tires = time but how much?

Hello Hello

I've cruised around the internet and couldn't find any hard data so I thought I'd throw this question out there.

I recently purchased a 4000 lb luxury sport sedan which I wish to run in RTR (FS stock).

The previous owner did me the favor of placing completely wrong tires on the car (caught the lip on front pass side even) and I need to get new tires ASAP.

Currently on the car are 235/50-18 Mich. Latitudes and it should have 255/40z-18's (Pirelli pzeros).

My question is (and I realize the ambiguity) if I were to do a straight swap out to brand XYZ performance summer tires what kind of time difference could I expect. I'm really just looking for some veterans to ballpark what they've seen or experienced.

I'm on the fence about keeping the car. I ran an event yesterday where I was 3 seconds out of second and 5 out of first in RTR (PAX time) but 9 seconds behind first in FS. I know I can pick up some time just in driving style. I never ran a rwd car before and haven't autocrossed in over 3 years.

Depending on how I feel about buying tires will make my decision about keeping the car or starting over with something more practical. I don't need to be #1 but I'd like to be in the top 25% of my class.

Thanks!

Your crappy tires aren't THAT crappy in tire terms. If I had to make a guess, and this is only a guess based on my experiences switching from stock tires to Star Specs on my 2 which is about as different as you can be to your car, I'd say a maximum of two seconds. This is about what I've see with others making the same jump as well, though they have pretty much all been on the steep part of the learning curve when they did it.
For another data point, a friend of mine who is a very good autocrosser was able to beat me by about a second while driving his 2 on stock tires. Whatever advantage I had with good tires was more than erased by his superior driving.

I would spend time and money on the driver. That investment will transfer to every car you'll own from now on.

BTW, what does a 4000 lb. luxury sport sedan look like?

Dan

Ballpark guess, assuming the driver can get the most out of the car/tire combo, is a ~2-second improvement going from the existing tire to one of the top ST tires.

In a 235/50R18, I assume those are going to be Michelin Latitude Alpins, correct? Those are winter tires. They aren't "completely the wrong tire", but they aren't designed for summer use. I would recommend purchasing a separate set of wheels and mounting the summer tires on the new wheels and keeping the winter tires on the OE wheels assuming you live somewhere with snow. If not, I would take the winters off, purchase summer tires and then try to sell the winter tires come winter. Here is an article explaining the magnitude of the difference, they aren't the exact make/model of tire, but you get the idea: http://www.automobilemag.com/features/news/1211_rubber_matter_tires_test/

914Driver wrote: BTW, what does a 4000 lb. luxury sport sedan look like?

Either like it was designed by Chris Bangle or like a German taxi

(my first guesses were Fisker Karma and Tesla S, but those are both much heavier )

Those two tires sizes are approximately the same height, and if the rim width is correct, entirely interchangeable(as a set). Any clearance issues are likely stemming from it having the wrong offset wheels on it.

According to the rimsntires calculator the 235/50's are 1.3" larger in diameter than the 255/40's. That is why it would be hitting the fender lip.

You could use 235/50's but you would need a wheel with a different offset.

Thank you for the input.

For the inquiring minds, it is an 02 XJ Sport. 4.0 N/A.

nolson wrote: Thank you for the input. For the inquiring minds, it is an 02 XJ Sport. 4.0 N/A.

This being GRM, you know most people are going to think this.