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.

 

[TANTO]

Thanks for reinforcing this rear lower pressure. Looks as though I’ve been overinflated for decades he-he. Going against the manual “was” tough but now i finally get it. Running my Falkens 215/255’s at 31/38, I’ve always felt that the rear felt overinflated. When folks/family want a “test ride” where I don’t have time to get the tires warm enough, the rear gets loose. Riders love it – me not so much! Will now do 34 or so all around. Regarding crash historys, around 1991 when test driving the “new” RX-7 (along with Mr2’s/Esprites) a Mazda sales guy slipped and told me that they had only released 30 rx7’s to dealers for test rides and already 27 of them had left the road! No shade intended - very fun car and I’m sure owners have learned to tweak or adapt.

 

[docjohn]

Ha I had a 93 R1 and it had some evil oversteer stock.

 

[Wild Turkey]

Ha I had a 93 R1 and it had some evil oversteer stock.

Things hadn't improved by 01. My '01 929RR could be a handful in the oversteer department as well, even after I solved the tank slappers with a steering damper. Although once I got used to it, I could slide it more reliably with Metzlers. A couple of Michelin's I tried just hooked up suddenly with much rectal puckering, so back to Metzlers. Honda tried some crazy disconnection of the swing arm from the frame for those 2 years - insane! Why I loved that thing is beyond me. Well except for the insane power at any RPM from 3 to 11k perhaps... Who cares what gear you're in, just turn the throttle and try to hold on, and keep the front wheel from sailing. No courage for any of that anymore..

 

[Old Guy]

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.

Ha! Marketing speak. Reminds me of one journalists description of the oversteer on an early 911 as "violent and final". Probably not going to see that in the marketing literature.

A few years ago I stumbled across a blog from a guy who experimented with altering tire pressures and measuring tire contact area and approximate effects on lap times. Within the limits of safe pressures, reduced pressures increased contact area which had a mild correlation to lap times. That blog involved a front wheel drive car.

I understand the concept of dropping the tire pressure and its potential effect on adhesion. However, I do wonder if the high rear tire pressures which potentially reduces rear adhesion might have been a Honda measure to try and induce rear over steer a little more gradually / earlier. In my very limited experience, the transition to oversteer on the NSX falls into the sphincter tightening category rather than entertaining category (I am thinking of you BMW 3 series RWD). Improving rear adhesion with lower tire pressure at the expense of making that transition to oversteer even sharper is not a place that I would be keen to visit.