I have 2 40% 615's and am looking at getting 2 60% neova's. They are all 235/40/18. Looking at the info online, the 615's are .2" taller in total diameter, but they are also more worn. So total diameter should be very very similar. All other spec's such as section and tread width are or are almost identical. Going to run the neova's in the front and 615's in the back.

How does this setup sound, any thing i should look out for?

1/4" circumference difference is OK according to the manual somethingsomething

Cool, i think this will make for a sweet auto-x/track setup.

I would measure the tires to be sure, and then I'm not even sure about it. If you are tracking the car with different size tires any diff wear will be exacerbated. I also don't know how the AWD will like the two different tire types, someone else will have to chime in with that info.

I would REALLY avoid putting 2 different types of tires on to an AWD vehicle. You might be lucky and nothing happens to the car. HOWEVER if something does happen to the car it is going to cost you a WHOLE LOT more then a matching set of tires would cost you.

Consult your owners manual about the tire information or I am sure you could look at the Tire FAQ and find out.

If you do decide to run them, good luck and I really hope nothing happens to your car.

Ok, understood.

What should i look at besides overall diameter and section width? Those seem to be the only two factors in how similar a tire is.

Lets say in theory, that these tires are the exact same size, what would i need to look at otherwise? I know the tires differ in weight by about 3lbs a piece. Could this have an effect?

Ill measure if and when i get the neova's, and ill make a new thread for some better consultation since ill have 1st hand measurements.

As was already discussed, the overall circumferance makes a difference.

Sidewall strength also affects what the effective rolling difference will be, but you should be able to "correct" this by playing with pressures.

Then there is the handling. The tires have different characteristics, so that will affect how well you corner. And different grip levels may cause one end to slip before the other, accelerating center diff wear.

As was already discussed, the overall circumferance makes a difference.

Sidewall strength also affects what the effective rolling difference will be, but you should be able to "correct" this by playing with pressures.

Then there is the handling. The tires have different characteristics, so that will affect how well you corner. And different grip levels may cause one end to slip before the other, accelerating center diff wear.

1)What other factors effect overall circumferance besides overall diameter and section width?

2)The sidewalls on these tires should be very similar, maybe with neova's being a bit stiffer. But it is no different then someone playing with air pressures on their car at an auto-x or track event. With the neova's being a bit stiffer, i can probably get away with staggering the pressure between the front and back less.

3)The majority of subaru's out there understeer. I have no allstar set-up, my car understeers as well. I give the slight nod to the Neova in overall grip...which means my car will understeer less. The only end that is going to be slipping before the other is the front end, in which case, in your theory, i would have a set-up that sustains a healthy diff better then a set of tires that are all identical...no?(i dont know if thats true or not, just going off what you said)

In Australia, at one time it was considered a "smart" thing to run summer tires up front and all-seaons in back to help improve front grip vs. rear grip and thus reduce understeer. Here is a paragraph from the Autospeed Magazine article "Rex on Rails" from the Whiteline website:

On our MY94 WRX - which is fitted with Toyo Proxes T1-Ss on the front and Toyo Proxes FZ4s on the rear - we found the best handling balance is achieved with a considerable difference in front-to-rear tyre pressures.

http://www.whiteline.com.au/articles/AS_Rex_on_rails_1.pdf

silly thing to do IMO - that's why we have adjustable swaybars, adjustable dampers and varying methods for changing alignment - but it didn't seem to screw up the rear diff.

on the other hand, you have an '05 STI, a far more complicated -and expensive - beast than a '94 WRX.

good luck...

In Australia, at one time it was considered a "smart" thing to run summer tires up front and all-seaons in back to help improve front grip vs. rear grip and thus reduce understeer. Here is a paragraph from the Autospeed Magazine article "Rex on Rails" from the Whiteline website:

On our MY94 WRX - which is fitted with Toyo Proxes T1-Ss on the front and Toyo Proxes FZ4s on the rear - we found the best handling balance is achieved with a considerable difference in front-to-rear tyre pressures.

http://www.whiteline.com.au/articles/AS_Rex_on_rails_1.pdf

silly thing to do IMO - that's why we have adjustable swaybars, adjustable dampers and varying methods for changing alignment - but it didn't seem to screw up the rear diff.

on the other hand, you have an '05 STI, a far more complicated -and expensive - beast than a '94 WRX.

good luck...
That depends. On an older wrx it may be "worth" it for autox, definitely not track. THe USDM sti though has no problems with hanging the tail out with power like the wrx's do. You don't need to run different kinds/sizes of tires to keep it from understeering under power.

Guys im not running this as an ideal set-up. I am getting the neova's cheap and i already have the 615's. I wouldnt be considering this at all if i had the cash to buy the tires new.

The 615 is very similar in grip to the Neova's. Hell if sizing is the same, it might as well be like me having neova's in the back with a couple psi to much pressure.