What is the lap time difference between 17x8 and 17x9
 

I see a lot of posts stating that 17x8 is quicker, but I wasn't able to find actual lap time comparisons. Ideally advanced driver (I'm not), close to stock power levels, Enkei RPF1, EHP tire, at Laguna Seca, Thunderhill, or Sonoma Raceway would be best.

If the difference between 17x8 and 17x9 at my local tracks is minuscule, I'll just go with 17x9 in case I want to compete seriously in STX.

Offset is also a factor. Since we're also factoring in offset, suspension clearance ALSO becomes a factor.

That's why there is no clear cut answer; it's not an apples-to-apples comparison

It doesn't look like rubbing is an issue for 17x9 +35mm with 245/40R17 on stock suspension. And from Essex's post, both 17x8 +45mm and 17x9 +35mm would clear BBK.

I'm just curious what kind of magnitude of difference would there be. A couple hundred milliseconds? A couple seconds?

Test the two options yourself against the clock.

Barring that, pick one and learn how to make it fast.

Thanks all. The answer I got from this thread is that "difference between 17x8 and 17x9 at my local tracks is minuscule".

Are you competing on track in some way? If you are just doing HPDE then what does it really matter? If you are trying to be competitive in STX, then get 17x9s.

I think you need a few extra mm in the 17x9+35 RPF1. TC105N has plenty of clearance on the same size - this is for the Sprint kit.

As noted, for AutoX 17x9 is what you want, but typical builds sacrifice handling elsewhere to maximize transition speed. If your track demand the same, the wider setup may win. I could see a 225 on 17x8 being faster in a lot of cases though.

Maybe 86CUP next season? Looks like I would need 17x8 225 to compete in the stock class.

The question is more what tyre sizes are fastest. 225 are faster on some tracks and yet some tracks benefit from 245s. Locally Z221s on a 235 and 255 seem to be very much similar in times because of our track layouts which seem to be rather different to most tracks you guys in the USA get to use. 235 in a 8 vs 9 is a good question and is a bit like the 4 pot WRX brakes around. They will feel different but act much the same to stock brakes.

This definitely depends on the track. On a more technical course the added traction from 17x9 245 will probably get you through the corners a bit quicker and shave off some time. On a less technical course with longer straights, the lighter rotational mass of the 17x8 225 will help you accelerate a bit faster and shave off some time. I'd be interested to see some test results if anyone has two sets of wheels and can get times with all other parameters held constant.

In reality I doubt the difference would be too consistent/repeatable since most of us are probably not good enough to fully utilize the added traction of the 245s. Just my two cents.

I'd be surprised is it was rotational mass and not drag, both mechanical and aero. I think where you'll see the difference is at the end of long straights.

I can confirm this to a degree. At my local track on the OEM wheels and tires, my entry onto the back straight was at 55kph and my top speed at the end was 160kph. On my 17x8 +35s running 245 Rivals, my entry onto the straight was 60kph and my top speed was 155kph. In the end, my time on the back straight is basically the same regardless.

Of course, I'm comparing old Rivals to Primacy's so take that with a grain of salt. Newer designs have better rolling resistance and higher ultimate grip but just letting you know that it does exist and is measurable in extreme cases.

Now if you are talking about comparing a 225/45r17 to a 245/40r17 in the same compound, the 225 tire may have slightly less rolling resistance by virtue of being lighter and having less contact patch but it's also slightly taller which would negatively impact acceleration. In the end, I think it would be a wash on straight line speed with an edge in cornering for the 245 tire. The only good reason I could see going to a 225 tire would be cost related.

Going slightly off tangent, my 255 40 17 RS3 have the same top speed as stock tires at a local track. Keep in mind these straights are quite short (video in track videos thread).

Forgive me if I misunderstood you're OP, are you asking about the difference of a 245 on either an 8" or a 9", but also changing the tire width as well? Others brought in the size difference, so I wanted to make sure I'm clear on what YOU are asking.

In the Miata world, we run very wide wheels compared to tire size. The speed difference is marginal, but the feel is better. For EHP and Comps, 205 goes on an 8", 225 on a 9", 245 on a 10".

Testing has showed the 225 won't be much slower, if at all on an 8 vs a 9", but the feel consistency is better so it's typically a touch easier to get "all of it" all the time.