and 29 in the back. They both weigh the same, why is there a difference in recomended psi?

That's funny, my Volvo 770 took 110psi. Think mine might have been bigger though.

:shocked: Did it run on bicycle tires?

Have you been taking inflation advice from fleabag?

At 110 psi surely your tyres would look worse than this ... RHS.

That is also over double the recommended maximum pressure.

That diagram is of BIAS-PLYS you retard.

Took you long enough to show up here