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Different F and R rolling diameters - anyone know why?

GT 550

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Hey folks,

I'm researching tyre options for a 2017 PP1 and it looks like the OEM 255/40/19 fronts are 3/4" smaller in diameter than the 275/40/19 rears. Could be many reasons for this including whatever tire deal Ford could make with Pirelli, but I was wondering if anyone knows of another reason like ABS tuning etc.

Thanks in advance...
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cmxPPL219

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Hey folks,

I'm researching tyre options for a 2017 PP1 and it looks like the OEM 275/40/19 fronts are 3/4" smaller in diameter than the 255/40/19 rears. Could be many reasons for this including whatever tire deal Ford could make with Pirelli, but I was wondering if anyone knows of another reason like ABS tuning etc.

Thanks in advance...
Depending on what you meant to say, your question may have been better asked, "Why the 2017 PP1 has a staggered tire setup" - Rolling diameter differential being a side-effect.

There are many reasons for an OEM to choose a staggered tire setup on a RWD car (over a square setup) with certain desired handling characteristics being a primary reason. Another, is the fact that a higher power RWD car will need to be able to put that power down the ground, so a wider tire provides more contact patch allowing this to happen, which also needs to be balanced out and determined based on actual tire performance category and tire specific compound.

On the handling characteristic side of things, in the case of our cars, there is a decent amount of power going to the rear, and as a rule, fitting a wider rear tire introduces a certain amount of understeer when the car is driven at the limit. This is also an intentional choice for Ford, since these cars will naturally be tail-happy, having a wider rear tire on certain models, coupled with the driver aids like traction control and VDC, will allow precise tuning of at-the-limit handling characteristics, and the engineers can dial the car in to where they want it.

In the case of the OEM Pireli's for the 2017 GT PP1s, the specific compound type for this class of tire (which the 2018+ models have Michelin Pilot Sport 4 S installed in the identical size, and is the same tire performance category as the Pireli's) the Ford engineers may have chosen this wider rear size, balancing cost with x amount of at-limit grip with certain weather performance with other handling targets.

Meanwhile, a 2019 GT PP2 has a square setup, with wider 305s all around - however, aside from other reasons, one of the reasons for not having a wider rear tire in this application, is that the compound of the OEM Sport Cup2s may have provided the amount of grip that was desired, without sizing up in the rear. Then, in the case of the GT350R, you also have Cup2s, but 305/30 in the front, 315/30 in the rear - this car having an entirely different set of handling and grip targets.

With respect to tire brand chosen, this often comes down primarily to which tire company for the given and desired size bids the cheapest, but this isn't always the case - for certain performance applications, like a GT350 for example, Ford may not go with the lowest bidder necessarily - the high level of handling and grip that is desired necessitates a certain tire and compound, and in the tire performance category that the Sport Cup 2s live in, choices will be limited, for something that is streetable yet provides superior grip and handling response when driven at the limit.
 
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@FruityJudy doh, thanks original post edited.

@cmxPPL219 thanks it's primarily the rolling diameter difference I'm interested in, as I said in my post this can be for many reasons but I'm trying to find out if anyone knows for sure why it's the case with the PP1.
 

cmxPPL219

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@FruityJudy doh, thanks original post edited.

@cmxPPL219 thanks it's primarily the rolling diameter difference I'm interested in, as I said in my post this can be for many reasons but I'm trying to find out if anyone knows for sure why it's the case with the PP1.
Right, I understand what you're asking - however, OEMs don't specifically spec rolling diameters - what they are concerned with is the tire sizes themselves, and whatever difference in rolling diameter the fronts end up being, compared to the rears, is just a side effect - rolling diameters are not something that engineers outright chose to spec, if that makes sense.

Things like speedometers and ABS wheel sensors will may be calibrated for the revolutionary unit that the wheels will output, but again, there is no specific diameter measurement that is sought after. ABS and VDC will then look at the difference in wheel speed to make certain adjustments.

The rolling diameter difference of the 255/40/19, vs the 275/40/19, is not something specifically chosen by Ford - the tire sizes are. The different tire sizes will naturally have a different rolling diameter.

Also, another reason why rolling diameter is not a measurement that is looked at, is because whether rolling diameter or rolling radius, these can also differ between various tire manufacturers for a given size. So in other words, if someone measures the Pirelli's rolling diameter for a given tire and then Michelin's equivalent tire in the same size, both tires will not have the same measurement. So, when it comes time to replace tires, and the OEM specc'd a rolling diameter, but the factory tire brand and size is no longer available, more than likely, any other tire that will be fitted will not have this same factory OEM "rolling diameter" measurement. In this case, no replacement tires could end up meeting the replacement spec. This is part of the reason why we have the tire size system we do (Width in mm; Sidewall Height as % of width; and Rim Size in inches).
 
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but I'm trying to find out if anyone knows for sure why it's the case with the PP1.
why do you care? it's irrelevant. ABS systems are plenty sophisticated to compensate for any mix of tires you put front/rear. Hell VW for example doesn't use TPS at all because they are smart enough to look at wheel sensor drift and reliably ascertain when one tire is under-inflated.

Only Audi Quattro seems to care about holding 'close' tolerances on rolling diameter. How much of that is real vs the fever-swamp of non-engineers waxing on the Internet I don't know.
 
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@cmxPPL219 ok so an OEM will decide that it wants a XXX/XX/RXX tyre at each end (or on all 4) and gives no consideration to rolling diameter as an input?

@shogun32 I don't know if I need to care or not hence the question on whether there's a confirmed reason for the difference. If there is, I'll care but if not happy days as it gives me more tyre choice. Re the Audi Quattro being held to close tolerances I don't think it's unrealistic as it's AWD, the GTR becomes unhappy if I put different diameter wheels on each end and it's only part time 4wd.
 
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shogun32

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Ok so you're saying an OEM will decide that it wants a XXX/XX/RXX tyre at each end (or on all 4) and gives no consideration to rolling diameter?
I didn't say that. 2% delta is nothing though. I would expect 'cosmetics' are a bigger factor in tire size choice than engineering. Within reasonable bounds of course.

There is zero merit to "the rolling diameter has gotta match". It never mattered.
 

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Yeah I’ve run 275/35 275/40 285/35 and 305/30 rear tires at different times no issues with any of those due to rolling diameter.

Ford wanted a stagger and picked 255F 275R for the PP1 to establish some slight understeer (possibly in part due to the aggressive 3.73 gearing), and they didn’t want to go smaller than a 40 sidewall, for comfort, appearance, IDK. Rolling diameter itself isn’t magic or sensitive. If they had used a 275/35, which is the next step down, the sidewalls on the rear tires would have been shorter than the sidewalls on the front tires, which would have looked odd.
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