Different Tyre sizes front and rear
Discussion
My 2003 Boxster 2.7 came with 235/35/19 on the front and 275/35/19 on the rear when I bought it a year ago. I have just changed the rears and the garage said that I should really have 275/30/19s on the rears.
I did about 10,000 miles on the previous tyres which were on the car when I bought it. I have really struggled to unstick the rear in the dry and even in the wet it handles well. How might the lack of reduced aspect ratio(?) between the fronts and rears affect the car?
My 997 has 35s on the front and 30s on the rear.
I did about 10,000 miles on the previous tyres which were on the car when I bought it. I have really struggled to unstick the rear in the dry and even in the wet it handles well. How might the lack of reduced aspect ratio(?) between the fronts and rears affect the car?
My 997 has 35s on the front and 30s on the rear.
To answer your specific questions;
The rolling radius of the tyre is larger so you would have had a speedo that was under reading and a car that was slightly higher geared.
In terms of handling, the higher aspect ratio would allow more give in the sidewall which would effectively soften the rear a little, making breakaway slower.
The rolling radius of the tyre is larger so you would have had a speedo that was under reading and a car that was slightly higher geared.
In terms of handling, the higher aspect ratio would allow more give in the sidewall which would effectively soften the rear a little, making breakaway slower.
A tyre size calculator shows that if you had 30 profile tyres at the rear, front-rear your diameters would be practically identical.
As is, your rears are 1" larger.
This means that the rear of your car is sitting 1/2" higher than it should.
I would not say that this is a big deal, unless I could flog the tyres for good money I would probably just wear them out and get the correct ones then.
Not sure if there is any stability management on your car than may get confused by the differences, if there were, it would think that the front of the car is going faster than the rear, and who knows what it would make of that!
As is, your rears are 1" larger.
This means that the rear of your car is sitting 1/2" higher than it should.
I would not say that this is a big deal, unless I could flog the tyres for good money I would probably just wear them out and get the correct ones then.
Not sure if there is any stability management on your car than may get confused by the differences, if there were, it would think that the front of the car is going faster than the rear, and who knows what it would make of that!
First, read the manual it tells you about tyre sizes.
You need to understand what the "aspect ratio" means. The figure 30 represents a % of the width which gives the depth of the tyre that protrudes from the rim. With this information the rolling circumferences of the front and rear tyres can be matched.
For example a 225/40 on the front would be a close match for a 265/35 or a 285/30 on the back.
I understand, from what it says in the manual, that on a Tiptronic vehicle rear and front wheel rotational speeds are matched. If the gearbox management senses that the rear wheels and front wheels are rotating beyond a certain limit, it changes up. It is therefore important that the overall size of the wheel is the same for front and back. Also, as another poster has said the speedo will be wrong with tyres that have a wrong aspect ratio on the back.
I do not know if 19" wheels are a spec for your car. Check the manual to find out.
Putting the wrong size tyre on is a modification to the vehicle.
You need to understand what the "aspect ratio" means. The figure 30 represents a % of the width which gives the depth of the tyre that protrudes from the rim. With this information the rolling circumferences of the front and rear tyres can be matched.
For example a 225/40 on the front would be a close match for a 265/35 or a 285/30 on the back.
I understand, from what it says in the manual, that on a Tiptronic vehicle rear and front wheel rotational speeds are matched. If the gearbox management senses that the rear wheels and front wheels are rotating beyond a certain limit, it changes up. It is therefore important that the overall size of the wheel is the same for front and back. Also, as another poster has said the speedo will be wrong with tyres that have a wrong aspect ratio on the back.
I do not know if 19" wheels are a spec for your car. Check the manual to find out.
Putting the wrong size tyre on is a modification to the vehicle.
Boxstercol said:
SFO said:
986 Boxsters did not have 19 in wheels OEM
275 sounds too wide, it was 265 for the 987
Agreed.275 sounds too wide, it was 265 for the 987
My 987 on 19's are 265 on the rear, 235 on the front.
LaSource said:
With ref to the speedo being wrong: I suppose this depends on which wheels are used for measuring speed (ie just the front, the back, or some clever average of all wheels)
I know that when I changed one of the front tyres the tiptronic box started to play up. It noticed the difference in speed between the new tyre with 8mm of tread and the part worn tyre with 4mm of tread. So I wonder if the relevant sensors are on the front axle?LaSource said:
With ref to the speedo being wrong: I suppose this depends on which wheels are used for measuring speed (ie just the front, the back, or some clever average of all wheels)
I have used a couple of different GPS devices and the speedo is completely accurate. I wonder if this means that the speed is measured from the front wheels?The speedo should over-read as is normal on all cars, so if you're getting a GPS verified accurate reading from your speedo then it proves that the speedo's showing a lower speed than it should.
The maximum tyre sizes ever spec'd for a 986 were 265/35 18 rear and 225/40 18 front, so yours are somewhat bigger. Search online for a "tyre calculator" where you can enter one of these sizes and your size and it'll tell you by how much yours are "out", usually showing green or red for whether you're within an acceptable tolerance.
On the 986 3.2 race cars we used to run Michelin road based tyres of the sizes mentioned above, but this year the control tyre is a Pirelli slick. There wasn't a 225 available so we had to run 235 instead and when we first used them we had big problems with the front tyres fouling the bodywork, so do be careful that you have enough clearance for all the tyres on your car. Ideally use a 4 post ramp and then compress each corner (and then each axle) by using ratchet straps or similar (to mimic braking and cornering loads on the suspension), and check for clearance throughout the range of steering wheel positions.
The last thing you want is a tyre being very gradually rubbed away without you noticing until one day it gives up altogether, most likely to happen when it's under more load i.e. when you're driving quickly and need it to grip and it'll be hard to control if you lose it.
The maximum tyre sizes ever spec'd for a 986 were 265/35 18 rear and 225/40 18 front, so yours are somewhat bigger. Search online for a "tyre calculator" where you can enter one of these sizes and your size and it'll tell you by how much yours are "out", usually showing green or red for whether you're within an acceptable tolerance.
On the 986 3.2 race cars we used to run Michelin road based tyres of the sizes mentioned above, but this year the control tyre is a Pirelli slick. There wasn't a 225 available so we had to run 235 instead and when we first used them we had big problems with the front tyres fouling the bodywork, so do be careful that you have enough clearance for all the tyres on your car. Ideally use a 4 post ramp and then compress each corner (and then each axle) by using ratchet straps or similar (to mimic braking and cornering loads on the suspension), and check for clearance throughout the range of steering wheel positions.
The last thing you want is a tyre being very gradually rubbed away without you noticing until one day it gives up altogether, most likely to happen when it's under more load i.e. when you're driving quickly and need it to grip and it'll be hard to control if you lose it.
Boxster from 1997 Front: 205/55 ZR 16 Pilot SX MXX3 N2 2.0 (29) – 2.0 (29) –
Rear: 225/50 ZR 16 Pilot SX MXX3 N2 – 2.5 (36) – 2.5 (36)
Boxster from 1997 Front: 205/50 ZR 17 Pilot Sport N2 2.0 (29) – 2.0 (29) –
Boxster S Rear: 255/40 ZR 17 Pilot Sport N2 – 2.5 (36) – 2.5 (36)
from 1997 Front: 225/40 ZR 18 Pilot Sport N1 2.0 (29) – 2.0 (29) –
Rear: 265/35 ZR 18 Pilot Sport N1 – 2.5 (36) – 2.5 (36)
From Michelin, listing approved tyres in Porsche fitments.
Rear: 225/50 ZR 16 Pilot SX MXX3 N2 – 2.5 (36) – 2.5 (36)
Boxster from 1997 Front: 205/50 ZR 17 Pilot Sport N2 2.0 (29) – 2.0 (29) –
Boxster S Rear: 255/40 ZR 17 Pilot Sport N2 – 2.5 (36) – 2.5 (36)
from 1997 Front: 225/40 ZR 18 Pilot Sport N1 2.0 (29) – 2.0 (29) –
Rear: 265/35 ZR 18 Pilot Sport N1 – 2.5 (36) – 2.5 (36)
From Michelin, listing approved tyres in Porsche fitments.
I had to do this a few years ago when I needed some winter tyres and could only get ones with a similar difference.
It does affect the handling, especially if it is the back ones but not a massive amount (i only really noticed it at high speed on the autobahn).
Depends how ocd you are, after a while it did start to annoy me.
It does affect the handling, especially if it is the back ones but not a massive amount (i only really noticed it at high speed on the autobahn).
Depends how ocd you are, after a while it did start to annoy me.
Just felt like there was more float at the back in high speed corners. Also made the front feel slightly odd. Sorry I can't explain it well but was never enough to warrant changing them.
Makes me miss Germany, I remember the Porsche dealer apologizing that they were "only" rated to 240kph.
Makes me miss Germany, I remember the Porsche dealer apologizing that they were "only" rated to 240kph.
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