A couple questions on tire sizes and diameter for the experts

Dragster · Aug 13, 2017

Hey everyone! I have never put aftermarket wheels/tires on a car before, and I had a couple questions for those that have done so. I currently have a GT PP with the stock wheels and tires, and I was looking at Michelin Pilot 4S tires to go with MRR M350 staggered wheels (10 and 11). The sizes I'm looking at are 275/30/19 and 305/30/19.

1) When changing sizes, how much do I want to pay attention to the relationship between front and rear diameters? For example, the stock GT PP has a rear diameter .6" bigger than the fronts. The combo I'm looking at now would be about the same at .7", but the overall diameter of the tires is about 1.5" smaller (wider, lower profile tires on wider rims). My car is lowered on Eibach Sportlines, so I'm thinking that the smaller diameter might help with clearance--is that correct, or am I just making that up?

Also, how much difference does changing this relationship affect the handling of the car? I had initially wanted a 285 front tire, but the 4S only comes in a 35, so the overall diameter is 1.3" greater than the 275/30, and it would result in the front diameter being larger than the rear by .6". Am I putting way too much thought into this?

2) My car is tuned by Lund--does anyone know if they can calibrate the speedo for the different sized wheels and tires through a tune? I know I can contact them to find out, but I figured someone on here probably knows since I'm already asking.

Thanks in advance for the help, I appreciate it!

plc268 · Aug 14, 2017

1) You don't want a taller tire up front, that's for sure. If it's taller, it'll look goofy. The relationship between the front and back isn't super important, and people go with a 28.5" tire in the rear (305/35/20 rear) and 27-27.5" front all the time.

That said... do you really want to go with tires that small in diameter? You really should be shooting for close to 27" diameter tires, as that's roughly what it is stock across the board (with a few minor variations). Only the GT350R uses a diameter smaller than that. I haven't done the research on tires available in 19" wheels, but shoot for close to 27" overall diameter.

You're also on a GT PP with 3.73 gears. You're going to run out of gear even faster with a 26" tire in the rear.

2) No they can't calibrate the speedo. No tuner can. You have a few options to do so though. Easiest is to take it to the dealer, but you can do it yourself by modifying your "as-built." There's a tutorial on this forum somewhere, and if I find it, I'll link it.

Alternatively, you can buy whipple's wheel and axle flare tool which will do it too, but it costs $$.

NightmareMoon · Aug 14, 2017

Stock diameters for a PP car are 27.1 F and 27.7 R

Nobody runs a 275/30R19 in the front. That is just too short (25.6 diameter). Get the 285/35R19 or 275/35R19. 275/35 will be closer to your 305/30 rear in overall diameter. IMHO going smaller than 26" diameter would be a mistake. The wheel/tire would look just too small on the car, your ground clearance too short, and your fender gaps too large.

If you go 275/35 and 305/30 you're looking at 26.6 F and 26.3 R

If you went 325/30R19 rear, your rear diameter would be 26.7, but the tire would bulge a bit.

plc268 · Aug 14, 2017

Yea, 275/35/19 and 325/30/19 would work fine with aftermarket rims.

Or, hopefully with the release of the 2018 mustang, michelin will put out their 255/40 & 275/40 sizes since the 4s is the tire for the performance pack.

Need4SpeedMotors · Aug 14, 2017

Dragster said:
Hey everyone! I have never put aftermarket wheels/tires on a car before, and I had a couple questions for those that have done so. I currently have a GT PP with the stock wheels and tires, and I was looking at Michelin Pilot 4S tires to go with MRR M350 staggered wheels (10 and 11). The sizes I'm looking at are 275/30/19 and 305/30/19.

1) When changing sizes, how much do I want to pay attention to the relationship between front and rear diameters? For example, the stock GT PP has a rear diameter .6" bigger than the fronts. The combo I'm looking at now would be about the same at .7", but the overall diameter of the tires is about 1.5" smaller (wider, lower profile tires on wider rims). My car is lowered on Eibach Sportlines, so I'm thinking that the smaller diameter might help with clearance--is that correct, or am I just making that up?

Also, how much difference does changing this relationship affect the handling of the car? I had initially wanted a 285 front tire, but the 4S only comes in a 35, so the overall diameter is 1.3" greater than the 275/30, and it would result in the front diameter being larger than the rear by .6". Am I putting way too much thought into this?

2) My car is tuned by Lund--does anyone know if they can calibrate the speedo for the different sized wheels and tires through a tune? I know I can contact them to find out, but I figured someone on here probably knows since I'm already asking.

Thanks in advance for the help, I appreciate it!

I would agree with the other members here i would suggest something like

275/35/19 305/30/19
275/35/19 305/35/19
285/35/19 325/30/19

or if its a square set up go with 285/35/19..

Paul@PKAUTODESIGN · Aug 14, 2017

Dragster said:
Hey everyone! I have never put aftermarket wheels/tires on a car before, and I had a couple questions for those that have done so. I currently have a GT PP with the stock wheels and tires, and I was looking at Michelin Pilot 4S tires to go with MRR M350 staggered wheels (10 and 11). The sizes I'm looking at are 275/30/19 and 305/30/19.

1) When changing sizes, how much do I want to pay attention to the relationship between front and rear diameters? For example, the stock GT PP has a rear diameter .6" bigger than the fronts. The combo I'm looking at now would be about the same at .7", but the overall diameter of the tires is about 1.5" smaller (wider, lower profile tires on wider rims). My car is lowered on Eibach Sportlines, so I'm thinking that the smaller diameter might help with clearance--is that correct, or am I just making that up?

Also, how much difference does changing this relationship affect the handling of the car? I had initially wanted a 285 front tire, but the 4S only comes in a 35, so the overall diameter is 1.3" greater than the 275/30, and it would result in the front diameter being larger than the rear by .6". Am I putting way too much thought into this?

2) My car is tuned by Lund--does anyone know if they can calibrate the speedo for the different sized wheels and tires through a tune? I know I can contact them to find out, but I figured someone on here probably knows since I'm already asking.

Thanks in advance for the help, I appreciate it!

Hey brotha

So the sizing your saying is to short upfront.

You are much better off doing a

275/40/19 or 35 upfront and a 305/30 or 35 on the rear.

30 profile rear is closest to OEM OD if you want to keep it that route.

Now I believe they have a calibrator from what I've heard so they can adjust that for you, but I can ask my inhouse tuner Rick to triple check since hes very close with the guys at Lund.

You do have alternative options for tires what it comes down to is what you want to invest, how you want it to look and if you have a tire brand you desire.

Dragster · Aug 14, 2017

Thanks for all the replies, I appreciate it! The problem with the 4S tires is that--at least what I can see on Tirerack--they don't make as many sizes as they did with the Super Sport tires. When I had initially checked on the MPSS tires, they made a 305/35 rear, which is very close to stock, and a 285 that was a close match as well. Unfortunately, they don't make those sizes in the 4S tire... I would really like to do a 305 or 315 in the rear, but they don't make a 315, and the only 305 they make is the 30. Perhaps I'll try another brand; if anyone has any suggestions, I'm all ears. My car is not a daily driver, so I'm most concerned with dry performance, and as I said, I'd really like to do a 305 or 315 in the rear. The car is primarily used for weekend/evening use, and the occasional HPDE.

EDIT: I suppose I could also consider going with 20" wheels, but my car sits very low on Sportlines, and I was concerned about clearance and rubbing. After looking at the sizes of tires with 20" wheels, it looks like I can get a diameter similar to stock with the size tires that I was looking for. Thoughts? Any downsides to going to a larger wheel if I'm staying with a similar overall diameter?

A couple questions on tire sizes and diameter for the experts

Dragster

Well-Known Member

plc268

Well-Known Member

NightmareMoon

Well-Known Member

plc268

Well-Known Member

Need4SpeedMotors

Well-Known Member

Paul@PKAUTODESIGN

The Wheel guy

Dragster

Well-Known Member