By Barry Lee Brisco, David Jones, Alan Klingen, Ernie Puskas, Tom Martinez, and Al Zim
|If your tires are over 9 years old, replace them!
Not sure how old they are? Here's how to find out.
356 tire pressures are different from modern cars because of the lightness of our classic vehicles and the unusual weight balance: heavier in the rear, not the front. The factory 356A owners manual recommends 24/28psi front/rear, but that was for bias-ply tires 'back in the day'. The 1965 Elfrink Technical Manual suggests 24/26 for "normal" driving and 26/29 for "fast driving" on 165 tires for the C/SC coupe/cab that had a 2,061 lb. curb weight (the Porsche C Owner's Manual has almost the same figures for "braced tread" tires, the early name for radials). However, since that time, tires have changed dramatically — fortunately much for the better — so be careful applying old pressure recommendations to new tires. Certainly underinflation is to be avoided, as that can produce a loss of control in an emergency situation.
Al Zim went straight to the experts at Tire Rack and got this reply: the loading on the tire should be the criterion for inflation, and based on the lightweight 356, they recommend 30 psi front and 34 rear. Al notes that 26 psi is too low and 40 psi is to high!
Sometimes a bit of experimentation is required to find the optimal pressure. Tom Martinez says, "I use the the same pressure bias front to rear that the factory recommends but I increase it overall to 32-35. Much more and the car feels a bit twitchy, and much less make the tires squeal. Gas mileage is also improved as well as high speed and crosswind handling. I run 165-15 Vredsteins and Michelins."
Alan Klingen states: "Don't use older tire recomendations, instead use new ones. You need to use modern guide lines when using modern tires. The days of running bias ply tires are gone so you need to follow your tire company's guide lines. All modern radial tires are recommended to be run above 26 psi at least for street tires. The same goes for alignment, the old spec was all most 3 degrees of toe in, but this for older type tires. Today it is near zero. It is true a less inflated tire is softer riding but it will not grip as well. General rule of thumb, keep the pressures above 26 psi."
David Jones comes out for somewhat lower pressures: "Try a simple exercise. Increase the tire pressures to the higher values suggested and then drive the car at a reasonably quick speed over a road that is not really smooth. Then reduce the pressures and drive it again. I would suggest pressures in the 19 to 22 range front and 24 to 28 rear. I always keep about a 5 psi differential front to rear because of weight distribution. I find that with the higher pressures the car feels like a hard rubber ball with reduced compliance and with the lower pressures it has a more comfortable stance. It will depend on what the car feels like and also different tires and widths will make a difference. Wider tires will usually require higher pressures to help keep the center of the tread on the ground as a tire will crown out with more pressure and crown in with less."
"The 356C drivers manual recommends 23/26 to 26/28 for radials. I feel that the modern radials have a stiffer sidewall than those of the 60's so would not want to increase those pressures by very much if at all. I run 22/27 on my "A" with 165/80 tires and those that know me know I do not hang around and I do conserve my brakes."
"My experience on the race track suggests that different pressures suit different driving styles and trying a variation in pressures may lead to finding a setting which will enhance the feel of the car for some but some folks will feel no difference at all."
"I have found that tire pressures are a personal preference to some extent. Even on race cars different drivers with the same tire and car combination liked different pressures. The only thing that I find to be a constant is that over inflation leads to crowning of the tread and consequent faster wear of the center of the tread, and under inflation leads to wear on the outside edges of the tire. A differential of about 5 psi is needed with the fronts anywhere from 18 to 25 and the rears from 23 to 30. I have tried 32 psi and I think that is just a tad to high for good traction. The prefered pressures will depend on tire manufacturer, size, age, and then the weight of car and the feel through the seat of the pants as you traverse your favorite set of bends at higher than moderate speed. Try different pressures and see how it feels. Remember that going up on the pressure makes less difference than going down."
Ernie Puskas believes that: "Tire pressures recommended by auto manufacturers are derived from the combination of multiple considerations. Among these are treadwear, handling, fuel economy, ride comfort, noise and design load. Absolute max inflation pressure/load reflects the tire manufacturers safe guess at the limit which will not cause the bead to unseat under extreme maneuvers and avoid damage to the tire carcass and components from impact with foreign objects, potholes and curbs."
"Recommended pressures lower than the maximum printed on the sidewall reflect that the weight carried by the tire on this vehicle is less than the maximum capacity of the tire, thus requiring less force inside the tire to keep its shape while in use."
"When our 356s were built, most bias-ply tires were designed to run at about 28 psi, with max load at 32 psi. Improvements in bead and wheel design and development of radial tires have generally enabled tires to carry a higher inflation pressure for any given construction."
"The best pressure for you is the one which enables you to best meet your needs and desires. Most auto manufacturers bias their recommendations toward softer ride, to overcome shortcomings (cost savings) in their suspension systems. This detracts from handling and treadwear, and increases heat buildup, thereby shortening tire life. I suspect that Porsche gives much more weight to handling than soft ride, even advising increasing pressure about 2 psi for "spirited" driving. My own judgement is that I like my tires 2 to 4 psi higher than the auto maker recommends. I do not go below the recommendation at any time. You need not worry about bead unseating at recommended pressure, anything which would unseat the bead is likely to demolish the tire anyway. I like 28 psi in the 185/70 tires on my 356. It rides like a truck over rough road surfaces, but gives me sharper turning and handling than the recommended 24 psi would. I probably pay in getting more of a beating to the suspension and rest of the car (as well as my teeth), but that's a price I am willing to pay."
Other 356 Registry Articles on Tires
Calculating Tire Dimensions & Comparative Tire Sizes
Tire Safety and Date Codes
Tires With Tubes – Original Yes, Needed? No