I recently installed new tires (Pirelli Cinturato P7) on my Mazda3. I went from 215/45-18 to 225/45-18 (bigger selection in the 225 size, generally cheaper, and only throw the speedo off by 1 mph at 75 mph (this was also blessed by tirerack as being an acceptable alternate size). I'm aware that as a general rule, when tire volume increases tire pressure should decrease. The recommended pressure for the stock tires was 36psi. I'm assuming that the increase in size I went with is not too significant and only a slight decrease in PSI is called for. I know there are some complicated formulas to do this precisely, but I hope I can avoid that with some reasonably accurate seat of the pants adjustments. Does 34 or 35 psi seem OK?
PSI adjustment for larger tires
- Thread starter GaryMX5
- Start date
Tire Pressure Calculator
Easy to use tire pressure calculator. Use our tire pressure calculator to calculate the new tire pressure needed when changing tire sizes.
Huh? So if your 215/45-18 recommends 36 psi, what should my 165/65-14 Honda Insight tires be inflated to? Like 12 psi? No, the manual says 38/35 psi front/rear.
I wouldn’t change it by that much. Many different cars with the same size tires call for different air pressures. The Mazda leans towards the sporty side of things and there is no way I would decrease the pressure to 32. I usually go over a pound or two on everything I have. The GTI calls for 39 psi and I round up to a nice even 40.
There is a school of thought that says you should match the original pressure to keep the same spring rate in the tire so the suspension reacts the same.
The recommended pressure by the car manufacturer is for the spec'd tire size provided on the car at the factory. It is not always applicable to other sizes that might be used. The issue has to do with tire volume (original vs replacement). The reverse is also true. If the original tire is replaced with a smaller tire, the tire pressure in the smaller tire likely needs to be increased (if it can safely be done). The (very complicated) formula I read about was developed by tire engineers. It takes into account numerous parameters to determine the appropriate PSI. It's not solely based on ride quality or load capacity for that matter. I assume the calculator provided above takes those design parameters into consideration. It would be nice to find another calculator to compare results. I'm also aware that there's some subjectivity involved (the feel factor). I remember increasing tire pressures from 28 to 40+ psi when autocrossing my Miata years ago to keep that sidewall from rolling too much . I sure didn't run that pressure when using it as a daily driver.
Tire Pressure Calculator
Easy to use tire pressure calculator. Use our tire pressure calculator to calculate the new tire pressure needed when changing tire sizes.tiresize.com
DO NOT trust this calculator. I ran a sample calculation and got an incorrect answer.
Why? Best guess is that they are using the Load Index. That's not the way to do this!
Also, having programmed stuff my entire life, there are a number of ways to do what this calculator is supposed to be doing - and several that would be wrong!
One of the right ways would be a table lookup. You wouldn't need a formula to do this kind of thing.
I keep 45 psi in my Insights and many people inflate to 50 and 60 psi or more to maximize gas mileage.
AZjeff
$50 Site Donor 2023
Hypermiling has nothing to do with adjusting psi due to different tire volume.
The volume of air contained by those 2 tyres is likely not very different.
I'm surprised to read about these high pressure ratings. My 2017 Civic recommends 32 psi, and since upgrading to Si wheels, I've moved to 33 psi. Take this for what it's worth, though...
When I started autocrossing I attended several driver seminars conducted by well regarded divisional and national champions for rookies. At one, tire pressures were the main item of discussion. I remember one of the instructors comparing a tire to a basketball. The more you fill a basketball the higher it will bounce. A tire will basically do the same thing and as a result, puts significant stress on the shocks/dampers in order to control excessive rebound. Even the best performance shocks with all the adjustments in the world can be overtaxed (overworked) by excessively high tire pressures. I have to think when people significantly increase tire pressures for better fuel mileage are also causing accelerated suspension wear and/or damage.
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Thank you...Rather than lower it to the calculator result I went down from 36 to 34 psi last night. At 36 I was getting more thumping from expansion joints than I cared for and the steering seemed a little lighter compared to the old tires at 36 psi. I will be playing with pressures a little more over the next several days (if outdoor temps cool down a little).
The margin of error in a tire gauge may be more than that 2 psi.
Maybe more, for consumer grade gauges.
I put together my own gauge recently, using a digital Ashcroft gauge and Autometer hose. It cost a little more, but the accuracy and ease of use is fantastic.
I'm normally door sticker PSI or slightly over in all my vehicles. My tire and rim sizes vary between winter and summer. Stay about the same and some are slightly wider/taller than original. I check pressure weekly and tread depth across full tire at least monthly. I'll adjust by 1-2 PSI based on that wear.
Only one vehicle I have 2 psi lower that has a larger size tire but that see's a lot of stop and go around town and was wearing in middle more at door sticker PSI. They are also A/T tires on a Jeep Renagade.
I also keep 40 PSI in all 4 for my GTI.
