How to determine proper tire pressure

[Jeff73Mach1]

How to determine proper tire pressure when using aftermarket wheels and 17 tires.

I don't know!

My best guess is that using the inflation guidelines for a modern mustang with comparable weight and tire sizes would be the most accurate, but that our different suspensions might have an effect that is unanticipated.

I feel confident that the maximum PSI of the tire manufacturer is not the right pressure and that the pressures for 70 series 15 inch tires aren't likely to be helpful.

What have you done to determine what the proper inflation level is for your oversized wheel tire combos?

I started by looking at cars with comparable weight and tire sizes and determined that 32 PSI front and rear was a close approximation of what the factory would have said about my tire sizes. Then I went to BFG's site and looked at the following chart:

For high-speed driving, additional inflation pressure and possibly reduced tire loading and/or upsizing is required. In the absence of specific recommendations by the vehicle manufacturer, use the following guidelines based on those in the European Tyre and Rim Technical Organization Standards Manual.

 


For speeds over 160km/h (100 mph), load and inflation must be adjusted according to the table below.

 


Y-Speed Rated Sizes:

 


Maximum Speed (mph) 118 124 130 136 143 149 155 161 168 174 180 186

 


Inflation Increase (psi) 0 0 0 0 1.5 3 4.5 6 7.5 7.5 7.5 7.5

 


Load Capacity (% of max) 100 100 100 100 100 100 100 100 100 95 90 85

By reviewing the chart, I find my tires are all within the load rating as published by the manufacturer, but that the top speed I have driven does get into the range that requires some additional inflation of 4.5 pounds. Since I rarely open her up long enough to reach or hold those speeds, I run 32 PSI front and 30 PSI rear at this time, but increase the pressure to 35 PSI front and rear if I know I am going to be pushing the car hard.

Anyway, I was hopeful my thought process might help others to understand that the maximum inflation pressure of a tire is not the right number to just pick.

Drive safe and have fun

 

[luxstang]

I dunno either.

For what it´s worth, on the 18 inch tires, I simply put in between 33 and 36 psi (2,3 to 2,5 bar, converted by google, so no guaranty on accuracy). That is about what you put into other cars as well and I have never had a problem with that pressure. After a while I check how the tires wear off and raise or lower the pressure accordingly.

By the way, I used the same pressure on the old 15 inch tires for years.

Just for additional info: When we made the video, I inflated the rears with 65 psi, to allow for easier fishtailing and burnouts.

 

[Jeff73Mach1]

Just for additional info: When we made the video, I inflated the rears with 65 psi, to allow for easier fishtailing and burnouts.

ROFLMFAO I never expected the thread to go here, but I'm glad it did!

 

[OMS]

My son's 2002 GT door tag says 30 psi cold

I don't see where it would be much different

on a 71-3

Watch your tire wear

Middle wear is over inflated

Outer wear in under inflated

 

[Don C]

There used to be a lot of load/inflation charts, but they are becoming more difficult to find. Here is one that I found from Firestone, that is a little limited on tire sizes:

http://www.tiresafety.com/images/Tire%20Replacement%20Manual.pdf

The load/pressure charts start on page 29, for our tire sizes.

Here's on from Toyo:

http://toyotires.com/tires-101/load-and-inflation-tables

If your tire size isn't shown, find what your tire's load rating (index) is and then find the closest one to it in the tables, for your tire's diameter and aspect ratio. Match your approximate weight on each tire to the chart. The load capacities change for every aspect ratio and diameter. This is probably why it has become more difficult to find these charts, as the number of variations in tire sizes has grown dramatically over the years.

You are correct, higher speeds require more pressure, to reduce the sidewall flex and heat build up. However, do not exceed the maximum tire pressure shown on the tire sidewall.

 

[Jeff73Mach1]

There are plenty of folks that over inflate because they don't know any better. I was trying to go through the exercise of arriving at an inflation level in a manner that was logical and utilized data provided by both tire companies and an independent technical organization.

I know where I want to run my tires and why (in spite of the statement to the contrary at the start of my post) even though I do not know all the formulas and parameters that go into tire inflation specifications.

I thought that it might help others determine what pressures to run on tires that were never intended for our vehicles and their antiquated buggy like suspensions


Thanks Don C. for elaborating on what I was trying to do. You get it, but a lot of folks don't.

 

[Don C]

The days of trying to decide if I wanted to replace my 670-15s with 710-15s are long gone. The number of variables in tire sizes, not to mention speed ratings, has gotten huge. Bigger tires do not necessarily mean being able to run a lower pressure, like it used to, its going to depend on aspect ratios, diameters, and speed ratings. Some of us can "read" the tires, tell if the alignment and pressures are correct, but that is something that it seems like fewer people are learning. The majority of people can't even figure out the pressure table on their door jam (just look at the number of the squished out sidewalls the next time you go out for a drive).

I agree with you, and this is a good time of the year to educate folks on tire pressures, what with the warm (hot here) weather coming. Heat buildup destroys tires.

 

[midlife]

For general street driving, this is how I determine the optimal tire pressure, which is entirely empirically based upon first principles.

To start, the idea is to maximize the patch area of the tire onto the ground, particularly from side-wall to sidewall, and make that patch have even pressure. I inflate the tires to what I think is reasonable, then drive the car carefully over a stream of water on concrete, and examine the wet tread pattern left by the tire exiting the water. You want that tread pattern to decay evenly from side to side as a function of distance from the water. If the last piece of wet tread is in the center of the tire, then the tire is over-inflated. If the last piece is on both edges, the tire is under-inflated. If the last wet tread is on one side or the other, get an alignment then repeat.

I got this trick from a person who worked years in a tire shop.

 

[Don C]

For general street driving, this is how I determine the optimal tire pressure, which is entirely empirically based upon first principles.

To start, the idea is to maximize the patch area of the tire onto the ground, particularly from side-wall to sidewall, and make that patch have even pressure. I inflate the tires to what I think is reasonable, then drive the car carefully over a stream of water on concrete, and examine the wet tread pattern left by the tire exiting the water. You want that tread pattern to decay evenly from side to side as a function of distance from the water. If the last piece of wet tread is in the center of the tire, then the tire is over-inflated. If the last piece is on both edges, the tire is under-inflated. If the last wet tread is on one side or the other, get an alignment then repeat.

I got this trick from a person who worked years in a tire shop.

Nice trick, sure beats waiting for the tires to wear...

 

[OLE PONY]

If it helps my 2004 came stock with 17 inch wheels and I run 34 psi in them. Probably a good psi to start with based on the input above.

 

[jadatis]

I live in Holland and end 2007 I got hold of the European formula and its addings for speed and camber-angle ( later) , that the tire-makers in Europe use for determining advice -presssure.

Declared the formula holy , and learned myself Excell to make spreadsheets for it.

Translated a few to English to go worldwide with it and now google dayly for tire/tyre-pressure to see If I can make my spreadsheets to use.

So came today to this topic and registered to give you information.

But by reacting and searching for information, I came to the conclusion that , the formula and system is not that holy as I declared it in the beginning.

Also discovered that sertain tires ( low aspect ratio, AR-The 40 in fi 235/40ZR 19) are calculated to high in their maximum load.

So I will give you the link to a map on my public map of skydrive, with spreadsheets to re-calculate, yust what you need here, you should think.

But in that the old formula is used and you have to substracht for low AR tires 20% of the maximum load, to get save answers for the tires.

that is why it comes to low pressures for those tires.

If you give me the needed data of tires and car, I can do it for you here as an example.

the needed map

To use a spreadsheet first download it by rightclicking on the document, then choose download, and not open in Excell or Exell Web app)

After download and eventual virus check, open it in Excell or Open Office Calc to use it. If you accidentially leftclick it will be tried to open in the Excell like program in the cloud, but this cant handle sheetprotection (to prefent overwriting formula's) and data validation ( to make dropdown-boxes) that I used in it.

So try it out and dont hesitate to ask me for help

Greatings from Holland

Peter

 

[Totalled]

For general street driving, this is how I determine the optimal tire pressure, which is entirely empirically based upon first principles.

To start, the idea is to maximize the patch area of the tire onto the ground, particularly from side-wall to sidewall, and make that patch have even pressure. I inflate the tires to what I think is reasonable, then drive the car carefully over a stream of water on concrete, and examine the wet tread pattern left by the tire exiting the water. You want that tread pattern to decay evenly from side to side as a function of distance from the water. If the last piece of wet tread is in the center of the tire, then the tire is over-inflated. If the last piece is on both edges, the tire is under-inflated. If the last wet tread is on one side or the other, get an alignment then repeat.

I got this trick from a person who worked years in a tire shop.

That's the nice way of doing it. The old school way is to lay rubber and adjust pressure till it leaves the widest, most consistent stripe.

 

[73vertproject]

I live in Holland and end 2007 I got hold of the European formula and its addings for speed and camber-angle ( later) , that the tire-makers in Europe use for determining advice -presssure.

Declared the formula holy , and learned myself Excell to make spreadsheets for it.

Translated a few to English to go worldwide with it and now google dayly for tire/tyre-pressure to see If I can make my spreadsheets to use.

So came today to this topic and registered to give you information.

But by reacting and searching for information, I came to the conclusion that , the formula and system is not that holy as I declared it in the beginning.

Also discovered that sertain tires ( low aspect ratio, AR-The 40 in fi 235/40ZR 19) are calculated to high in their maximum load.

So I will give you the link to a map on my public map of skydrive, with spreadsheets to re-calculate, yust what you need here, you should think.

But in that the old formula is used and you have to substracht for low AR tires 20% of the maximum load, to get save answers for the tires.

that is why it comes to low pressures for those tires.

If you give me the needed data of tires and car, I can do it for you here as an example.

the needed map

To use a spreadsheet first download it by rightclicking on the document, then choose download, and not open in Excell or Exell Web app)

After download and eventual virus check, open it in Excell or Open Office Calc to use it. If you accidentially leftclick it will be tried to open in the Excell like program in the cloud, but this cant handle sheetprotection (to prefent overwriting formula's) and data validation ( to make dropdown-boxes) that I used in it.

So try it out and dont hesitate to ask me for help

Greatings from Holland

Peter

Wow, that is a lot of information, Peter. Welcome to the forum and thanks for your contribution!

 

[midlife]

For general street driving, this is how I determine the optimal tire pressure, which is entirely empirically based upon first principles.

To start, the idea is to maximize the patch area of the tire onto the ground, particularly from side-wall to sidewall, and make that patch have even pressure. I inflate the tires to what I think is reasonable, then drive the car carefully over a stream of water on concrete, and examine the wet tread pattern left by the tire exiting the water. You want that tread pattern to decay evenly from side to side as a function of distance from the water. If the last piece of wet tread is in the center of the tire, then the tire is over-inflated. If the last piece is on both edges, the tire is under-inflated. If the last wet tread is on one side or the other, get an alignment then repeat.

I got this trick from a person who worked years in a tire shop.

That's the nice way of doing it. The old school way is to lay rubber and adjust pressure till it leaves the widest, most consistent stripe.

Sure! But it doesn't work on the front tires (unless you have converted to FWD) or if you have a peg-leg rear end, in which case only one tire can be measured.