Looks like it’s time for new tires on my baby so I was wondering if anyone here ran 305/35/20 on rear stock wheels?
My thoughts are:
1) 255/40-20 front, 305/35-20 rear
2) 255/35-20 front, 305/30-20 rear
3) 255/35-20 front, 295/35-20 rear
Looking to go with PSS. Any inputs are greatly appreciated.

 

Selection 1. will make your tire diameter over-sized both front and rear. Slower acceleration, mildly looser handling, and speedo error, though a softer ride.
Selection 2. will leave the stock size in front, look more aggressive in back and make the rear tire diameter just a hair larger than the front (the stock 295/30 rear is just a bit smaller in diameter than the front)
Selection 3. will result in a rear tire that will be visibly larger in diameter than the front.

Selection 2. would be my overwhelming choice. Would have gone that way if the 305/30-20 PSSs had been available when I was replacing the Pirellis.Got the stock size 295 instead.

 

Yeah don't put a 35 series in the rear. That doesn't make sense.

 

I'm wanting wider rear tires too. Anything to put more power to the road instead of having wheel slippage.

 

I doubt you'd even notice a difference with extra 1/3" width on the 305.

 

Then you want a taller tire and not a wider one. Popular misconception.

 

So how do you figure that?

 

Please elaborate. My assumption is more rubber on the road leads to more power transferred. A 305 is 10mm wider than a 295. So I'd assume that's better. But you're saying maybe I want a taller one?

 

You're 100% correct, more rubber on the road is better for traction. However widening your tire doesn't actually add any rubber to the road, it only changes the shape of the contact patch... the total area remains the same.

The tire (basically a big balloon) flexes when the weight of the vehicle is put onto it, and thus the contact patch is created. Since you haven't changed the weight pushing down on the tire, or changed the air pressure inside the tire, there is no increased force squishing the tire and subsequently expanding the contact patch area; it has only changed shape because you've widened the tire.

So you won't be changing the area of the contact patch when changing tire width (all other factors left constant), but you can modify contact patch shape to suit your uses. A wide/short tire will have a wide contact patch with a short length, while a skinny/tall tire will have a narrow contact patch with a greater length. See photo below for a visual.

The benefits of a wider contact patch is that the tire is more resistant to loosing traction laterally, such as when going around a corner. The wider is better philosophy mainly applies to the road racing crowd, where adding extra width can help lessen or eliminate understeer/oversteer. Conversely this type of contact patch is more prone to wheelspin. A longer/narrow contact patch is better to resist wheelspin because each rubber molecule has to loose grip over a great length (length of the contact patch) prior to the wheel spinning, rather than a short length with on a tire with a wide contact patch.

Basically there is compromise here. You can give up some lateral grip, for increased resistance to wheelspin or vice versa, but in some cases its best to let some air out of the tires and actually increase contact patch area, helping on both accounts. Obviously this only works to a point. Keep in mind also that when you widen your tire, the height of the tire changes also if you leave the aspect ratio constant. You can use this link to calculate contact patch size, and thus find what tire is best for your needs/desires:

Contact Patch Calculator

Now this isn't a perfect rule... there are some forces that come into play such as sidewall stiffness and load distribution, but this generally applies to most vehicle applications. Hopefully I've explained this well enough for everyone to understand!

 

 

My sentiments exactly. Hence, the skinny tires on race cars.

 

Bingo. Dragsters... Off road vehicles... generally anything that needs straight line grip gets taller tires. Those vehicles just don't turn well.

 

I must live in a different world. When PolkNole was talking about slippage, I assumed handling grip. On careful read, i now see he's trying to address straight-line challenges. Never been my concern and certainly not an issue on the V6.

 

I think he needs to find a way to run some of these:

 

Guys...if you're looking for straight line / straight line-ish traction the only thing that's going to reign in all those super charged horses is a drag radial, i.e. the compound of the tire.

Mickey Thompson Performance Tires & Wheels

I use a set at the drag strip on my f-type. If you can deal with the decrease in turning abilities (vast majority of people will be more than fine) and you don't drive in the rain...they're awesome

Here it is on a 305/35/19

 

Not really an acceptable choice for a street car however. But the same theory applies to a drag radial. Taller is better.

 

True - but there is no f-type making enough power to take advantage of a taller tire for straight line acceleration. In the case of the folks here - it'll be all about the compound. Don't want to run a drag radial...go for a sport cup tire. We don't all have that AWD luxury

 

Of course there are. Any car that spins the wheels can take advantage of a taller tire for straight line grip... Especially with our limited tire selection at stock sizes.

 

LOL, even the base V6 can spin the rears from a standing start.

 

Can anyone with a V6 get their tires to spin in a straight line with 20" PSSs?
At least with the manual, these tires have more traction than the clutch.