225/45/17 vs 245/40/17 Same Compound and Wheel, which is faster

[Pat]

Is it really more sidewall? It has to be really close. Insignificant.

[DarkSunrise]

Originally Posted by Pat

Is it really more sidewall? It has to be really close. Insignificant.

A bit more, little over 8%.

[Pat]

Originally Posted by DarkSunrise

A bit more, little over 8%.

Whoops, I was thinking 245, not 235.

[RT-BRZ]

Originally Posted by Pat

Is it really more sidewall? It has to be really close. Insignificant.

When talking about these kinds of things we do have to consider that a change in overall tire diameter does have an impact on the final drive ratio of the car.

The stock differential on our cars is 4.10:1 (4.30:1 on 2017+) and we'll use that as a base.

The stock tire is a 215/45-17 which is ultimately how that final drive ratio is calculated. For all intents and purposes, we'll start there.

If you use a 245/40-17 you move to a FD ratio of 4.08:1.

If you use a 235/45-17 you move to a FD ratio of 3.99:1

If you use a 255/40-17 you move to a FD ratio of 4.03:1

If you use a 225/45-17 you move to a FD ratio of 4.04:1

These differences do seem somewhat minor numerically but they do have a significant impact on both acceleration and top speed achievable per gear.

More importantly, while understanding these numbers is very important, it doesn't mean that a lower FD ratio is necessarily bad. It all really depends on so many other factors.

In the end, threads like this are largely pointless (just like the whole corner speed thread) because there are so many factors at play that you would have to have the same driver, same car, same track, same conditions and only change a single variable (like tire compound and/or size) to get any real data and that all assumes that the ultimate variable (the driver) is so consistent that none of the other data is affected by the variability of the driver.

This is all fun conversation and a great way to pass the time but all these conversations can really do are offer general pointers on how to get faster given the car you have. The ultimate mod here is the driver mod.

[jflogerzi]

I don't think it's useless. There is some merit to these discussions.

Sent from my Pixel 6 using Tapatalk

[Desertnate]

On a totally different train of thought, isn't 225 too narrow for a 9" wheel?

I would have thought 235 or 245 would be the starting point for a wheel of that width since 225 seems to be a better fit for a 7.5~8" wheel. I'd think 225 would be stretched and not fit very well.

[RedReplicant]

Originally Posted by Desertnate

On a totally different train of thought, isn't 225 too narrow for a 9" wheel?

I would have thought 235 or 245 would be the starting point for a wheel of that width since 225 seems to be a better fit for a 7.5~8" wheel. I'd think 225 would be stretched and not fit very well.

https://www.miataturbo.net/wheels-ti...e2/#post676544

Again, tires work best when the section width matches the wheel width.

[Tatsu333]

Originally Posted by RT-BRZ

When talking about these kinds of things we do have to consider that a change in overall tire diameter does have an impact on the final drive ratio of the car.

I think it would be more accurate to say it "has a similar effect to changing the final drive ratio".

A 4.1 final drive stays a 4.1 final drive regardless of what wheels and tires you have on the car because it still turns the same number of times vs. the input from the gearset / driveshaft, but yes, all other things being equal, going with a different overall diameter can affect the effective torque to the ground in similar fashion to changing the final drive ratio.

It has to be a pretty substantial change in size to match the effect though. If I'm doing the math right, using the rough outside diameter of 25" for a 225/45R17 combo someone else posted, that would mean running about a 235/35R17 (not sure such a thing even exists) to effectively match the change in FD ratio from a 4.1 to a 4.3, so the sidewalls would be about 20mm shorter.

Conversely, to effectively match the change the other way (4.3 to 4.1), you'd need to go to a 245/50R17...

[Desertnate]

Originally Posted by RedReplicant

https://www.miataturbo.net/wheels-ti...e2/#post676544

Again, tires work best when the casing width matches the wheel width.

Thanks.

That's why I asked. On other cars I've run 225 tires on 8" wheels and the tire casing appeared to be perfectly matched as you state. That's what led me to think 225 on a 9" wheel wouldn't be a good fit and it would lead to the casing being stretched a bit like the OEM 215's on my OEM 7.5" wheels.

[RT-BRZ]

Originally Posted by Tatsu333

I think it would be more accurate to say it "has a similar effect to changing the final drive ratio".

A 4.1 final drive stays a 4.1 final drive regardless of what wheels and tires you have on the car because it still turns the same number of times vs. the input from the gearset / driveshaft, but yes, all other things being equal, going with a different overall diameter can affect the effective torque to the ground in similar fashion to changing the final drive ratio.

It has to be a pretty substantial change in size to match the effect though. If I'm doing the math right, using the rough outside diameter of 25" for a 225/45R17 combo someone else posted, that would mean running about a 235/35R17 (not sure such a thing even exists) to effectively match the change in FD ratio from a 4.1 to a 4.3, so the sidewalls would be about 20mm shorter.

Conversely, to effectively match the change the other way (4.3 to 4.1), you'd need to go to a 245/50R17...

Sorry, I should have said "effective final drive ratio". You are correct that the ring and pinion doesn't physically change.

As for the minor differences, I would say they definitely matter. In Solo I ran 2021 and half of 2022 with 255/40-17's on my 2013 BRZ. I also did track days on those tires. Before that I ran 245/40-17's and also in the last half of 2022. What I personally observed is that the car was less "quick" in Solo and I lost about 3 mph in the back straight (GPS adjusted) by running the larger diameter tire. No other changes were made to the car that would have produced those results.

I know it's minor and so are the impacts. That doesn't mean that it doesn't really matter. YMMV

[CSG Mike]

Originally Posted by RT-BRZ

When talking about these kinds of things we do have to consider that a change in overall tire diameter does have an impact on the final drive ratio of the car.

The stock differential on our cars is 4.10:1 (4.30:1 on 2017+) and we'll use that as a base.

The stock tire is a 215/45-17 which is ultimately how that final drive ratio is calculated. For all intents and purposes, we'll start there.

If you use a 245/40-17 you move to a FD ratio of 4.08:1.

If you use a 235/45-17 you move to a FD ratio of 3.99:1

If you use a 255/40-17 you move to a FD ratio of 4.03:1

If you use a 225/45-17 you move to a FD ratio of 4.04:1

These differences do seem somewhat minor numerically but they do have a significant impact on both acceleration and top speed achievable per gear.

More importantly, while understanding these numbers is very important, it doesn't mean that a lower FD ratio is necessarily bad. It all really depends on so many other factors.

In the end, threads like this are largely pointless (just like the whole corner speed thread) because there are so many factors at play that you would have to have the same driver, same car, same track, same conditions and only change a single variable (like tire compound and/or size) to get any real data and that all assumes that the ultimate variable (the driver) is so consistent that none of the other data is affected by the variability of the driver.

This is all fun conversation and a great way to pass the time but all these conversations can really do are offer general pointers on how to get faster given the car you have. The ultimate mod here is the driver mod.

I've exclusively used 215 and 225 on my 2022 and 2023 BRZs

[timurrrr]

Originally Posted by Desertnate

On a totally different train of thought, isn't 225 too narrow for a 9" wheel?

I would have thought 235 or 245 would be the starting point for a wheel of that width since 225 seems to be a better fit for a 7.5~8" wheel. I'd think 225 would be stretched and not fit very well.

Depends on the exact tire.

225/45R17 GT Radial SX2 don't look stretched much on a 17x9 wheel:



I think I've seen 245/40 Michelins (PS4S?) that looked more stretched on 17x9 wheels.

[RedReplicant]

A 225 Hoosier looks like that on a 10, lol

[Capt Spaulding]

Originally Posted by RedReplicant

https://www.miataturbo.net/wheels-ti...e2/#post676544

Again, tires work best when the section width matches the wheel width.

Originally Posted by timurrrr

Depends on the exact tire.

225/45R17 GT Radial SX2 don't look stretched much on a 17x9 wheel:



I think I've seen 245/40 Michelins (PS4S?) that looked more stretched on 17x9 wheels.

Actually, it does to me. If the wheel is laid flat it looks like the combination will be resting on the outside of the rim even though the sidewall seems to bulge a bit. And, to my eye, the tread of the tire is even narrower.