245/40 vs 245/45 in STX
 

Is there any benefit of running one size vs the other. Running them on 9” wide wheels with 40 offset. Homing to get to -2.5 rear and at least -3. Front camber. Better selection with the 245/40. Still deciding on the tire for next season. Most likely Re-71 or Rivals. Thank you

The shorter sidewall gets you better torque. I’ve been trying to use up some 245/45 sized tires at the track: They came with car when I bought it; figured why not...

A basically stock BRZ walked away from me on a 1/2 mile long straight at the track. There’s almost no chance I was down on power compared to his car. Obvious difference was tire sizes- My 245/45 vs his 245/40.

The track isn’t the same as an autocross course, but the impact of that taller tire will still cost you power to the ground. I think you’ll be happier with the shorter sidewall unless you can save a bunch of money on the “wrong” size.

The overall tire diameter difference between the 245/40-17 and the 245/45-17 is 1". (24.7" vs. 25.7")

The stock rear differential is 4.10:1 when you use the stock tire size. When you put the "taller" tire on the car it changes the final drive rear differential to 3.95:1.

This change in gear ratio has a negative affect on acceleration and the power curve (effectively). The only positive effect it has is that the highest speed is increased but it will take longer to get there if nothing else changes.

In order to get the car to behave the same way with the taller tire the rear differential gear ratio would need to be changed to 4:26:1 gears (I don't believe this exists for our cars).

For STX we are allowed to change our rear gear ratio and LSD to anything as long as it's mechanical (not electronically controlled).

It looks like the next available gear is 4.44:1 and then a 4.67:1 based on some quick searching.

With stock tires the 4.44 will drop about 7mph off of 2nd gear @7000rpm (60mph vs. 53mph) and about 10mph off 2nd gear @7000rpm (60mph vs. 50mph).

The higher final drive ratio will allow for quicker acceleration but it does lower your top speed in 2nd gear which may force you to shift to 3rd if you run stock tire height. Running the bigger tires may be the way to address that. I don't think the juice is worth the squeeze when you consider how much you have to spend and what it will do to the overall performance in the given scenario of STX but I don't have the experience in this regard that national competitors have.

Regarding camber, I don't know that tire diameter will impact what you can get versus what the other components will do for you. STX rules allow you change most any of the suspension parts as long as you don't put in solid metal bushings but I would always consult the rule book very closely when swapping these parts.

I was running RE71's but I'm moving to Falken Azenis RT660's this season because they are cheaper by a good bit and the RE71s are going to be harder to get from what I understand. The RT660's are well reviewed on these cars. Rivals don't seem as popular based on everything I've read or seen.

wut
no you're not

edit; for OP, others covered it, taller tire, slower acceleration, higher speed in each gear before rev limiter kicks in. on a '13 raising the rev limiter to hit 60mph in 2nd is common for stx, and if you actually look back at the courses there are very few spots where you're riding a higher speed limit in 2nd long enough to outweigh the acceleration you'll use between every single element on the course so the 40 is preferred. A 45 might make sense with the later cars with the shorter rear end to scrabble back some of that top end, but I think I'd go test it and find out before recommending it.

I admit that I might be misreading the rules but, here's what it says:

However, now that I re-read the rules, I realize that I was off base on that. You may change the LSD with a unit that is like the OEM version which is mechanical, but since it doesn't specifically say that you can change gears unless it's STP.

My bad. I really wish they would make these rules easier to read/understand and not combine multiple classes into a single category unless they are so much alike that there are no significant differences.

STP is dead, as is STF, no gear changes in ST.

I suggest you get a newer copy of the rules, yours looks to be about 4 years out of date.
AWD diff rules got updated to match in the latest iirc

You're right. That was from the 2017 rules. Here is the excerpt from 2020:

Looks like they just removed STP and STF but your point is still spot on. Thanks for calling it out.

STH exists, and you should really use the 2021 rules since it changed the allowance for awd.

Looks like I will go with the 40. When I run NEPA region, we regularly hit 3rd at Pocono and only occasionally on one or two other courses. If it helps down low than that would be the better benefit. pocono has one tight hairpin where I’m lower in the rev band computer. Thank you everyone. I’ll be keeping my eyes open for reviews on new tires.

I wouldn't ever consider the 245/45. To me, your options are 245/40 and 255/40.
I run 255/40 with a MY17 and the factory 4.3 diff gear.

Beyond my nonsense about the rule book, I generally agree with this assessment. The 255 wide tire causes a slight increase in diameter which would make 2nd gear just a slight bit "taller" which would give you a little more speed and I can certainly see doing that.

oh hell yeah another brz in stx. all the other ones left the class but james who dumpsters me lol. also to add to this thread i run 255/40/17 but i also have a 17

I was kind of thinking the 255/40 was the preferred size for the update cars, given the shorter gearing. Any fitment issues to watch for?

i have csg tein coilovers and run 6ul 17x9 +40 and with RE71r i need a 5mm space to not touch at full lock

I'm running RCE T2's with 17x9 +40 wheels. 255/40 A052s
Currently -3.8 camber and 7.1 caster
I just don't turn full lock to avoid the inner rubbing from it.

How is your knee doing. Glad your staying STX. Can’t wait for the season to start.

i never got it fixed after spending 1400$ seeing what was wrong because it stopped hurting and fixed itself. i just looked up PT exercises on youtube lol