[aa406079]

As you all know.. the V8 M6 is geared towards the taller end of the spectrum for fuel economy...


I'm going down to a 19" wheel and tire combo to strip down some weight... I was going to go from

285/30R20 to 285/35R19

and

305/30R20 to 305/35R19


then I got to thinking... wouldnt it make more sense to go to 285/30R19 and 305/30R19?

The overall diameter would be reduced... slightly increasing final drive ratio... giving better midrange torque and faster spool up... sure... fuel economy would suffer the other way...

Is it really difficult to reprogram a new tire diameter so that the speedo/odometer becomes recalibrated?

I will post this now and then add the tire diameter calculations when I get them all together

heres the formula:


Tire Diameter = 2 x (section width/25.4) x (aspect ratio/100) + rim diameter

[aa406079]

http://www.onallcylinders.com/2015/0...l-drive-ratio/



EAGLE F1 SUPERCAR 3
Extreme Performance Summer

Rear: 305/30ZR20

2x 305/25.4 x 30/100 + 20 = 27.2"

27.2" total diameter (rears)


Front 285/30R20

2x 285/25.4 x 30/100 +20 = 26.7"

26.7" total diameter ( fronts)


total rake ... .5"

front to rear diameter difference 1.87 % ( important for PTMS)

drop an inch from both.... 26.2/25.7= 1.94 % ( I would like to believe this very minor change shouldnt affect PTMS which relies on the difference in tire diameter to NOT change much to function properly... the Corvette guys claim square set ups really mess up the PTMS since its not originally calibrated for a square set up...)


if I go with Michellin Super Sports ( since the numbers the new PS4S tire arent out yet)

a 305/30R19 would be exactly 26.2" 1" smaller diameter...

285/30R19 will be 25.7" ... preserving the 1/2" rake and diameter differential for PTMS

the formula means that the size should be exactly 1" smaller on the fronts as well... keeping the same rake from factory...


so... now I would have a car effectively lowered 1/2" ( and that better than springs because it includes a lower CG for the unsprung rotating weights) and a higher final drive ratio of...

27.2/26.2 x 3.73 = 3.87 final drive ratio ....

...instead of the stock 3.73


thoughts? comments? predictions of disaster?


essentially the speedo and the HUD will read 3.8% faster than it should...


Will an aftermarket speedometer calibration affect warranty?

[rotis]

I won a Solo National Championship in my 2016 M6 SS running 275/35-19, it's fine.

Less rotating mass, better gearing, lower Cg. All good things.

[Mr. Wyndham]

Originally Posted by aa406079

thoughts? comments? predictions of disaster?


essentially the speedo and the HUD will read 3.8% faster than it should...


Will an aftermarket speedometer calibration affect warranty?

No disaster. You'll effectively lower the car by a half inch or so, increase well to wheel gap by half an inch, and have a faster-than-real speedo. The ride *might* be a bit rougher due to sidewall reduction, but not by much.

On the other hand, as rotis said: lower rotating mass means you'll feel like the car accelerates quicker. Because it does. And the suspension won't need to work quite as hard to absorb bumps. Stopping distances may decrease as well (depending on the tire).

Any flash to the computer will appear if a dealer ever looked for it. Which they are required to do before GM will approve most powertrain warranty repairs. Typically...if they see the computer was changed in any way, they will void the warranty.

[vtirocz]

Originally Posted by aa406079

http://www.onallcylinders.com/2015/0...l-drive-ratio/



EAGLE F1 SUPERCAR 3
Extreme Performance Summer

Rear: 305/30ZR20

2x 305/25.4 x 30/100 + 20 = 27.2"

27.2" total diameter

if I go with Michellin Super Sports ( since the numbers the new PS4S tire arent out yet)

a 305/30R19 would be exactly 26.2" 1" smaller diameter...

the formula means that the size should be exactly 1" smaller on the fronts as well... keeping the same rake from factory...


so... now I would have a car effectively lowered 1/2" ( and that better than springs because it includes a lower CG for the unsprung rotating weights) and a higher final drive ratio of...

27.2/26.2 x 3.73 = 3.87 final drive ratio ....

...instead of the stock 3.73


thoughts? comments? predictions of disaster?


essentially the speedo and the HUD will read 3.8% faster than it should...


Will an aftermarket speedometer calibration affect warranty?

I would check with the dealer or truck forums where going to different size tires is probably more common. If the speedometer is reading 3.8% faster, that will affect odometer, and possibly even PTM & active rev matching some.

[aa406079]

Sounds like its time to pretend I own a C7 Grandsport and head over to the "Ask Tadge" section and ask them that very question in regards to PTMS

Perhaps this is something I'll have to take up with GM and my Dealership manager... they usually have tune flash counters to snoop on non OEM tunes... perhaps I can get the dealer to supervise the recalibration "tune" to confirm its not changing any other powertrain parameters and as long as the tune flash count stays at 1 I should be OK... IF they will be open to that suggestion...

[aa406079]

Originally Posted by aa406079

http://www.onallcylinders.com/2015/0...l-drive-ratio/



EAGLE F1 SUPERCAR 3
Extreme Performance Summer

Rear: 305/30ZR20

2x 305/25.4 x 30/100 + 20 = 27.2"

27.2" total diameter (rears)


Front 285/30R20

2x 285/25.4 x 30/100 +20 = 26.7"

26.7" total diameter ( fronts)


total rake ... .5"

front to rear diameter difference 1.87 % ( important for PTMS)

drop an inch from both.... 26.2/25.7= 1.94 % ( I would like to believe this very minor change shouldnt affect PTMS which relies on the difference in tire diameter to NOT change much to function properly... the Corvette guys claim square set ups really mess up the PTMS since its not originally calibrated for a square set up...)


if I go with Michellin Super Sports ( since the numbers the new PS4S tire arent out yet)

a 305/30R19 would be exactly 26.2" 1" smaller diameter...

the formula means that the size should be exactly 1" smaller on the fronts as well... keeping the same rake from factory...


so... now I would have a car effectively lowered 1/2" ( and that better than springs because it includes a lower CG for the unsprung rotating weights) and a higher final drive ratio of...

27.2/26.2 x 3.73 = 3.87 final drive ratio ....

...instead of the stock 3.73


thoughts? comments? predictions of disaster?


essentially the speedo and the HUD will read 3.8% faster than it should...


Will an aftermarket speedometer calibration affect warranty?

had to redo the calculations based upon Michellin PS4S tires initially only be available in

305/30R19
285/35R19
295/30R19

]http://www.onallcylinders.com/2015/07/30/tire-math-calculating-the-effects-of-tire-diameter-on-final-drive-ratio/[/url]



EAGLE F1 SUPERCAR 3
Extreme Performance Summer

Rear: 305/30ZR20

2x 305/25.4 x 30/100 + 20 = 27.2"

27.2" total diameter (rears)


Front 285/30R20

2x 285/25.4 x 35/100 +20 = 26.85"

26.7" total diameter ( fronts)


total rake ... .5" was original... now reduced to .35"

front to rear diameter difference 1.87 % ( important for PTMS)

drop an inch from both.... 26.2/25.85= 1.01 % ( I would like to believe this very minor change shouldnt affect PTMS which relies on the difference in tire diameter to NOT change much to function properly... YET while only 1% difference its almost double the %...the Corvette guys claim square set ups really mess up the PTMS since its not originally calibrated for a square set up...)


if I go with Michellin Super Sports ( since the numbers the new PS4S tire arent out yet)

a 305/30R19 would be exactly 26.2" 1" smaller diameter...

2x305/25.4 x 30/100 + 19 = 26.2"

2x285/25.4 x 35/100 + 19 = 26.85" ( BIGGER THAN FRONTS... definite NO NO)

2x295/25.4 x 30/100 + 19 = 25.96" ( half the normal rake ... guess it will have to do)

Looks like I would have to go with 295/30 fronts to keep the (reduced) rake


so... now I would have a car effectively lowered 1/2" ( and that better than springs because it includes a lower CG for the unsprung rotating weights) and a higher final drive ratio of...

27.2/26.2 x 3.73 = 3.87 final drive ratio .... (unchanged)

...instead of the stock 3.73




The other option is to go 295/30R19... but perhaps that might mess up the handling of the car by adding too much front tire....

are these changes minor and am I overthinking this?

[wnta1ss]

Keep in mind that the numbers on the sidewalls are only an approximation. The true diameter of the tire does not necessarily match the theoretical, you'll need to take careful measurements if you want the real numbers.

[grnflash]

Originally Posted by rotis

I won a Solo National Championship in my 2016 M6 SS running 275/35-19, it's fine.

Less rotating mass, better gearing, lower Cg. All good things.

This.

I'm running 285/35-19's square. No PTM/rev-match freak-outs, and you can easily feel the effects of each benefit mentioned above.

[Viper126]

How does the rim size impact your figures? 9, 10, 10.5, 11? I've been contemplating 18 and 17 for snow tires when I read your post.