[vtirocz]

Originally Posted by KingLT1

Taller tires reduces shift extension meaning you will fall back to a lower rpm on shift. You will go faster on a shorter tire if you can hook. I believe the top bolt-on A8 cars run a 26" tire for better gearing at the track, then move to a 28" with more power like H/C because the 26" will no longer hook. I ran a 305/35/20 555r even when bolt-on NA and it was 5-6 tenths quicker over the stock run flats but lost a little trap speed. Hard street tires put more power to the ground over drag radials. On a dyno you will see 15whp difference. So if all you care about is mph or roll racing then run a shorter harder street tire. If you want to hook then taller softer compound with a side wall that will flex is what you want. Sticking with something like a 275/35/20 555r2 is probably a good middle ground.

I'm just commenting on this one point in bold: "Taller tires reduces shift extension meaning you will fall back to a lower rpm on shift.'

I don't think that's accurate. You could increase (or decrease) tire diameter and it theoretically would not impact the engine speed drop between shifts assuming the engine speed at which the shift is initiated remains the same.

Similarly, you could change the rear end gear ratio and it also wouldn't affect the engine speed drop between gears.

The engine speed drop between gears is a function of the transmission gear ratios.

[s346k]

speed vs rpm for a given tire size. the other (mechanical) variables and trans gearing never change.

[lt4camaro]

Originally Posted by vtirocz

I'm just commenting on this one point in bold: "Taller tires reduces shift extension meaning you will fall back to a lower rpm on shift.'

I don't think that's accurate. You could increase (or decrease) tire diameter and it theoretically would not impact the engine speed drop between shifts assuming the engine speed at which the shift is initiated remains the same.

Similarly, you could change the rear end gear ratio and it also wouldn't affect the engine speed drop between gears.

The engine speed drop between gears is a function of the transmission gear ratios.

I do not think engine RPM drop when a shift takes place was the thought here. The camaro auto trans up shifts by MPH not rpm. So the question I asked would a shorter tire make the WOT shift point at a slightly higher RPM because MPH would be less at a given RPM no matter what gear your in. So hypothetically if the trans was looking at MPH to do a WOT shift, it may be doing just that at a little higher RPM now to reach the MPH its looking for. For intance a 1 inch shorter tire maybe cause a WOT upshift 100 rpm higher?

[wnta1ss]

Originally Posted by vtirocz

I'm just commenting on this one point in bold: "Taller tires reduces shift extension meaning you will fall back to a lower rpm on shift.'

I don't think that's accurate. You could increase (or decrease) tire diameter and it theoretically would not impact the engine speed drop between shifts assuming the engine speed at which the shift is initiated remains the same.

Similarly, you could change the rear end gear ratio and it also wouldn't affect the engine speed drop between gears.

The engine speed drop between gears is a function of the transmission gear ratios.

This is true, the rpm drop on upshift does not change with the taller or shorter tire. I ran the numbers for the stock runflats, a 28" MT, and a 26" MT. Shifting at 6500rpm with the A8 transmission. All 3 of them dropped to the same rpm in the next gear after the 6500 upshift.

[vtirocz]

Originally Posted by lt4camaro

I do not think engine RPM drop when a shift takes place was the thought here. The camaro auto trans up shifts by MPH not rpm. So the question I asked would a shorter tire make the WOT shift point at a slightly higher RPM because MPH would be less at a given RPM no matter what gear your in. So hypothetically if the trans was looking at MPH to do a WOT shift, it may be doing just that at a little higher RPM now to reach the MPH its looking for. For intance a 1 inch shorter tire maybe cause a WOT upshift 100 rpm higher?

I'm not disputing any of what you wrote above, only KingLT1's comment that "Taller tires reduces shift extension meaning you will fall back to a lower rpm on shift." I agree with the rest of King's post, just not that one point.

Originally Posted by KingLT1

S346k is correct. When you go to a taller tire it effectively changes your final drive ratio. So if you shift at the same rpm as the shorter tire, then the rpm will fall back to a lower rpm on a upshift. I have seen it first hand.

I disagree with that. Assuming the shift is initiated at the exact same engine speed (6500rpm for instance), the engine speed after the shift is completely independent of rear axle ratio and tire size (instead it's a function of the transmission gearing). I've calculated this out just to confirm I'm correct here. I guess there could be vehicle acceleration differences between the shift that could play into this, but I'd expect that to be a small difference. I agree with the other points from your prior post.

[KingLT1]

The confusion is on my part when I said that the rpm drops lower on gear change but that is incorrect. The shorter tire will put you at a lower mph at the same rpm as the taller tire. or another way to say it is the taller tire will be at a lower rpm at the same mph as the shorter tire...that is what I was thinking when I referenced it to RPM drop. My mistake you are correct.

[L78toLT1]

Originally Posted by vtirocz

Good pass. I consider 1.95 good for a 60’!

Oh hell yes, 1.95 is a good 60'! I'll have to check my time slips, but I believe 1.90 is my best to-date, and that's on your old drag pack, Greg. I love my m6, but A8 or A10 is the only way to roll on the strip.

[vtirocz]

Originally Posted by L78toLT1

Oh hell yes, 1.95 is a good 60'! I'll have to check my time slips, but I believe 1.90 is my best to-date, and that's on your old drag pack, Greg. I love my m6, but A8 or A10 is the only way to roll on the strip.

In my stock '17 SS M6 on original tires, my best 60' is 1.91s. I believe other M6 cars have dipped into the 1.8s, but based on the fast list you can see a clear difference/trend between the auto and manual.

With a drag radial, I'd hope for a 0.2sec improvement.

[lt4camaro]

Originally Posted by Yosh6

Couple of points. I'm not a tuner, but I'm pretty sure of these two things. 1. If the tire height in the rear is changed from stock, the speedo calibration in the tune has to be changed because a taller/shorter tire will change the VSS revolutions per mile. Not making this change to match tire height will give the transmission false data for shifting. 2. Mismatching front to rear tire height can result in a StabiliTrak nightmare. This one I know first hand. I tried 26's in the rear and left the 28's on the front. StabiliTrak went mad and put the car in limp mode. I don't know how much of a mismatch you can get away with, but my car will not tolerate a 2" difference. I'm running 28's on all four corners after Bret @ Pray educated me on this. BTW, you cant just assume that you will get EXACTLY 28" tires when you buy them. The number for the calibration comes from an actual measurement of the tire height on the car, on the ground with the correct air pressure. My Micky "28's" are actutally 26.83". Photos FYI only for reference, suggest you get a tuner to get things right, or consult before you go in a certain direction.

The car will tolerate 1 inch tire diameter diff no problem. Can go to a 27.7 to a 26.7 with no issues and tire size diff front to rear will tolerate the 1 inch difference also.

[KingLT1]

I don't have any issues with a taller tire on the back either. I run stock size 245/40/20 front with 305/35/20 rear. They are rated 28.4 tall but measure 27.9 loaded so that is what I plugged into the tune to calibrate the speedo and shift points. The speedo is dead on with my Dragy. I do agree you want to let the ECU know that you changed tire height mainly so it can adjust the shift points and correct the speedo.

[wnta1ss]

As far as tire size and Stabilitrack, I found the same thing as KingLT1 did with tall rears on my SS. The taller rear tire (305/45-17) with the stock size front tire did not bother it. However, a smaller rear tire (275/40-17) with stock size front tire freaked the car out like Yosh6 mentioned. Warning lights lit up and car went into some weird limp thing. Due to this, ended up turning the nannies off with the ~10sec hold of TC button to drive the car with the shorter rears. With the shorter tire, I had adjusted the speedometer per their spec before driving the car, but it freaked out in less than a mile.

[wnta1ss]

Subject of changing the speedo for a different tire, I had tried it when my TCM was still locked (pertinent to the 2020+ auto owners). Car was 2017 SS A8 with MT 305/45-17 DRs on the back. I had found that these were taller than stock, and the car was able to get 116mph in 4th gear at 6500rpm with the stock tune. With these tires on, traction was better, but the shifting with the locked tcm was worse than with the stock tires. I did try changing the speedo cal for them, but shifting remained roughly the same as with the stock speedo calibration. Illustration below, the 109mph is with stock speedo calibration and the 114mph is with speedo calibration adjusted closer to the MT's actual size. The cursor is at top of 4th gear.

[RobbyBeefcake87]

Originally Posted by vtirocz

In my stock '17 SS M6 on original tires, my best 60' is 1.91s. I believe other M6 cars have dipped into the 1.8s, but based on the fast list you can see a clear difference/trend between the auto and manual.

With a drag radial, I'd hope for a 0.2sec improvement.

I've seen some dip into the 1.8s as well. There's a guy that just posted a 1.7 60' on a 12.0 run with a stock m6 1le. I figured stock for stock eventually a 1le would probably get the best m6 60' due to a tire advantage.

Excited to see what the new a10 1le's do with the 2.85 rear end.

[lt4camaro]

Originally Posted by RobbyBeefcake87

I've seen some dip into the 1.8s as well. There's a guy that just posted a 1.7 60' on a 12.0 run with a stock m6 1le. I figured stock for stock eventually a 1le would probably get the best m6 60' due to a tire advantage.

Excited to see what the new a10 1le's do with the 2.85 rear end.

I would love to get a 2.85 rear for my 2021 LT1 10 speed auto that is being built this week 11/2