Lower gearing? The tire circumferance should be about the same. Just that you will have a higher profile than with the 18s. So you are planning to run 45s on a 17? Just run with 225/40-18s and you will get the same "gear reduction." IMHO this is a really bad idea. Where do you race your car?

 

If you have an equal profile and equal width, the 17s will have a smaller diameter than the 18s. As you note, changing the profile from a 45 to a 40 will accomplish about the same thing, although the 17x..45 will still have less rotational inertia than the 18x..40. Going to a 40 reduces your slip angle a little, which will make the car a little "twitchier" but will also bleed a little less energy through the turns. For auto-x, I don't think the difference between a 40 and a 45 is too important. For high speed events, the reduced profile could make the car a bit trickier to handle.

But going from a 225mm width to a 275mm width will significantly change your tire circumference. If you go from a 225mm to a 275mm but assume you stay with a 18in and a 45 profile, your sidewall is going to increase 0.45*50mm, or 22.5 mm which is nearly an inch, so your overall radius will also increase that inch thus increasing your gearing. Going to a 17 will help offset this increase somewhat.

Simple examples of the radius from the axle to the tire patch:
Radius = 1/2 wheel diameter + (tire width * profile)
so
stock: 18in/2 + 225mm*.45 = 9 in + 101.25mm ~= 229mm + 101 = 330mm
18x275R45: 229mm + 275*.45 ~= 352mm
17x275R45: 8.5in + 275*.45 ~= 216mm + 124mm = 340mm
17x275R40: 216mm + 275*.40 = 216mm + 110 = 327mm

The circumference will be 2*pi*Radius, but that isn't important. If the radius goes up %2, then the circumference will go up the same %2.

All 3 of the 275mm examples have a taller sidewall than stock simply because the tire width goes up ~2in., so the sidewall goes along with that unless you reduce the profile.

If that is all obvious and boring, I apologize. I just find it easier to think of this with some actual numbers.

I'm not planning on racing my 8 this year, but I'll be racing a kart out on the left coast, near Portland, OR. Chassis setup on a kart is a bit different, but racing is racing. On the kart, everything just goes out of control a bit more rapidly. Not that the 8 wouldn't be a lot of fun to race, but if the guy next to me wants to bump the side of my kart, new side pods cost about $40.

 

I've been called thick, but sometimes I might not be thick skinned enough.

 

frankly you sound like a guy who read a bunch of books, but otherwise lacks practical experience in the topic he's trying to discuss.

for starters, setting up a kart is a lot different than a production car, a lot ...

and in the real world, the general theory of tire inertia is not so important as compared to the actual magnitude of comparable differences relative to all the other factors that come into play with regard to tire performance on a 3000# car, many of which outweigh what you're going on about ...

further, maybe you'll find it easier to think about practical tire sizes like 275/35-18, 285/30-18, etc. and other annoying real world practicalities such as available wheel sizes and their weight/cost. The manufacturers actually provide the data for you; no calculator is required

perhaps the most practical of all, consider asking questions to start rather than espousing ... i.e. ask if 16" wheels fit before going on about it pointlessly when in fact they don't. You're more likely to get the kid-gloves treatment that way ...

 

To bad there are no 275-45-18 track tires, nice try, now if you use real race tire sizes that you can buy it looks like this:

Tire Size Sidewall Diameter Circumference Rev/Mile Difference
225/45-18 4.0 in. 25.97 in. 81.59 in. 777 0.0% stock size
245/35-18 3.4 in. 24.75 in. 77.76 in. 815 -4.7%
265/35-18 3.7 in. 25.30 in. 79.49 in. 797 -2.6%
285/30-18 3.4 in. 24.73 in. 77.70 in. 815 -4.8%
245/40-17 3.9 in. 24.72 in. 77.65 in. 816 -4.8%
245/45-17 4.3 in. 25.68 in. 80.68 in. 785 -1.1%
275/40-17 4.3 in. 25.66 in. 80.62 in. 786 -1.2%

 

Give screamindean a break, he asked about the 16s and got an answer that they didnt' fit. Sounds to me like everything else he wrote thereafter is not untrue; if it's of no interest to you personally, leave it be. I personally find this topic interesting, and would hope that additional thoughtful discussion/input would not be discouraged.

 

^^^yeah, but everything he asked has been covered in depth many times on here. He could of searched to begin with.

 

It's only discouraging for those who choose to be overly sensitive to the style of delivery. Keep in mind, we've got a bunch of smart guys here with real-world knowledge trying to help steer. People should take advantage of that, along with the wealth of knowledge already loaded in these forums.

A problem with espousing and speculating and running calculations to come up with theoretical numbers is that other new people will read those & think its true. As mentioned before numbers aren't the gospel if the combination doesn't work with everything else.
-dk

 

Two of the choices you mentioned were:
245/40-17 3.9 in. 24.72 in. 77.65 in. 816 -4.8%
245/45-17 4.3 in. 25.68 in. 80.68 in. 785 -1.1%

Hypothetically, would it be safe to say that the 245/40 is a "better" choice out of the two above for autocross because it has slightly less intertial mass (for quicker acceleration) as well as better handeling (shorter sidewalls means less tire roll)?

 

I've enjoyed reading over the discussion and am not too worried about some of what has been entered that could be interpreted as, well, not overly kind. Those notes that point out the deficiencies in my posts are useful. Those that ignore what I have to say because I am a noob, or because the poster somehow divined my racing experience based on a few posts, well, those posts are useful too, at least in understanding the poster.

And to be clear, no I don't know diddly about the specifics of RX8 chassis setup. I'm new to the car and learning along with many of the rest. And absolutely, chassis setup for a fully independent suspension on a 3000lb car is very different from a solid rear axle, 360 lb (including driver) kart.

But I do have a fair bit of experience with tires, including car tires and the basic effects of width and profile are the same everywhere. Gear ratios are also very straightforward. The beauty of changing tire sizes is that they don't significantly change chassis geometry (except for a trivial bit of ride height). So you don't necessarily need a detailed knowledge of a specific car's suspension to discuss tires and wheels.

OK, enough of that.

I will give credence to what somebody says based on their demonstrated experience. When somebody can say "05' SCCA BS Natl Champ", I gotta pay attention to that. Interestingly , ULLLOSE did point out there are 275-40/17 and 285-30/18 (wow!) racing tires. Also ULLLOSE said, "Right.. A 17X8 would be a step back. Now a 17X9-10 with a 275 would be nice" With that Enkei 17x9.5 wheel and the 275-40/17 wheel you could do that and keep yourself slightly below stock gearing.

And before I get jumped on, no I don't know that it would necessarily be faster, but clearly it can be done on the RX8

"Are you pissing in someone's pool again?'

"Yeah. And we're fresh out of chlorine"

 

One small followup. My previous post is not intended to imply that the only smart guy on this board is ULLLOSE. It's just that I don't know much about anybody on this board yet. I don't know who knows their stuff and who just has a lot of opinions. I do know that a national champ knows his stuff.