Tidybuoy

I increased my tire size slightly when I changed to Spyder wheels, which are larger. My new tire sizes are 245/35/19 and 275/35/19. I'm wondering if I should be increasing the tire pressure or does it stay the same as the stock tire sizes. I'm currently running 32 front and 36 rear.

Any advice would be appreciated.

Dave in Chicago

The tire pressure sticker on driver's door jam should list the optional size you list above (it does on my '05 Boxster S). Does the Cayman not have that sticker or, perhaps, those sizes listed? Also, these are listed in my owners manual. Ex, on mine it lists 18's, 19's, and 18 M&S (winter tires sized to allow chains on rear).

On mine, the 19's list 32 F / 37 R. I'm a tire pressure maniac and have found the Porsche recommended pressures to be spot on.


Dave

'05 987S
'92 968 SP3
'88 944 NA (gone)

storii

I am currently running 35 all around in my 245/35/20 and 275/35/20 and the car has been pretty neutral. This setup was the recommendation from the Porsche specific indy who mounted the tires on the car.

The tire pressures can be used to alter car's handling characteristics. The rule of thumb is to reduce the pressure on the end where you want to increase grip. For example, in order to reduce understeer, you either reduce the front tire pressure or increase the rear. Do the opposite to tame oversteer.

Tidybuoy

Thanks all.

I did some internet searching and I found that 32/37 is what the factory recommends and increasing the tire size does not change pressure requirements. Basically, a larger tire takes more air volume but the pressure should remain the same as the factory determined the pressure by the weight of the car. So, because I increased my tires from 235/19 to 245/19, the tire will hold more air but I need to keep the tire pressure the same.

I do agree however, that minor adjustments can be made to make handling changes.

Thanks...I'm learning something new every day.

Vern

Scooby921

Tire size shouldn't change pressure requirements. The volume of the tire might have increased, but the air pressure required to maintain shape is pretty consistent across all tire sizes. If you go a little higher the tires will be a little harder and provide less rolling resistance. But going too far will cause them to wear more down the center. Going a little lower on tire pressure will make them softer and the ride quality might improve. Going too far will wear the inside and outside edges of the tire more than the middle and leave you more susceptible to cracking or bending a wheel on a large impact (pothole).

For what it's worth I ran the OE 20" Pirelli PZero at 32psi front and rear. I now have 19" Bridgestone Re71R's and I run them at 30psi front and rear. The newer tires have a stiffer sidewall, so I run a little lower pressure to compensate and improve the ride quality a tiny bit.

ghartke

Hey, gang,

I do very close to what Scooby does. I set 32 psi all 'round on my 2014 CS w/MT, also running RE-71Rs (245-35/20 and 285-35/20), finding that to be a good compromise pressure. When I race (AX), I run hot pressures of 30 psi rear and 29 psi front, and typically drop the front to 28 psi for the last couple of runs. I even cool my tires with a water sprayer, but still wound up dropping the pressure 10 psi during the course of 6 runs recently on a hot day. Amazingly, the tire pressures still hadn't stabilized by the last run and I had to drop the pressure 1 psi all around to get to my targets. Note that I always pump 'em back up before I drive back home, though - don't want them to get even hotter when driving at highway speed for a while.

Regards,

Greg