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I want to use Toyo RA 1's for but they only come in 275 35 for the rear. Will this effect my speedometer reading, and if so how can I correct it?
Any help would be greatly appreciated.
I doubt there is significant speedometer error for that size tire. It will have significantly less error than the 265/35-18 R comps that many people are running. The outside diameter on the 275/35-18 are much closer to stock although I believe they are still half an inch shorter than a 265/40-18. 265/35's are about an inch shorter than stock.
I don't know how you could correct it but do you plan to switch out from the RA1's to stock or will you run the RA1's full time?
I run Nitto NT01's in that size for my autox wheels and left them on the car for a couple of months last summer. I never noticed any error but I didn't use my GPS to check it either.
One of the most important things to consider is that you need to keep your overall tire diameters within 3% of stock, and within 3% variance front-to-back, otherwise not only will you have problems with your speedo - which is probably the least of your worries - but you run the risk of the ABS and PSM systems not working properly.... it's not a good thing to mess with the workings of your ABS.
brad
PSM - Porsche Stability Management
While it can’t overcome the laws of physics, the revolutionary Porsche Stability Management (PSM) system does lend an added degree of balance and control to the Cayman’s mid-engine driving dynamics, inspiring surefooted confidence in corners and extreme situations.
A standard feature on the Cayman and Cayman S, PSM continuously monitors steering input, road speed, yaw velocity and lateral acceleration to calculate the actual direction of travel. If the car begins to steer off line, PSM instantly intervenes with precision brake inputs on individual wheels to help bring the car back onto the driver’s intended path.
If braking alone isn’t enough to correct the vehicle’s cornering line, PSM then calls on the Cayman’s engine management system, adjusting engine output as needed to help stabilize handling. PSM can also compensate in an instant for mid-corner changes in load resulting from deceleration or braking. When Sport mode is selected with the optional Sport Chrono Package, PSM’s threshold for intervention is raised, allowing for greater driver involvement. If you prefer driving without automatic PSM assistance, the system can be set to standby at any time. In this case, it will only intervene under heavy braking, where both front wheels exceed the ABS threshold.
For all of its technical ability, PSM goes virtually unnoticed in everyday driving situations, preserving the Cayman’s natural agility.
One of the most important things to consider is that you need to keep your overall tire diameters within 3% of stock, and within 3% variance front-to-back, otherwise not only will you have problems with your speedo - which is probably the least of your worries - but you run the risk of the ABS and PSM systems not working properly.... it's not a good thing to mess with the workings of your ABS.
No ABS or PSM problem running 245/40-18 front and 275/35-18 rears. I have a lot of experience in pushing my 987 Boxster S with this combo through autocross courses and all the systems work perfectly. However, I gave up running autocross with the PSM on even in the Sport Chrono sport mode. I am much faster with PSM off and PASM on the stiffer, sport mode.
PASM - Porsche Active Suspension Management
This active damping system offers continuous adjustment of individual damping forces based on current road conditions and driving style.
The driver can choose from two setup modes, ‘Normal’ and ‘Sport’, using a separate ‘damper’ button on the center console. ‘Normal’ mode is designed for general road driving and circuits with uneven tarmac. ‘Sport’ mode is intended for smoother track surfaces, where the harder settings help eliminate pitch and roll.
In either mode, PASM continuously evaluates the current conditions while automatically selecting the corresponding damper rates from the respective set of mapped values.
A range of sensors are used to monitor the movement of the body under acceleration, braking and cornering maneuvers, as well as on poor road surfaces. The PASM control unit then evaluates this data and modifies the damping force on each individual wheel in accordance with the selected mode. The result is a significant reduction in body movement as well as a better grip on the road.
For example: if ‘Sport’ mode is selected, the suspension is automatically set to a harder damper rating. If the quality of the track surface falls below a certain threshold, the system immediately changes to a softer rating within the ‘Sport’ setup range. When the quality of the tarmac improves once more, PASM automatically returns to the original, harder rating.
Need more information about PASM? Click this link: FAQ for PASM
Sport Chrono and Chrono Plus
This optional package is a valuable addition for trackday use. Available in conjunction with the CDR-24 CD radio, it includes a swivel-mounted analog and digital timer unit which is centrally located on the dashboard. All functions are easily accessible via the control stalk for the on-board computer. Analog dials measure hours, minutes and seconds, while a separate digital field displays whole seconds, tenths and one hundredths of a second. A second digital display runs in parallel in the instrument cluster. Click this Link to visit the FAQ entry for Sport Chrono.
PSM - Porsche Stability Management
While it can’t overcome the laws of physics, the revolutionary Porsche Stability Management (PSM) system does lend an added degree of balance and control to the Cayman’s mid-engine driving dynamics, inspiring surefooted confidence in corners and extreme situations.
A standard feature on the Cayman and Cayman S, PSM continuously monitors steering input, road speed, yaw velocity and lateral acceleration to calculate the actual direction of travel. If the car begins to steer off line, PSM instantly intervenes with precision brake inputs on individual wheels to help bring the car back onto the driver’s intended path.
If braking alone isn’t enough to correct the vehicle’s cornering line, PSM then calls on the Cayman’s engine management system, adjusting engine output as needed to help stabilize handling. PSM can also compensate in an instant for mid-corner changes in load resulting from deceleration or braking. When Sport mode is selected with the optional Sport Chrono Package, PSM’s threshold for intervention is raised, allowing for greater driver involvement. If you prefer driving without automatic PSM assistance, the system can be set to standby at any time. In this case, it will only intervene under heavy braking, where both front wheels exceed the ABS threshold.
For all of its technical ability, PSM goes virtually unnoticed in everyday driving situations, preserving the Cayman’s natural agility.
The boxster uses different size tires. The Cayman uses different height tires front to rear and going to the 275/35R18 can be a problem with the ABS. They would end up being 3/4" shorter than stock
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Jim Holloman | Sales Specialist
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