THSCC Forums

I've narrowed it down to running 225-45-17 or 245-40-17 V710's on the Si Sedan this season in GS and was a little concerned about upsetting the gearing since the Si tops out at around 56 which is going to require a shift to third on some of the more open courses. I'd imagine that running a staggered setup would be ideal but I want to be able to rotate then throughout the season.

So, my real question is, would it be safe to say that the 245's have the same circumference (despite the lower aspect ratio) compared to a stock size of 215-45-17?

Is there a good web page that has some sort of a configurator for something like this based on the average size for each spec; I know different manufacturers can have a little wiggle room.

Also, I know it's a stab in the dark, but if anyone has a set of 17x7 (5x114 & 45 offset) that are less than the stock 22 lb'ers, I'd like to talk.

thanks,
Matt

http://www.miata.net/garage/tirecalc.html

245/40-17 and 225/45-17 are about the same - that's the default stagger on the M3. 215/45 will be a tad shorter.

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Dustin Fredrickson
-- I'm a nobody --

^ Awesome!! Exactly the toy I was looking for. Thanks Dustin.

That confirms what I though about them being really close (1/3" in circumference).

At Tirerack.com open up the specific tire and on click "spec". That will give you an idea of what the overall diameter of each tire is and in some cases the rpm per mile. I saw 24.7 for 245/45/17 and 25 for 225/45/17. I didn't see any 710s at 215/45/17, just 24.1 for a 215/40/17. In any case have fun.
Don

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Don, Yeller Cobra

Hey Matt - as far as I can remember, a 245-40-17 Hoosier is nearly the exact same diameter as a 215-45-17 street tire. There was a difference, but it was a small one. If you'd like, we can do a test fit using one of my CCWs. They're 17x7, 5x114, +49 offset, with 245-40-17 Hoosier A6s. Not a perfect match for what you're after, but somewhat close. I've never run Kumhos, but I know Keith is at least considering them. He may be able to answer the circumference question directly.

The S2000s Keith and I drive top out at a little under 60mph in 2nd gear. The speedo on mine can't make up its mind between 58 and 59 on the limiter in 2nd. Both of us tend to ride the limiter rather than upshift to 3rd, unless it's more than 2 or 3 seconds worth of limiter. Given that an S2000 has a bit more straight line squirt than a Civic Si, you might not need to upshift to 3rd as much as you think. Tough to say for sure until actually on course.

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Karl S.
2014 Baby, 2014 House, 2013 Ford Focus ST, 2013 BMW 328i, 1994 Mercedes E320
(Insert passive aggressive signature line here)

While the diameter is important, actually mapping the diameter and applying it to the gear ratios is most important. For any car that I am looking to put race rubber on (or considering buying), I pull the potential tires diameters and plug in the numbers.

When Heartland Park was being used, for a DS/GS car, you were likely fine to rev out at around 57mph (by gear ratio, not speedo with error). While I didn't go to Lincoln for Nationals (just the May ProSolo), I would say based upon the layout that for the same car I would be looking for more speed, say 60-62mph.

It's all balance between tire width, gearing and ability to fit them. - AB

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'14 Toyota Sequoia Platinum 4WD
Super Westerfield Bros - '93 Integra - LeChump Du Jour
STX 93 - Scion FR-S

FWIW, Spratte and I only hit the limiter in once place on one course Lincoln in the ITR (4-5 pops maybe?). And we had a turbo-boosted air box. From the sounds of it, it doesn't look like we'll be seeing speeds much higher than that at Nationals any time soon. However, I don't think Matt is too concerned with a site 1200 miles away.

Locally, there have been very few courses (always at Sanford) that we've needed 3rd in the ITR. As I'm sure you know as a former owner, the ITR hits the limiter in 2nd right at 60mph so I think you will be fine in the Civic Si at a slightly lower speed. I'd pick whichever tire puts the most rubber on the ground and call it a day. Even if you could extend the Si to 60mph, you'd probably need 3rd anyway on the courses I was referring to.

Jim

What does that tell me? When I look at the results, you finished near or at the bottom

I do agree that getting the widest tire on a car is the best for about 95% of the situations, except where it would make your gearing awful. - AB

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'14 Toyota Sequoia Platinum 4WD
Super Westerfield Bros - '93 Integra - LeChump Du Jour
STX 93 - Scion FR-S

It tells me you're too scared to buy another DS car even though you would have a decided advantage at Pros. Considering our heads-up track record, I can't say I blame ya...



That's a good point. Assuming the tires in question are about the same diameter, go wide. With the Kumhos, your tire guy will thank you every time you see him!

Jim

Thanks everyone for the advice; you all pretty much confirmed what I was hoping- it's not THAT big of a deal.

Actually, I probably don't have a choice because Tire Rack is out of the 245's and it could be 30 days or 3 months.

I appreciate the feedback on the limiter speed Jim. I couldn't remember what it was in the R and I feel that there's no doubt that my 4-door baby hauler won't exactly be going quite as fast so the whole up to 3rd gear shift vs. pegging the limited a few times will most likely be moot. Now I have a nifty red blinking light that I'd venture to say that most Si owners never see.

Now... off to research how quickly I can expect to see excessive outer shoulder wear on the new 'squared' tires on my camber challenged McPherson strut front end. Hope it's not too harsh!

I like this calculator better since you can easily compare different setups side by side.
http://www.wheelsmaster.com/rt_specs.jsp

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2003 Subaru WRX Sportwagon = daily driver/boost and booster seats
2000 BMW 540iT = wife's kid hauler
2012 Toyota Highlander = the boring reliable vehicle

BTW, tire diameter is a pretty simple calculation. I've done it enough times that I have it memorized and just grab a calculator if it's easier than getting to it on the internet.

Tire Diameter = (2 times tire sidewall height) + Wheel Diameter

or

Tire Diameter (inches) = 2 * (AR/100) * W / 25.4 + WheelDiameter, where:

AR = tire aspect ratio
W = tire width (mm)

For example, the diameter of a 245/40/17 tire would be:

2 * 0.40 * 245 / 25.4 + 17 = 24.717 inches

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Keith Q.
2008 Top Gun
http://www.youtube.com/watch?v=KmX8XuZ_DCo&NR=1

What about putting your current TPMS sensors in a sealed PVC tube in the trunk?

I actually tried this w/ the mustang and it worked for about a week until it failed. For what ever reason there was a disconnect between the turning wheels and the TPMS being static (not rotating). I had a dash light come on stating a service failure. Once I had the TPMS monitors reinstalled, no issues.

I'm assuming that the TPMS rotating on the wheels keeps them charged to maintain the signal send.

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Rodney

'08 Bullitt mustang, CAM 7
Autox VP '09-'10, President '11-'12, interim President 2nd half of ‘14
proud recipient of the Bowie Grey service award '12
Now just a guy driving a mustang....

True, this is only going to work for passive sensors. I'm not too up on TPMS technology, but I'd assume some are active and some are passive.

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2011/2012 Autox VP
2013/2014.5 President
2013 Top Gun

2015 Fit

22R-EC => 4G63 => D16Y7 + D16Y8 => EJ255 + K24Z2 => K20Z3 + K24Z2 => K24Z2 + M54 => L15B