[RESOLVED] Shouldn't the tire pressure on an AWD be the same all around?

[Jdiddy2k4]

Just a quick question. Shouldn't the tire pressure on an AWD model be the same all the way around. I just got my tires changed at discount tire and they show 32F and 29R, but I was under the assumption that AWD models should have everything the same on all 4 tires.

 

[98GSXtreme07]

Naw, not necessarily. You have more weight in the front because of the engine, so it requires a little more pressure than the rear. At least, thats how I always thought of it.

 

[97 4G63]

Look on your drivers side door there should be a sticker there on the bottom of the door and it shoul"d tell you what your pressure should be.

 

[91talon27]

It says 32 front and 28 rear on my drivers door.

 

[Jdiddy2k4]

Ok thanks for the quick replies. I was always under the assumption that everything needed to be the same for the AWD models or you would risk messing up drivetrain parts.

 

[TunaTalon]

The rolling radius of all four tires needs to be the same on AWD cars.
With the same rated size tires and same air pressure front and rear the greater weight on the front will cause lower rolling radius there.

The specified pressure difference front and rear is to keep the rolling radius and revolutions per mile the same front and rear.

Note that rolling radius takes into account the deflection of the tire under load. Two tires with the same specified size will have different rolling radius if they are loaded differently.

 

[TJs91tsi]

Yes, you are very right that the diameter/size/BRAND need to be the exact same when shopping for an awd, hell even the drive tires of a front or rear wheel drive should be the same size,BRAND etc...

But along with tunatalon, if you pump all four to 32 psi, then load the front tires with an engine/trans/tcase... then the fronts will deflect or deform more compared to the rears. The extra psi is supposed to balance that out. Im pretty sure that the tires actual diameter wouldnt change enough to worry about because of a few psi.

BUT ALWAYS ALWAYS ALWAYS get four tires of the exact same BRAND and size. best to get a spare tooo and rotate it in with the rest. You cant get two 235 50r 16 pirelli, and two 235 50r 16 goodyear tires and expecty them to be good enough for an AWD. Its exactly like your shoe size, it varies depending on what type of shoe you get. I can wear size 12 in vans, and only 11 in penny loafers. haha Every manufacturer is different.

And always check your psi after going to tire places. They sometimes go by the door pillar psi ratings regardless. Even if you install off road baloon tires, or low pro tires ... can easilly overinflate.

Tyler./

 

[DGajre777]

I run 36 psi on all 4 tires on my AWD.

 

[1g_tsi_awd]

Yea, the pressure is somewhat negligible. As long as you are ballpark. It changes with the air temp anyways.

 

[rarson]

best to get a spare tooo and rotate it in with the rest.

I wouldn't do that. The steel belts inside radial tires wear in directionally. If you run a couple thousand miles on a tire with it on the left side and swap it to the right, you risk separating the tread from the tire (I've experienced it more than once with old tires on beater cars).

When you rotate your tires, you shouldn't be crossing sides. Front moves to rear and vice versa, that's it. For a spare, you might as well keep the donut because they're not designed for high mileage use. One of the reasons for that is because the spare might be placed on either side of the car.

 

[Twistaye]

I run 36 psi on all 4 tires on my AWD.

Which probably adds an extra 2mpg, I do the same thing but not quite as much 35 here

 

[FuelInjected]

i also run around 34-36psi all around, all tires the same. I've used different tire brands too before, no big deal, i dunno why u all saying it should be same brand, blah blah. I wouldn;t change just one tire, but i'd change 2 at a time, I am sure its not the best performance wise, having different front from rear, but for daily driving its tottaly fine, if u take into account that u have different tires, and play threar pattern in effect as well, if different. But i got 2 back tires use, and had used fronts from before, my rears went, so i got 2 rears very close thread to match, and hardly noticed a difference, i drove for another 8 months, and then got all new tires all around. the used ones cost me 90 bucks installed.
anyways thats my input on the tires.
i even had mine pumped at 38psi before, and drove fine too for a while, got more mileage too thats why i mainly did it, but now i got some eibachs and the ride is too stiff with that psi, so i lowered it to 34-35

 

[TJs91tsi]

I wouldn't do that. The steel belts inside radial tires wear in directionally. If you run a couple thousand miles on a tire with it on the left side and swap it to the right, you risk separating the tread from the tire (I've experienced it more than once with old tires on beater cars).

When you rotate your tires, you shouldn't be crossing sides. Front moves to rear and vice versa, that's it. For a spare, you might as well keep the donut because they're not designed for high mileage use. One of the reasons for that is because the spare might be placed on either side of the car.

Every tire shop today almost exclusively rotates side to side, UNLESS of course they are directional. The only time ive heard you need to be carefull swaping side to side is if you dont rotate your tires for a very long time, then decide to rotate them. Then tread separation is a possibility. Everyone recommends side to side, especially for awd and 4wd.

When you have a fullsize spare and rotate it in (mostly just for awd and 4wd) you have all your tires worn equally, so if you have a blowout/flat/hole in the sidewall, you can put on your fullsize, same tire spare and carry on. (Yes difficult with directional, but id rather have a tire dismounted, flipped and remounted, rather than buying all new tires) If you dont have the spare, you now have 3 options,
--run a different sized spare tire-- bad for the car,
--replace only one tire and have different tread depth---bad for the car--,
--or replace all four at once--- bad for the wallet.

FuelInjected i also run around 34-36psi all around, all tires the same. I've used different tire brands too before, no big deal, i dunno why u all saying it should be same brand, blah blah. I wouldn;t change just one tire, but i'd change 2 at a time, I am sure its not the best performance wise, having different front from rear, but for daily driving its tottaly fine, if u take into account that u have different tires, and play threar pattern in effect as well, if different. But i got 2 back tires use, and had used fronts from before, my rears went, so i got 2 rears very close thread to match, and hardly noticed a difference, i drove for another 8 months, and then got all new tires all around. the used ones cost me 90 bucks installed.
anyways thats my input on the tires.

yes psi has minimal effect to an extent..

You seem very confident. The only reason why you say that everything was fine running mismatched tires is because your center diff is taking up all the slack. you just couldnt tell you were potentially screwing up your car.
I have a 1980 subaru dl, Part time 4wd. If i run mismatched tires; note same numerical size, different brand, similar tread) and put it into 4wd and move more than a few feet, it becomes VERY difficult to get it back out of 4wd because of the drivetrain bind.. I have to lean on the lever, and when it does go, its followed by a loud clunk. when i have all the same brand tire, it is literally like butter both in and out of 4wd.
I know and could tell its bad for it because it doesnt have a center diff to mask the problem. If it did, i could continue ruining my car unaware.

Tyler./

 

[rarson]

Well, when I worked at a mechanic's shop 12 years ago, there were clear instructions that tires should only be swapped to the other side if they are bias-ply. Not to mention, I know for a fact that the steel belts wear into the tire directionally. I have had a 2-foot section of tread separate from the tire while I was driving an old truck (and it stayed attached to the tire... wap wap wap wap!!! That would do some serious damage to a nice DSM). I've had a quite a few tires that separated without coming loose, creating a nice, nasty bulge in the tire. I've also seen the belts break through the sidewall.

These were relatively old tires, but depending on both the age and mileage of your tires, there can still be a significant risk. Do whatever you like, but I highly recommend that radial tires be rotated front to rear.

A lot of shops out there simply don't know what they're doing (I know, it's a shock, isn't it?). Kind of like the DSM dealership that can't find a transfer case leak or doesn't replace the balance shaft belts during a timing belt job because "it looks okay."

 

[TJs91tsi]

Jdiddy, best bet is to go by what the manufacturer says. Service manual, door post... The fronts are loaded more and need more psi, rears have much less load. If you want to reduce rolling resistance, sure you can run all four the same psi. Might be a reason why no car manufacturer does that though..

Jarson... Did all the tires that youve had bulge/shred on you do so right after you rotated them? Ive had MANY bulge or shred on me too, but they were all sitting for years at a time, whether checked, or abused offroad rallying arround. not after i swapped side to side. hell i didnt even rotate those tires.

Man the only exeption is unidirectional, or A symetrical Tyres that are purposefully meant to only rotate one direction...

~ Big O Tires, Inc.
~ Tire Rotation - Discount Tire Co.
~ Sullivan Tire - Learning Center : Tire Maintenance : Tire Rotation
~ Goodyear Tires | Support | FAQs | Driving Habits
~ Tires & Wheels: Tire Rotation
~ Tire Tech Information - Tire Rotation Instructions

I agree, not every mechanic shop knows what their doing.

Tyler./

 

[disciplekyst]

I personally like to run my car with a pretty big difference between front and rear tire pressure.

The rationale goes like this. Our cars (tsi awd) have about 60% of the weight in front
--> 1400kg x 0,6= 840 kg

I personally put 30psi in the rear (the contact patch seems optimal at that pressure, without any bulging of the tire, so low rolling resistance too).

Considering that the rear tires support 560kg, that makes 18,6 kg/psi
Theoretically, you'd have to put 45,2 psi in the front to have the same ratio
BUT, there are other variable which cannot be calculated that would probably lower that number to a more reasonable 36-37 psi, which is the pressure I put in the front.

So at about 37 psi front and 30 psi rear, You get a radius that is almost exactly equal between front and rear (confirmed by precise measurement), which is what you want for low rolling and drivetrain resistance.

(By the way, the 37 psi not only helps mileage, but It also makes turn in A LOT more responsive)