Tire pressure for 20's

I run 19's so you may discount what I say, but for daily driving I run 36psi front/back.

20s need a bit more pressure because they are bigger, in order to fill up the added space. It's a volumetric thing.

5LHO said:

20s need a bit more pressure because they are bigger, in order to fill up the added space. It's a volumetric thing.

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Hmmm....are you sure about this? I can see where more volume (square inches) of air is needed if the internal size of the tire is larger but wouldn't you fill that volume with the same amount of pressure (Pounds per Square Inch (PSI) no matter what size the tire is? So more air is required, but not more pressure.

Just asking.

5LHO said:

20s need a bit more pressure because they are bigger, in order to fill up the added space. It's a volumetric thing.

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Wat? Pressure is not relative to the volume of air required to fill given area. The VOLUME of air required changes, but that is not what is being referenced or measured here.

OP- Run 35 psi and you will be fine. I run that in my 275/35 and 305/35 - 20s

It is going to vary per tire. I've ran tires in my last truck that recommended mid 30s and tires that were the same size that recommended mid 40s. Compounds, patterns, load rating, speed rating... etc go into pressure requirements.

The height of your wheel has little to do with pressure, so the 20s comment by 5LHO makes no sense, especially since an 18" tall wheel will have taller sidewalls (more volume) than 20s. The ideal gas law is PV=nRT, so your pressure and volume are inversely proportional. If you have a larger volume, you simply need more air for the same pressure.

I'm running 18" wheels, so I may not be the same, but I stay 35-36 psi, which is about what most of my past cars recommended with OEM wheels/tires.

I have 255/35/R20 in front and 295/30/R20 in the rear and run 35 psi.

I run 35 psi on front and rear. Fronts are 19", and rears are 20". If that feels a bit harsh, drop them down a pound or two each but that would be it.

Pete

I think vehicle Type, Use and Curb Weight are the important factors. Not the size of the tire.
Ford recommends 32 psi (cold) for 2014 GT. Front and Rear

The following is from Tire Rack dot com -

Regardless of its size, every tire's load capacity, durability, traction and handling is dependent on using the right inflation pressure for the application. Since both too little and too much inflation pressure sacrifices some of the tires' performance, maintaining the "right" inflation pressure is very important.

While a wide variety of tire sizes are available to fit the many different vehicles in use today, almost every tire size can be used on more than one vehicle. Therefore it is the vehicle manufacturers that ultimately determine the tire inflation pressures they believe best fine-tune the tires' capabilities to their specific vehicle makes and models.

The pounds per square inch (psi) pressure number branded on the tire's sidewall identifies the maximum cold inflation pressure that specific tire is rated to hold. However, the tire's maximum pressure is not necessarily the correct pressure for every vehicle upon which the tire can be used (almost all vehicle manufacturers' recommended tire inflation pressures are less than the tires' maximum pressure).

Therefore when checking and adjusting tire inflation pressures, the "right" inflation pressures are those provided by the vehicle manufacturer, not the "maximum" inflation pressure branded on the tire's sidewall. The vehicle manufacturer's pressure recommendation can be found on the vehicle's tire information placard label, as well as in the vehicle owner's manual.

It depends on the size of your tire, the load range of your tire and the weight of your car. 99% of the time its gunna be 35PSI. IF you had a really low profile tire it maybe like 38 or 40. So put 35 cold in your tire. If there hot put 38

TommyV said:

What pressure are you guy's running in your 20's for the street?

Invo's 275/35's & 315/35's

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I have the tables you need to match load capacities across different tire sizes and load definitions.

Assuming that your car was originally equipped with 235/50-18's in Load Index 97 (standard load) and the door sticker recommends 32 psi . . .

Invo 275/35-20's are Load Index 102 in Extra Load, which would call for 31 psi to match the load capacity of your original tires @ 32 psi.

Invo 315/35-20's are Load Index 106 in Standard Load, which would only call for 23 psi to match the load capacity of your original tires @ 32 psi. But for sure that'll throw a TPMS message, and I feel that's too low for maintaining acceptable handling characteristics anyway. Maybe something more like 28 psi, and you'll still probably have a tendency to wear the tread centers a little faster (and a suddenly cool day will probably still throw a TPMS message).


Basically, I'd suggest starting at 31 front/28 rear and tuning from there to suit your own ride quality and handling preferences and what you can live with for evenness of rear tire wear and TPMS activity.


Norm

Hey Norm - where did you find the tables? Can you provide a link to them. Sounds like great information.

Rob

I've run 35 psi rear on 295/35 and 36-37 psi on 255/35 fronts for years now. Perfect when warmed up, run a few psi lower if out having fun.

5LHO said:

20s need a bit more pressure because they are bigger, in order to fill up the added space. It's a volumetric thing.

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?? I hope you're being sarcastic.

RoboStang said:

Hey Norm - where did you find the tables? Can you provide a link to them. Sounds like great information.

Rob

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I think it was from either a Yokohama or a Toyo site but can't be sure.

It'll be easier if I just attach the pages from a little spreadsheet of mine - this site does say you can attach Excel's *.xls format files.

I'll post them later.


Norm

Here are the tables.

Worth noting is the fact that the rated load line for any given SL Load Index is not the same as for the same Load Index line in the XL table. Basically, to get the same load rating in an XL tire at pressures at or below 36 psi, the XL tire will carry a Load Index number 4 higher than the SL tire. The extra load comes in at pressures above 36 psi. Yes, this means that the same size tire could exist with two different Load Index numbers, depending on whether it was rated as a SL tire or as an XL tire. If you do a search on 235/50-18 over at Tire Rack, you'll find some tire models rated 97 (SL) and some rated 101 (XL).


Norm

Norm Peterson said:

Here are the tables.

Worth noting is the fact that the rated load line for any given SL Load Index is not the same as for the same Load Index line in the XL table. Basically, to get the same load rating in an XL tire at pressures at or below 36 psi, the XL tire will carry a Load Index number 4 higher than the SL tire. The extra load comes in at pressures above 36 psi. Yes, this means that the same size tire could exist with two different Load Index numbers, depending on whether it was rated as a SL tire or as an XL tire. If you do a search on 235/50-18 over at Tire Rack, you'll find some tire models rated 97 (SL) and some rated 101 (XL).


Norm

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Great information Norm. So if I understand this correctly, my original 235/50/18 tires had a load index of of 97 with a recommended pressure of 32PSI. This would yield a load capability of 1455 pounds.

Now if I take the information from my new staggered setup, I think I get this recommended inflation pressure:

Fronts - 255/40/19, EXTRA load index = 100. This means that to attain an equivalent load capability, I would inflate the tire to 33 PSI giving me a load capability of 1466 pounds which is as close as I can get to 1455 without going under.

Rears - 285/35/19, EXTRA load index = 99. In this case I would inflate the tires to 35 PSI to get a load capability of 1466 (again, the closest that I can come to 1455 without going under).

Is this correct?

OP - I apologize and hope I am not hijacking this thread. I think everyone will benefit from this info.

Rob

Yes. For the Load Index data given, those are the same rated load and inflation pressure numbers I get using the full spreadsheet (which has its own lookup functions), and picking a pressure corresponding to a slightly higher load would generally be the way I would choose unless the shortage was only a couple of lbs.


Norm

Norm Peterson said:

Yes. For the Load Index data given, those are the same rated load and inflation pressure numbers I get using the full spreadsheet (which has its own lookup functions), and picking a pressure corresponding to a slightly higher load would generally be the way I would choose unless the shortage was only a couple of lbs.


Norm

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Thanks again Norm. I am a lot smarter about tires now thanks to you!

Rob

Thanks Norm, 28 seems low, but I'll try it. I'm not sure if it makes any difference, but the car originally came with the Brembo 19" wheels.