After seeing several threads and much arguing over inflation pressures and deviating from factory specs of 39/36psi F/R, I let my curiosity get the best of me and I came across this guide from Toyo on how to determine tire pressures to accommodate a different tire size by industry standard load ratings when switching to a different width or plus/minus size rim:
https://www.toyotires.com/media/2125/application_of_load_inflation_tables_20170203.pdf
Going through the exercise, the stock tire pressures are 39psi/36psi on a 205/40R17 Bridgestone Potenza RE050A 84W XL (reinforced) load range.This appears to be an ERTO metric tire, as there's no XL on the TRA P tables. Maximum per tire loads as specified by forsfor those pressures on the "reinforced" table are then 1036/981 lbs for Front/Rear
Swapping in a 215/45R16 RT660 or Rival 1.5S, which have an 86V SL (standard) load range (also ERTO) to get the equivalent load ratings in lbs gives you 1058/981 lbs at a surprisingly low 32psi front, 29psi rear.
Toyo also recommend that the same pressure difference be maintained from the factory tires (+3 psi front in our case), with one or the the other pressure being bumped up so both axles still meet the minimum load ratings or higher
Doing the same for a 205/45/R16, you'll find load ratings of either
87 (most summer tires) - pressures of 30(F)/29(R), or 32(F)/29(R) if you maintain the 3psi stagger
83 (most all season) which are barely adequate in "standard" load rating for the OEM load in lbs, correspond to pressures of 36(F)/32(R), or bumped up to 36(F)/33(R) for a 3psi delta.
Same exercise for nice meaty 15" tires, typical load ratings are:
89 (most 205/50R15 200TW tires) - 29(F)/26(R), (already at 3psi F/R)
88 (most 205/55R15 summer tires available) - 29(F)/28(R) which goes to 31(F)/28(R) (3psi)
86 (most other 205/50R15 Tires) - same as 215/45R16 above
87 (most 225/45R15) - 31(F)/28(R) (already at 3psi F/R)
Note you can always run higher pressures, but standard load rating tires don't increase in load capacity over 36psi even if maximum inflation pressure is higher. Of course for racing applications, you can always optimize for the conditions.
The much lower pressures are surprising when I first worked them out, but are not actually shocking given the rubber band low aspect ratio of the OEM tires. You can do the same exercise with any tire if you take the load rating number in the "service description" and look up on the tables in the link above. Note, you could run into real problems trying to run ultra low profile tires without reinforced (XL) load ratings, as the load ratings tend to be even lower.
the difference between Standard (SL) and Reinforced (XL) seems to be that at low pressures, SL tires have good load capacity up to 36psi, but the standard caps them for maximum load at that point - going higher does not increase load capacity. Reinforced tires on the other hand have initially *worse* load capacity at the same pressure, but are rated for higher pressures up to 42psi, which allows them to have higher total load capacity. The OEM tires are of the second type, which explains the unusually high pressures for a small car.
What’s really interesting is that the Toyo document outright says that a tire shop fitting different sized wheels and tires to a car should actually place a new sticker over the factory one indicating the correct tire pressures for the new size (giving the example of a Mitsubishi Evo moving from 17” to 18” wheels and tires). I doubt that’s ever been done, but it lends some validity to the idea that many people with minus size tires have had good results with lower tire pressures using standard load tires with higher profile (which also increases load rating)
Its worth noting that if you really plan to flog the car or have stiffer suspension, you’ll probably be applying higher loads to the tires than ford originally accounted for, which is evidently determined by driving conditions like “max load transferred to front left tire in a right turn at 60mph”, and not the static corner weight or GVW on the door sticker, so slightly higher pressures than the ones I listed might be appropriate.
https://www.toyotires.com/media/2125/application_of_load_inflation_tables_20170203.pdf
Going through the exercise, the stock tire pressures are 39psi/36psi on a 205/40R17 Bridgestone Potenza RE050A 84W XL (reinforced) load range.This appears to be an ERTO metric tire, as there's no XL on the TRA P tables. Maximum per tire loads as specified by forsfor those pressures on the "reinforced" table are then 1036/981 lbs for Front/Rear
Swapping in a 215/45R16 RT660 or Rival 1.5S, which have an 86V SL (standard) load range (also ERTO) to get the equivalent load ratings in lbs gives you 1058/981 lbs at a surprisingly low 32psi front, 29psi rear.
Toyo also recommend that the same pressure difference be maintained from the factory tires (+3 psi front in our case), with one or the the other pressure being bumped up so both axles still meet the minimum load ratings or higher
Doing the same for a 205/45/R16, you'll find load ratings of either
87 (most summer tires) - pressures of 30(F)/29(R), or 32(F)/29(R) if you maintain the 3psi stagger
83 (most all season) which are barely adequate in "standard" load rating for the OEM load in lbs, correspond to pressures of 36(F)/32(R), or bumped up to 36(F)/33(R) for a 3psi delta.
Same exercise for nice meaty 15" tires, typical load ratings are:
89 (most 205/50R15 200TW tires) - 29(F)/26(R), (already at 3psi F/R)
88 (most 205/55R15 summer tires available) - 29(F)/28(R) which goes to 31(F)/28(R) (3psi)
86 (most other 205/50R15 Tires) - same as 215/45R16 above
87 (most 225/45R15) - 31(F)/28(R) (already at 3psi F/R)
Note you can always run higher pressures, but standard load rating tires don't increase in load capacity over 36psi even if maximum inflation pressure is higher. Of course for racing applications, you can always optimize for the conditions.
The much lower pressures are surprising when I first worked them out, but are not actually shocking given the rubber band low aspect ratio of the OEM tires. You can do the same exercise with any tire if you take the load rating number in the "service description" and look up on the tables in the link above. Note, you could run into real problems trying to run ultra low profile tires without reinforced (XL) load ratings, as the load ratings tend to be even lower.
the difference between Standard (SL) and Reinforced (XL) seems to be that at low pressures, SL tires have good load capacity up to 36psi, but the standard caps them for maximum load at that point - going higher does not increase load capacity. Reinforced tires on the other hand have initially *worse* load capacity at the same pressure, but are rated for higher pressures up to 42psi, which allows them to have higher total load capacity. The OEM tires are of the second type, which explains the unusually high pressures for a small car.
What’s really interesting is that the Toyo document outright says that a tire shop fitting different sized wheels and tires to a car should actually place a new sticker over the factory one indicating the correct tire pressures for the new size (giving the example of a Mitsubishi Evo moving from 17” to 18” wheels and tires). I doubt that’s ever been done, but it lends some validity to the idea that many people with minus size tires have had good results with lower tire pressures using standard load tires with higher profile (which also increases load rating)
Its worth noting that if you really plan to flog the car or have stiffer suspension, you’ll probably be applying higher loads to the tires than ford originally accounted for, which is evidently determined by driving conditions like “max load transferred to front left tire in a right turn at 60mph”, and not the static corner weight or GVW on the door sticker, so slightly higher pressures than the ones I listed might be appropriate.
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