[wnta1ss]

Originally Posted by Umbriel

I just looked it up and the stock tire is 27.58 and the 305/45/17 is 27.8 which is not enough difference to even notice.

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MT lists the 305/45-17 that the OP was talking about as a 28.1 diameter, which is definitely taller than the stock SS tire. I have seen the effect of the taller tire in the data, an unadjusted speedometer reads lower with the taller MT on it. I have had both tires on the car, the difference is noticeable.

[Umbriel]

Originally Posted by wnta1ss

MT lists the 305/45-17 that the OP was talking about as a 28.1 diameter, which is definitely taller than the stock SS tire. I have seen the effect of the taller tire in the data, an unadjusted speedometer reads lower with the taller MT on it. I have had both tires on the car, the difference is noticeable.

If you say so, I have been running that same tire for years on two different cars and there was no noticeable difference in anything, that small of a difference can be the same as having 10 less or more pounds of air in it. If you do the math that is 27.8 like I said versus 27.58 and you will but notice anything.

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[Umbriel]

Originally Posted by wnta1ss

MT lists the 305/45-17 that the OP was talking about as a 28.1 diameter, which is definitely taller than the stock SS tire. I have seen the effect of the taller tire in the data, an unadjusted speedometer reads lower with the taller MT on it. I have had both tires on the car, the difference is noticeable.

I just used a tire size speed calculator and it says it will be .5 mph off, you aren't going to notice that.

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[BDPhoto519]

Thanks guys,

When I toss the ET Street's on this week sometime I will take a measurement of them sitting at 20psi on the car vs the stock wheels on the car sitting at 32psi and post it up here for reference.

If there is any difference I am sure my OCD will have me make a 2 second adjustment in HPTuners for it.

[wnta1ss]

Originally Posted by Umbriel

I just used a tire size speed calculator and it says it will be .5 mph off, you aren't going to notice that

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That is just generic stuff there, using nominal sizes like that. Perhaps you didn't realize it, but not all 275/35-20 or 305/45-17 tires are actually the same size just because it's stamped on the sidewall. I have scanned the car on the track using both the stock SS runflats, and the MT 305/45-17 tires that are the subject of this thread. What I saw was that with no change in the speedometer calibration, the MTs gave lower readings on the speedometer than the runflats did. Real speeds were known because it was done on a track. Dropped a lot more than half a mph in the reading going to the taller MT. Tire pressures were 27psi for the runflats and 17psi for the MTs, and yes, this means that the MT was made a little shorter by that lower pressure. If you ran a lot of air in the MT then you'd likely see an even bigger difference than this.

[Umbriel]

Originally Posted by wnta1ss

That is just generic stuff there, using nominal sizes like that. Perhaps you didn't realize it, but not all 275/35-20 or 305/45-17 tires are actually the same size just because it's stamped on the sidewall. I have scanned the car on the track using both the stock SS runflats, and the MT 305/45-17 tires that are the subject of this thread. What I saw was that with no change in the speedometer calibration, the MTs gave lower readings on the speedometer than the runflats did. Real speeds were known because it was done on a track. Dropped a lot more than half a mph in the reading going to the taller MT. Tire pressures were 27psi for the runflats and 17psi for the MTs, and yes, this means that the MT was made a little shorter by that lower pressure. If you ran a lot of air in the MT then you'd likely see an even bigger difference than this.

I understand tires aren't exact, I also understand I have experience with both tires, different cars with those tires and have been running for them for years and have never seen a difference enough to even notice. Run what you want though, I'll not worry about it with mine since I have them on my car right now and my speedo isn't off.

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[Umbriel]

So I played with this calculator to get it even closer to the actual tire height of 28.1 Mickey Thompson says and it will be off by 1 mph, like I said not even noticeable.

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[Snowblind]

Originally Posted by BDPhoto519

Is anyone changing the tire size scaling in HPTuners to keep all the mph/etc in proper calibration when running the DR's vs Street tires?

I haven't physically measure the height difference between the stock wheels, although I'm sure it close.

You can get away with not scaling and be ok, for informational purposes the ecu reads the factory tire height at 26.83 so changing the tire height to 28 will net you a difference of only 1 mph on the 1-2 upshift parameter and it's scaled to 4 mph on the 4-5.

[BDPhoto519]

Originally Posted by Snowblind

You can get away with not scaling and be ok, for informational purposes the ecu reads the factory tire height at 26.83 so changing the tire height to 28 will net you a difference of only 1 mph on the 1-2 upshift parameter and it's scaled to 4 mph on the 4-5.

Thanks Snowblind,

That was my main concern I had with the tire scaling affecting the trans shift points. I haven't sent in my TCM to be unlocked yet so I dont want to make the problem worse.

[wnta1ss]

Originally Posted by Umbriel

So I played with this calculator to get it even closer to the actual tire height of 28.1 Mickey Thompson says and it will be off by 1 mph, like I said not even noticeable.

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Your 731 revs/mi figure for the stock runflat is taller than it really is. The factory calibration is 745 or 748, depending on the model year. By calling the runflat taller than it really is, that makes it seem like the 2 tires are closer than they really are. The true difference, as I've track-tested, is larger than you are portraying. Was not talking about whether you notice it driving on the street with no benchmark, only talking about the true size difference. Whether someone wants to work on their calibration for this tire or not is of course completely up to them.

[Umbriel]

Originally Posted by wnta1ss

Your 731 revs/mi figure for the stock runflat is taller than it really is. The factory calibration is 745 or 748, depending on the model year. By calling the runflat taller than it really is, that makes it seem like the 2 tires are closer than they really are. The true difference, as I've track-tested, is larger than you are portraying. Was not talking about whether you notice it driving on the street with no benchmark, only talking about the true size difference. Whether someone wants to work on their calibration for this tire or not is of course completely up to them.

That 731 revs/mi figure isn't mine it was calculated by the program I used that was made by a company that sells tires for a living so I'm sure it's correct. Either way I run these tires, have been running these tires and my speedo is fine, you run what you want however you want with yours.

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[wnta1ss]

Originally Posted by Umbriel

That 731 revs/mi figure isn't mine it was calculated by the program I used that was made by a company that sells tires for a living so I'm sure it's correct. Either way I run these tires, have been running these tires and my speedo is fine, you run what you want however you want with yours.

Still just a theoretical calculation. If you are sure that a company that sells tires for a living is correct, how about the company that actually made the runflats, namely Goodyear? I looked it up, and Goodyear lists 758 revs/mi for the 275 runflat. Hmm, that is a lot different. Now the truth, as previously noted, is that the Chevy calibration of 745 to 748 is very close for this tire on an SS. How do we know that? It's seen by how close the speedometer reads to accurate in stock form. If either your calculation number, or Goodyear's number, were true, then the speedo would obviously not be as close as it was with this tire mounted.