Thread: 16" Tire Size

We have 2 very tiny lots in Chattanooga. My parents NEVER left 2nd. There were only a few events where they were bouncing off the 7800rpm rev limiter. (which sucks for me since I have a RR 6500 FTL.) I do have a B13 Transmission and Chassis though. Do you think it would be better for me to run 205/45/16s or 205/50/16(if that is even a reasonable choice)? The only option I have for now are 16x7.5. If I change sizes next year I will run 15x8 instead of 7.5 since those are a pain to find.
How did you like the XS autocrossing? There are mixed reviews on the ST thread on the SCCA forums.



Yes indeed. I really want to get some 15x8s or possibly try FSP if the B15 gets moved there and go for some 15x9s once I get more power.


For the record I know a RR B13 gets put into SM but our region is running a local class called STM which is SM but you have to follow ST tire rules. for me!

Not that tiny if 2nd gear bounces the rev limiter, that's 72 mph @7500 rpm using stock gears and tires. The problem with 2nd is on the bottom end, say 20-25 mph, you're not really in the power, I've very occasionally used first in a box corner or similar for exit IF the next section really opened up, and the 1-2 shift around 40 mph was worth any potential time loss because you had room to roll 2nd up to 55 mph or more.

Otherwise 2nd has plenty of headroom, even 6500 is over 60 mph and most autocross courses you don't see much more than that.

It's officially FSP now if you fit the SP mod criteria, in the 2009 SCCA Rulebook. Technically any motor swap other than a direct highport replacement is a automatic bump to SM, or possibly more depending on what's under the hood. Even a cam upgrade like my JWT C2's sitting in a box bumps my car to SM. After gaining the down-class to FSP where the competition is a LOT more on par, I'm hesitant to take the pounding in SM running street tires and at best maybe 160-170 whp, most of that all above 5500 rpm. That's great track power, not so great autocross power.

I haven't seen a SM street tire class, at least not its own class, closest is a catch-all ST class w/PAX. Personally I like raw time and I think PAX in general really sucks. Some events have a open PAX'd street tire class based on your class, which just has the same issue, assumes you've fully taken advantage of SM allowances and you get a tiny PAX adjustment. That's the bitch with the B13; take it out of a pure STx class and it becomes increasingly less competitive, FSP however is much better classing than DSP last few years.

I only have 1 event on the XS @autocross and it was very cold, started out high 30's and didn't clear 50 deg all day on a slippery lot, the tires didn't really get warm enough to tell much. They also only had 100 miles or so on them. Tire temps were below 70 deg F on all but one run, where they pushed high 80's. Btw if you don't have a pyrometer-thermometer, Harbor Freight has a screaming good deal on a CEN-TECH unit for $39.99. It is so helpful esp w/new tires and cooler temps than you see in summer, small air pressure adjustments really show up across the tire in terms of temps.

My first runs with the XS showed stone cold outer tire even though it was showing wear contact, I had them over-inflated for the cool morning @40 psi or so. It's so hard to know that your AP is really on w/o checking the tire temps inner, middle, and outer. Gotta take every advantage you can get, and $40 is nothing compared to anything you put on the car.

I think they stick better than the MX, which was pretty good but not tops for a street tire - 220's vs. 180's. Having run them on the track in GA they were very good, and DD use they are definitely much better than the MX - good as Bridgestone or Dunlops, don't know yet. They got pretty favorable press end of last season, just didn't get to run ST at Nats because they were introduced too late.

If your only option for now is 16, get the smaller of the two in at least a 205, that's enough tire for the most part unless you have some serious power.

My dad told me that if the B15 is moved to FSP than that makes it's engine legal to be in my car for the class.

My parents run the Solo II here and they along with the solo II board decided to make this class. They even came up with a pax for it. I forget what they did but the averaged 2 classes and it was extremely close to another class. We really only use pax times for Pro class though. Everyone else gets raw+cones.

I have access to other peoples pyrometer.

I didn't know they came out last year. I only recently started looking on the ST forums and noticed there was LOTS of discussion about them.

You think I should go with 45s?

Your Dad is almost certainly right, if the motor was ever sold with a B15 here, then there's a case for it. I really also really don't get the B14 not being in FSP.

Tires - 205/45/16 works out 23.26" diameter, the 205/50/16, 24.07" diameter, both offer almost exact same section width. My B13 I would take the smaller of the two, but your gearing is almost certainly not as tall as mine.

B15 final drive, first gear, and second gear are all shorter on your car, for 6000 rpm I can see 58 mph in second, plenty of room left. For you, more like 50 mph.



These are based on c of 77.75", or a about a 24.7" diameter wheel/tire.

205/45/16 - subtract 6% off the chart speed in gears
205/50/16 - subtract 3% off the chart speed in gears

Since I worry more about where I sit in 2nd gear, i.e. don't run too low or too high w/re to best power, look at the mph just based on the difference in our gearing:

6000 6500 7000 rpm
me 57.9 65.4 72.8 mph
you 50.9 57.3 63.7 mph

Then look at you running the two different sizes:

6000 6500 7000 rpm
45's 47.9 53.9 59.8 mph
50's 49.4 55.6 61.8 mph

You could make a case for slightly stiffer sidewalls with the 45's but depending on where you hit peak power, you may run out of 2nd gear by 6500 rpm with the 45's, and you're probably winding the motor higher than your peak hp.

I'm ok from 6200 to 7400, hp is almost flat, it peaks and stays pretty constant so while I don't make more power it doesn't hurt me to wind the motor up past 7000 if I need to. This is why I want to drop a B15 trans into my B13 which thrives on 6000-7500 rpm, my effective gearing is too tall for tighter courses. But a different final drive, gears, then...SM class. I'd only do that if I had more motor and fully exploit the class allowance to give me a fighting chance.

Bottom line is I think you may run out of where you want to be on the tach with 45's, check the numbers again but I'd go with the 50's given your stated rpm redline and guessing your peak power is more like 5500 rpm. You may find yourself running out of 2nd gear on faster sections, and too high in the rev range elsewhere.

I run +1 sizing (over the stock 14" rims) on my U13 which has gearing closer to your B15, 205/55/16. I've used it as a backup car for autox and it did well with its 6500 rpm redline (6800 fuel cut). I think the 50 is a better choice but you know that advice is like....I forget

HTH! Lmk if the trans ratio image doesn't show, sometimes the new and improved Mac hosting sucks. Recent whp vs. A/F on my motor:



For reference these are Kuhmo MX's on 205/55/16's on 16x7" BBS/Moda Rims, so you can see even the 55 isn't huge. Get 50's.

Well, I'm running the B13 Trans on the RR, is that what you did the numbers with?
Also I double checked and the Khumo XS isn't available in anything smaller than 215/45/16. So I might just have to deal with running those this year unless I decide to run the not so great Azenis. I mean this will be my first season running so it's not like I'm going to be that competitive anyways.

Didn't expect that. My numbers are based on the taller gears of the B13 trans. If that is what you have, definitely go smaller.

Sorry, I didn't see anything to lead me to expect you were not running a B15 trans.

You sure you have clearance for a 215 tire that large? I don't and my fenders are partially rolled, but I do drive a very low B13, not a B15.

No way even with CO's I could fit 16" 215's. Maybe with your ride height you could, but struts typically take up more room and are more prone to tires rubbing. Even with stiff springs on autocross I'd buzz my front fenders with a 215, it would not fit within the wheel well. If you have the drop and travel, by all means 215 would be better than a 205, just check the specific tire very carefully as it will exceed an 8" footprint.

You're driving a B13? You need tires sized smaller than 45 with a 16" wheel, most likely unless the section width is 205.

Seriously, now you're finding out why 16's and 17's suck for competition with a B13, the tires cost more and your selection is very limited.

You're trying to save money sticking with a set of wheels that require tires sized like your parents ran, and that means $$$.

You can't have it both ways - get an inexpensive 15x7 wheel and tires to suit, or be prepared to pay. Not just money, but compromises. You may find they fit "ok" in the front or rear but not both, or if you're not running a good set of CO's find the height adjustment is just the drop of the springs you run. I don't see how you expect to run 15x8's on the street, either, unless you run some really high spring rates, or go with ridiculously low profile sidewalls.

IDK about you, but running street tires, they do not last a season for me, I give my Kuhmo XS's until the end of June and they will be toast - 6-8 more autocross events by then , and at least 3 track days. If I don't haul my NT01's with me, the Kuhmos will be toast by then. That's another $400, cheaper than Bridgestone but still a decent amount of money.

Get a cheaper set of wheels and tires, and go drive, the season has already started. Be conservative if funds are limited, pushing the envelope of fitment with too wide street tires is a sure way to early season frustration.

Just don't do 215's, you will be so sorry unless you've test fitted a set very close to what you want to buy, close enough you can just mount them up with confidence and go. Best thing is to get something decent, not worry about "best possible" given your constraints, and let seat time make the difference.

GOSH_shiit, I've been beaten by a really good Subie RS running all season 400-wear level hard as nails tires, it's the driver, it's the driver, ... so go drive and worry about best tires when you're knocking down some class wins.

The only way I could get 15's would be to sell the 16s and I don't think I would be able to do that.(parents)
Would I be better off with azenis for now so I have decent tires until I get more seat time? Those are available in 205/40/R16

Great info in this thread! Thanks guy I have learned a lot from your discussion.

superblackz can you go into some more detail about your ideas on the 8in/9in 6ULs? What kind of spring rates would you suggest just for an idea? Do you just suspect clearance issues with only the fenders, on say something like Progress C/Os? Would just stock ride height check these clearance issues? And at that point would the added wheel width be worth the trade off of ride height, particularly with increase power?