Lap time vs tire size
 

The general consensus on the forum seems to be that a 225 tire tends to give the best laptimes as a rule of thumb for relatively stock cars. Most people seem to think its because any larger is too heavy but i find that hard to beleive.

Looking on tire rack a 225 17x8 setup is only about 2-4 lb lighter per corner than a 245 17x9 of the same tire and rim. I highly doubt 8-16 lb of rotating inertia makes a measurable difference especially considering your adding ~10% more tread width.

My only other thought is that maybe we cant get the wider tires up to proper temps but that too i find unlikely.

Anybody have experience or data they could share from comparing different tire sizes?

Are you willing to accept a mathematical answer for the acceleration performance difference? If so I can run some numbers tonight if you can give me weight and sidewall specs for both setups.

Voshlag just tossed on 18x11 w/ 315/30/18 square set-up on their stock FRS to do some testing before they LS swap it. Will try and update this with their findings.

The problem with that is that Terry didn't run any laps with decent tires before he did that (all his testing was with some cheap Firestones).

I run 255/40-17 tires (mostly for autox), I tried some used 225's at the track and found them slower than the same tire and compound in either 245 or 255.

This will also depend both on the track and the driver. If you aren't using all your grip all the time, the bigger tires are just extra weight (unsprung and rotating mass is the very worse type of weight to have). As a general rule of thumb, wheel weight is worth about 5x static weight (1lb of wheel weight is like 5lbs of weight anywhere else).

I ran the numbers myself and for 3 lb heavier a corner it equated to a 0.03 m/s acceleration difference at 100 km/h or roughly 1.3 whp equivalent loss. I can post the spreadsheet if anybody is interested in it.

One thing to consider is also how much of the tire you can effectively use and how the tire "feels". I've been in the camp of wider = better for a while, mainly because I run a lot of tight tracks, but this year I've had trouble getting enough heat in 255 tires and found 235 a tad easier to drive as well. On the same wheel (9"), driving the 235 tires was very easy, very fun, I could direct the car wherever I wanted, very sharp response. The 255 felt like a boat sometimes, its hard to describe. Over a single lap I found the 235s gave their best lap around 4-6mins into the session, while the 255 gave the best lap times at the very last lap usually.

Im not convinced that wider = better in the tire department, but I can totally see a wider wheel making a tire give a better feel. So maybe wider = better only the wheels are wider as well?

I live in Texas, so getting tires hot isn't an issue for me (my last two track days saw highs of 101F and 111F or 38C and 44C).

I run an STX prepped BRZ with 9" wheels.

It's not just weight, wider tires also add more rolling (and air) resistance.


A lower speed track with a lot of corners will probably be better (just like autox) with a 245 or 255, a higher speed track with more long straights will likely be better with a 225.

Also, tires stick to the track so wider tires require more power to unstick from the track. Weight + aero drag + adhesion all add up...

I hadnt considered the aero drag or rolling resistance. I suppose if you add up all these effects you might get something measurable.

Anyone have datalogs from the track with different tires they would be willing to share? It would be very interesting to see how much the lateral and longitudinal g's change

Like it's been said, wider tire have more rolling resistance, especially "sticky" tire.
Also an increase in weight on a tire is worse than on a wheel, because the weight is farther from the center on rotation, so higher inertia.

I didn't have my AIM data logger when I took these videos; however, the first two were within 2 months of each other, the first is with 245/40-17 RE71r's (1:26.6) the second is with 225/45-17 RE71r's (1:27.6) and the third is my personal best at that track on 255/40-17 RE71r's (1:24.7).

The g's look pretty similar. All three videos are with different brake pads, but other than pads and tires there weren't any changes to the car.

245's

[ame]https://www.youtube.com/watch?v=NFUUkOPVxKU&t=729s[/ame]

fastest lap starts at 5:53

225's

[ame]https://www.youtube.com/watch?v=2fDUhX4R9fM&t=864s[/ame]

fastest lap starts at 13:31

255's

[ame]https://www.youtube.com/watch?v=ZMznz70P2I8[/ame]

fastest lap starts at 3:53

um, so other than the 2 most important things on the car when it comes to tracking nothing changed.

Or was the tire and brake compounds the same?

Tire compound was the same (Bridgestone RE71r) but in different sizes (that was OP's question). All three tires were mounted on 17x9 wheels, I've got two sets but I think all three videos were using the same set (I use one set for the track and one for autox, it's rare for me to use my autox wheels at the track).

The brake compounds were different, in the first video I was using CL RC6 in the second video I was using some Hawk DTC 70's and in the third video I was on some Ferodo DS1.11's

This isn't the best comparison, but no one offered a better one. I've got the AP Sprint kit in the front but no ducts. The RC6 and DS1.11 are very similar pads but the DTC 70's were complete overkill (coefficient of friction was too high). I used either Ferodo DS2500's or Hawk Street/Race pads in the rear (I'm not sure, but I think all 3 videos were with the Hawk pads).

I don't think pads are a huge factor in lap times provided that they don't overheat and that they don't upset the balance of the car.

I should have try swapping wheels and tires with one of my DS friends next time at the track. It'd be easy enough to do it between sessions but I'd have to make sure to run the same compound (I don't run RE71r's at the track anymore after I ruined a set in a single day, but I've always got some for autox and I'd be willing to use them for a session for science).

I've convinced myself that at my local tracks, the wider tires are faster for me. I don't think that there is a huge difference between 225's and 255's of the same compound though, so if I was looking at a big enough price difference between them I'd run the cheaper size (or if there were classing points/issues to consider).

Your results and opinions may vary, I'm just sharing my experiences.