JCathers wrote:

int2str wrote:

Why?

From the rules:

**Code:**

Tire Size: Size 245 or smaller (DOT-approved) = +0.7

Size 275 to 250 (DOT- approved) = +0.3

This makes sense. That doesn't look the wrong way around at all?

So you're at 11.58:1 right now - if you run 275's you add +0.3 so you now have to be at 11.88:1 weight/power (ignoring the weight points for simplicity).. but if you run a narrower tire you have to add 0.7, bringing your weight/power ratio to 12.28:1 right? How does that make sense? Being penalized and forced to run a higher weight/power ratio for running a narrower tire. Unless I'm missing something glaring, I feel like I'm living on Bizarro.

Also, you don't start with your cars power to weight ratio before adding modification factors - you start with the base class ratio of 12:1 then add/subtract the modification factors - once you figure out that ratio, you have to get your car at or above it. It's been discussed and confirmed that everyone is starting at 11.7:1 in TT4 because we're all getting the 0.3 break for running 275 or smaller, since tires larger than 275 are banned.

They used to have a TT1-3 class calculator in the form of an excel spreadsheet posted on the NASA site but I can't find it now :/

I think you're misunderstanding how to do factors.

To run in TT4, you're final adjusted wt/hp has to be above 12:1.

So let's just take tires as only modification that your car has for simplicity.

First you start with your unadjusted wt/hp. Average hp divided by actual wt. Before all adjustments/modifiers.

If your unadjusted wt/hp is 11.31, then you would have to run 245's DOT or skinnier (+0.7) to have a final adjusted wt/hp of 12:01.

If your unadjusted wt/hp is 11.71, then you would have to run 275's DOT (+0.3) to have a final adjusted wt/hp of 12:01.

So, basically running narrower tires allow you to run a lower unadjusted wt/hp which basically means either more starting power or less starting weight.

I'm in TT3. My average hp is 338. My weight is 3145. This gives me an unadjusted wt/hp of 9.305. I run 245 Hoosiers (+0.7) and non-OEM aero (+0.0) which puts me at 10.005 final adjusted wt/hp. If I tried to run 275's, I would be at 9.605 and would have to either add weight, cut power, or move to TT2.

Hopefully this explanation helps.

Denis

That makes sense, I guess I've been looking at it from the flip side since I'm well above 12:1 in B trim so I've been working toward getting it to the actual 11.8:1 I'll be at with the "+0.3" I'll get for <275's and the "-0.1" for being <3000lbs - I misunderstood the OP and thought he was under the impression that he had ballast/detune up to 12.08:1