26" or 28" for boosted car?

[SolarFlare]

Anyone have any experience between 26 and 28" tires? I have 26.3s now, wondering if its worth the money to go 305/40-18 (27.6). Car is an auto, procharged, longtubes, 3.55 geared car. Ive been told at or close to 28" is where I need to be at but want to gather more data before swapping. shifting at 7k, 26.3 allows me to go ~135mph is 4th. I believe 27.6 would be slightly over 140. I ask here because some of you have way more coyote and drag racing experience than I do. Thanks.

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[Jay-rod427]

Always as tall as possible no matter the situation except maybe an 80 hp ricer.

 

[WestCyril]

I would only recommend a 26 for an NA bolt car just due to not making a lot of power. I would recommend you go with a 28, but also step down to a 17 rear tire so you can get some sidewall.

Sidwall will drastically improve traction if youre having issues with your current setup.

305/45R17 Et Street Rs on my Auto car went 1.3X not breaking a sweat. I believe they also make a set of 18 ET Streets that are around 27.5/28 tall as well.

 

[dubster99]

Most people tend to run 28's. That's what I'm going to run. 305/45/17 ET street R.

 

[Bartly]

So question on this and the S550. Is there anything to worry about as far as the ABS or Traction control if I was to go with larger diameter tires. I know my speedometer would be off whatever the new % is, just wondering if any of the navigational or computer systems in the car would trip up. What if the front tires were an inch shorter than the rears, would think if anything, that might be the setup that could freak out the ABS or even the traction control, but probably not? Anybody care to comment on this?

Car is a manual transmission, not sure if that matters.

 

[dubster99]

Everything will be fine. GPS speed on the nav will still be correct. Only issue is the speedo.

 

[Bartly]

Everything will be fine. GPS speed on the nav will still be correct. Only issue is the speedo.

Thanks, was guessing that would be the case, felt like asking just in case.

So here's another kinda question along the probably worrying about a problem that doesn't exist. So was thinking if the rear tires are bigger I might want to run a lower psi in the rear, does the tire pressure gauge system throw any warning if the difference in the rears is a few psi less than the front?

 

[1quick]

If you have a tuner you should be able to change the revolutions per mile so the speedo reads correct

 

[dubster99]

The tuners cannot adjust the speedo for tire size or gear changes. You need the whipple flare tool, or modify the as built data. There's a thread on this either in the drivetrain forum or the diy forum...i forgot which.

 

[dubster99]

Thanks, was guessing that would be the case, felt like asking just in case.

So here's another kinda question along the probably worrying about a problem that doesn't exist. So was thinking if the rear tires are bigger I might want to run a lower psi in the rear, does the tire pressure gauge system throw any warning if the difference in the rears is a few psi less than the front?

If the PSI is low enough to trigger a low pressure warning, it will probably illuminate the light.

 

[1quick]

The tuners cannot adjust the speedo for tire size or gear changes. You need the whipple flare tool, or modify the as built data. There's a thread on this either in the drivetrain forum or the diy forum...i forgot which.

That's the first I've herd of that, what changes when you adjust the revolutions per mile for tire size in the tune when you flash the car?

 

[WestCyril]

the speedo and other parameters are controlled by the body harness in S550 not the engine harness like previous generations. You'll need the whipple flare tool to adjust tire size and gear ratios. It can also change other things too.

 

[1quick]

I was just curious because I just changed to a 28" tire so I went to adjust the revolutions per mile in the tune like I used to with my 14 gt, and it was the same deal in the sct x4 let me pick the size and flashed it in, I never did check the speedo to gps to verify it was right though

 

[TooSoonJunior]

not sure why it never gets mentioned but to change tire/gear sizes you have the following choices:

-Custom email or dyno tune (requires buying SCT or nGage)
-HP Tuners (can purchase your own version for $399, scan your tune and reflash with correct tire/gears)
-Whipple Flash Tool

Whipple may be the cheapest route if ALL you are doing is changing tires and gears, but if you are considering slicks you probably already have a tune, which would mean the first two options are the cheapest.

 

[1quick]

not sure why it never gets mentioned but to change tire/gear sizes you have the following choices:

-Custom email or dyno tune (requires buying SCT or nGage)
-HP Tuners (can purchase your own version for $399, scan your tune and reflash with correct tire/gears)
-Whipple Flash Tool

Whipple may be the cheapest route if ALL you are doing is changing tires and gears, but if you are considering slicks you probably already have a tune, which would mean the first two options are the cheapest.

So if I have a custom tune and the tuner left the options open for me to change tire size, I can see the revs per mile in the options and it let me flash that in, that does or does not work?

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