Calculating Tire Pressure with Larger Rims / Tire Load Ratings

[Wild Turkey]

My 2001 came stock with 215/45ZR16 (33 PSI) in Front, 245/40ZR17 (40 PSI) in the Rear.

Rims have been swapped to a 17/18 setup, 215/40R17 87W in Front, 265/35R18 97W in the Rear.

When I tried to use the Tire Pressure Calculator at https://tiresize.com/pressure-calculator/ to correct the tire pressures, it asks for the Load Rating, which I can't find. Assuming its SL (standard load) and not XL (extended load), the calculator says I need to move to 36 PSI in the front, but that The max load capacity of the NEW tire (1074 lbs. @ 36 psi) is below the OEM tire load capacity at 33 PSI and that the load is above the original OEM load capacity so I shouldn't use these tires. The weight on the front tires is certainly below 1074 lbs/tire though.

At the rear, it's even more confusing as the pressure calculator says it's OK, but recommends only 34 PSI!

I didn't know about changing the tire pressure with resized rims, so I've run the standard recommended 33 PSI front and 40 PSI back for the last year and everything seemed fine. Tires are about 60% worn, with fairly even wear. I'm not driving near the limit so I can't comment if the tires are performing OK when stressed, but when I go around a traffic circle a couple of times, it seems like the front and rear end let go together - it seems like I can push the front or the rear out fairly predictably. I guess I'll go to 36 PSI in the front from here on in, but I don't know what to do about the rears, and how can I ensure my next set of tires is acceptable from a load rating perspective?

 

[centerpunch]

Here’s a convenient (that’s a joke) 31-page document I discovered when considering the same question.

Seems like Tire Rack, who offers “plus 1,” “plus 2,” etc., wheels/tires, should provide guidance, but I couldn’t discover any on their site.

Another path I pursued was looking for cars that used those tire sizes and seeing what pressures they used. I think I looked at Porsche and Ferrari before I ran out of time and/or enthusiasm and decided to just stop thinking about it lol.

https://www.toyotires.com/media/2125/application_of_load_inflation_tables_20170203.pdf

 

[centerpunch]

I thought maybe Porsche 993 would provide some guidance (I actually have one of those) since they offered either 17” or 18” wheels, but it turns out their recommended 17” pressures are bizarre and no one thinks they make any sense. Or maybe it’s the 18” pressures that are bizarre. I forget.

 

[stuntman]

I would never run an NSX with 40psi of rear pressure. If you do any sported driving, those pressures will quickly increase to mid 40s and will deliver far less grip. Personally I target 36psi HOT both front and rear, and will adjust my cold pressures accordingly.

 

[Honcho]

Agree with Billy, who also advised me long ago to run 36 psi cold all around on the street and aim for 36 hot at the track. It's worked wonderfully.

Remember that the OEM pressure spread was intended solely for the factory Yokohama and Bridgestone NSX tires, which were designed specifically for the car. The tread pattern, sidewall and compound were tuned to work with those pressures to create a "lively" and "playful" character, which is marketing-speak for oversteer. Many journalists crashed NSXs during their initial tests (including famously Csaba Csere from Car & Driver on the PCH) because the rear end broke loose and caused a snap oversteer condition that was hard to recover due to the slow steering and unfamiliarity with the NSX handling dynamics (I even get caught on Turn 11 at HPR from time to time). Anyway, I wish the tire pressure sticker had an asterisk that said "for factory tires only." Just run 35 or 36 cold all around and you'll be fine.

Also, the OEM rear tire size for a 2001 was 245/40/17, not 265. In 2002, it went up to 255 (largely because Keiichi Tsuchiya complained that the rear tire capacity was "too small" for the chassis), but Honda never gave us a 265.

 

[centerpunch]

Another long document on this subjest.

https://www.rubbernews.com/assets/PDF/RN86607218.pdf

 

[stuntman]

In general, the wider you go and the less tire pressure you can run to increase the size of the contact patch and reduce the tire load per square inch of that contact patch. I have 305s on the back of my NSX and run 29psi.