different tyre sizes

FRANKYDOOM · 07-10-2001, 06:23 AM

Hi all

Possibly a silly question - I gonna replace my tires and noticed that I have 225/50/15 rear and 205/55/15 front - question: why 2 sizes 50/55 ???? I asked a friend and he thought it has something to do with the speedo ?..... mmmhhh dunno anymore.... Car is a 924 with the carrera gt body kit - I cant ask the previous owner anymore has he legged the country...

Thanks

Frankydoom/London/UK

ehpower · 07-10-2001, 08:35 AM

The reason is the first part of the tire size is the width. You have 205 front and 225 rear. The second number that appears is the (/55 or /50) is the side wall height. The side wall height number is acutally a ratio of the width of your tire. So in order to keep the over all height the same they had to use a 50 series tire in back because it was wider than the front. So the front they used 55 series.

p0r2chby · 07-10-2001, 08:55 AM

... it's really pretty simple, a wider tire is used to provide more traction & control
(contact area)

the other benefit is that the added width tends to fill out the fenders better

IceShark · 07-10-2001, 09:14 AM

Planter, The rears are wider for improved performance. Tire contact patch, pounds per square inch, dry performance, wet performance, cornering and all the other tradeoffs that occur. The big reason on a 944 is that it is a rear wheel drive and the wider tire gives more traction on acceleration.

Different size tires is a pretty common tactic when maxing performance. And as noted earlier the fronts have a higher aspect ratio to keep the rolling radius of front and rear approximately the same.

Franky could change his replacement tires but he should read up on what size would be the optimum for his situation from someone that has actually run alternative tires. And he can go to a tire that has a different rolling radius but then he is going to run into speedometer accuracy issues. And clearance, and gear ratios and ........

FRANKYDOOM · 07-10-2001, 09:37 AM

Thanks EHpower

I thought first it was abit dodgy - lol thanks to all for the replies.

Regards

Frankydoom

Blackfoot · Blackfoot's Garage

911's need every millimeter of rear tire contact patch area they can get. :-)

IceShark · 07-10-2001, 10:07 AM

Blackfoot, that is not actually true. You can get too wide for your own good. Bad news performance issues crop up when the contact patch is real wide but short lenghtwise.

Otherwise everyone would be running monster steamrollers.

Blackfoot · Blackfoot's Garage

if you watched the old 911's racing, that's exactly what they had-monster steamrollers. look at the rsr's. last year i watched the speedvision gt series, and to even it up the top 911 was forced by officials to run a smaller tire in the rear. some cars had extra weight added, but to get the 911 they cut down the size of the rears.

IceShark · 07-10-2001, 11:21 AM

Blackfoot, true for dry pavement, I was talking about what street cars encounter and wide hurts in rain and snow.

Dave951M · 07-10-2001, 06:26 PM

If you keep the weight of the car the same, and increase the area of the contact patch, there is less lbs/sq in on the ground for a given area. In armor, this is known as ground pressure, it allows multiton vehicles to traverse mud pits. In cars, it can result in poorer handling than keeping contact area balance relative to the weight of the car and the tire compound. Everything is a trade off. Different size tires like this are not unheard of when looking to tweak a car.

Dave951M