Smaller Diameter Rears (a lot smaller)?

[jymcat1]

Pretty sure this is a bad idea, but here we go...

Running 325/30/19" (11 inch width) rears = 26.7" diameter. Only size 325 I can find.

Threw PP1 rear wheels (w/5mm spacer) up front. 275/40/19" = 27.7".

Adjusted coil-overs. I think it handles amazing; don't care that it looks stupid. From a pure handling perspective, would I be better off with 275/35/19" = 26.6" - in front? What am I screwing up by running this combo? I'm thinking the traction control/abs (obv speedo) would still be "off" with the 275/35's up front (but could fix w software if it's a serious concern).

Also, once the rears go I have the following options: stick w same size 325's, 315/35's = 27.7" (but Hoosiers are only tire available), or 305/35's = 27.4".

Why/how am I doing this? Went to my first autocross with no clue what I was doing + terrible asphalt; destroyed new PS4S up front. Down to threads/wire on inside of tire. Tire Rack got mounted/balanced/tpms 325's on 11" rims to my house for less $ than price of 2 PS4S 255's. So yea.

Appreciate the advice, am ready for the insults ;). Thanks!!

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[NightmareMoon]

what type of rear tire is it?

If the rears are shorter and you don't compensate with ride height, then you'll get some reverse rake, which may cause understeer on entry (as far as I understand), but with 275s up front and 325s in the rear, you're already in massive understeer territory.

Generally you want as much front tire as possible for autocross. A lot of people run 305 or 315s even on the front. So a 305/30 on an 11" for the front would be preferred over a 275/35.

The easy button for autocross is square 305 tires on 11" wheels front and rear. You can do this with 25mm spacers on the front (preferably slip on spacers over extended wheel studs). You want the rear tires to grip on corner exit, but you also want the car to turn on corner entry and mid corner. Too much tire stagger and the car won't turn well.

So IDK what you're trying to do precisely, but generally the car works well with big wide tires on both ends.

 

[luc]

Pretty sure this is a bad idea, but here we go...

Running 325/30/19" (11 inch width) rears = 26.7" diameter. Only size 325 I can find.

Threw PP1 rear wheels (w/5mm spacer) up front. 275/40/19" = 27.7".

Adjusted coil-overs. I think it handles amazing; don't care that it looks stupid. From a pure handling perspective, would I be better off with 275/35/19" = 26.6" - in front? What am I screwing up by running this combo? I'm thinking the traction control/abs (obv speedo) would still be "off" with the 275/35's up front (but could fix w software if it's a serious concern).

Also, once the rears go I have the following options: stick w same size 325's, 315/35's = 27.7" (but Hoosiers are only tire available), or 305/35's = 27.4".

Why/how am I doing this? Went to my first autocross with no clue what I was doing + terrible asphalt; destroyed new PS4S up front. Down to threads/wire on inside of tire. Tire Rack got mounted/balanced/tpms 325's on 11" rims to my house for less $ than price of 2 PS4S 255's. So yea.

Appreciate the advice, am ready for the insults ;). Thanks!!

“Handle amazing “. ??????
Your notion of handling need some serious recalibration…the mustang is a front heavy pig with terminal understeer that you made worst by your choice of tires witdh….

 

[jymcat1]

what type of rear tire is it?

If the rears are shorter and you don't compensate with ride height, then you'll get some reverse rake, which may cause understeer on entry (as far as I understand), but with 275s up front and 325s in the rear, you're already in massive understeer territory.

Generally you want as much front tire as possible for autocross. A lot of people run 305 or 315s even on the front. So a 305/30 on an 11" for the front would be preferred over a 275/35.

The easy button for autocross is square 305 tires on 11" wheels front and rear. You can do this with 25mm spacers on the front (preferably slip on spacers over extended wheel studs). You want the rear tires to grip on corner exit, but you also want the car to turn on corner entry and mid corner. Too much tire stagger and the car won't turn well.

So IDK what you're trying to do precisely, but generally the car works well with big wide tires on both ends.

Rears are Potenza S007A's. $142 each, so nothing to lose. I've adjusted the coilovers so there is a bit less rake than stock (BMR lockout kit too, if relevant). No 'Carolina Squat.'
Use case: I get onto country roads 1, 2x a week if I'm lucky for 6 months out of the year. That's it.
I realize the setup is stupid, but it has more grip than stock.
If you were me, would you keep driving on the 40's, or get 35's?

I'm getting the impression that it's such a huge mismatch that it won't matter handling-wise. But is the 1" diameter difference screwing anything up in/of itself?

I really appreciate you taking the time to respond. Have a great weekend!

 

[jymcat1]

“Handle amazing “. ??????
Your notion of handling need some serious recalibration…the mustang is a front heavy pig with terminal understeer that you made worst by your choice of tires witdh….

And how do you expect me to achieve that, sir? My notion, same as anyone else's, is based on my experience and point of reference: the rear breaks loose much less than before. I don't loose traction every single time I shift into second. It feels better. Furthermore I explained my "choice" - the wheels were on stupid-sale, and the rears fit up front. I came here looking for some advice.
Good job with the gatekeeping.

 

[CR151S]

I had taller front tires originally and it did well enough…

when it came time to replace the fronts, I went with a more appropriately sized tire and the car feels much better and more balanced

 

[mavisky]

Honestly, run what you have now until they're dead and you can get a proper balanced setup. You're in heavy understeer territory with a 275/325 stagger. You're not doing any damage to anything, but make a change in sidewall height alone won't fix the massive width difference you have front to rear.

 

[luc]

And how do you expect me to achieve that, sir? My notion, same as anyone else's, is based on my experience and point of reference: the rear breaks loose much less than before. I don't loose traction every single time I shift into second. It feels better. Furthermore I explained my "choice" - the wheels were on stupid-sale, and the rears fit up front. I came here looking for some advice.
Good job with the gatekeeping.

“Handling “ is commonly understood, when referring to vehicles, as the car dynamics going through turns, not how good a car hook up in a straight line when you step on the gas

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