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I always pick the tire I want to run and then pick the widest legal wheel I can afford. The tire choice is usually easy, for I want the best gearing possible for my car, in combonation with the widest contact patch. Thus, the tire with the most revolutions per mile is what I'm going with, for I need to keep the rpms as high as possible for my car. Then I'll choose the wheel.

That's why I went with a 15x7 with a 205/50/15. I would love a 15x7.5 with the same tire, but that wasn't possible for under than $300 per wheel in my car.

There's another guy out west running a GT-S in STS. He's running a 215/45/16 on a 16x7.5 SSR Competition. Very light wheel, but the tire is not only a little taller (23.61" diameter, compared to my 23.07" diameter) and the contact patch is about 3mm wider.

To me, that's not worth the a) extra weight b) higher effective gearing c) extra cost of tires/wheels. Plus my 15x7 is a very easy to sell wheel on top of it all, especially with all the road racers out there - AB

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

There is another potential factor that I don't think has been mentioned so far. In general the larger diameter wheel with the same diameter tire and the resulting shorter sidewall will be more sensitive to camber changes. The effective tread width can be reduced if there is too much camber change with body roll. A taller sidewall can be more "forgiving".

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Dick Rasmussen

FS 50 2018 Mustang GT

Aaron,

I understand what you are trying to do with regards to keeping the car in the power band, but how do you factor in ultimate top speed in 2nd gear? Do you shoot for a specific speed (i.e. 60) or do you feel it is OK for you to allow the gearing to cause you to top out at less than 60 at times in 2nd gear (all of this being said with no idea what the gearing is for your car)

Oh, and thanks to everyone for their comments. If I ever put together that spreadsheet on rotational acceleration, I will post a copy.

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Richard Casto
1972 Porsche 914
2013 Honda Fit Sport
2015 Honda Fit EX
http://motorsport.zyyz.com
Money can't buy happiness, but somehow it's more comfortable to cry in a Porsche than a Kia.

If you're going for absolute fastest acceleration with a narrow powerband, you don't worry about whether you have to shift to 3rd gear on course. You just accept it and get used to it. If your car already tops out at 54mph in 2nd with stock diameter tires and has comparatively huge torque (like an Impreza 2.5RS), then you just go with what you prefer - shifting to 3rd 3 or 4 times per run, or just barely hitting the rev limiter a couple of times per run.

If I were running national-level events in the RS, I would go with the smaller diameter tires I used last year (215/45/16) for the acceleration and steering response even though 2nd would top out at 48mph.

In regard to gearing and hitting the rev limiter vs. shifting: Within reason, it is frequently better to run a slightly shorter gear (using gears or tire diameters) and then sit on the rev limiter ocassionally for a second or two before braking for the next corner. I've done the math using lots of what if's and tested the math in competition. No stick shift car accelerates during a shift. Try to realistically measure your shift time. Very few autox cars accelerate enough for a second or two in third gear, even if it is a real short third, to reduce the elapsed time for that straight significantly. Especially if you assume that the shift will probably happen at a slower speed than the rev limited mph. Think about the drag racers who have a shorter elapsed time (i.e. win) against competitors with higher top speeds but longer ET's. Even if the upshift makes you slightly quicker, the downshift may cost you time/concentration.

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Dick Rasmussen

FS 50 2018 Mustang GT

Hmmm... the rule I was taught was that if you hear more than 5-7 blips against the rev limiter, you should be shifting. When I was using the smaller diameter tires, at some events if I didn't shift to 3rd I would've had to bang against the rev limiter for more like 4-5 seconds. Not... too... good...



If it's only a couple of seconds though, I agree that it's not going to make much difference.

[quote="Kevin Allen"] if I didn't shift to 3rd I would've had to bang against the rev limiter for more like 4-5 seconds. Not... too... good...
quote]

I don't see the problem... Chaka like rev limiter - AB

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

Yeah, even if I'm not looking, I can always tell when you're on course, Aaron.

I agree, 4 or 5 seconds would be a loooong time on the limiter. Not good for times (usually) and probably not good for engines. The only possible exception I've run into is a course segment where the speed on the limiter is as fast as the car can/should go from a cornering perspective. Very rare.

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Dick Rasmussen

FS 50 2018 Mustang GT

While we're talking about this, let's say driver A goes to 3rd for 2-3 seconds at 3 places on course, and driver B bangs against the rev limiter (48mph) at those same places. I THINK my car can accelerate from 48 to 60 or so in a few seconds in 3rd gear.

How much time are we talkin' here? (assume perfect upshifts & downshifts)

not sure on the actual gain, but each shift costs you about .5 - .8 second on the course. An extra shift up will cost that time, where the downshift can usually be attained in a braking section, so it's not as costly... still, that's a lot of time to make up. - AB

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

Okay, so I kinda figured how far ahead the shifting car should be assuming that A banged on the rev limiter @ 48mph for 3 sec and B shifted & averaged 55mph through the same 3 sections.

I came up with B getting around 30 ft farther into the course at each section and eventually cutting around 1 sec by shifting. So if you really do lose around 0.7 sec per shift, it would offset this and the guy who shifted would lose around a second to the guy who just wailed on the limiter.

Wish I was consistent enough from run to run to put this to the test at Sanford in a few weeks...



So is this the kind of stuff they show you in an Evo school?

Kevin,

I recommend that you actually measure the acceleration time between various speeds. My technique with street legal cars is to pick a speed range in a gear and then accelerate from a few mph below that speed (to eliminate lag) and time between the target speeds. Some road tests will have the data also. Once you have the data you can figure out the acceleration rates in ft/sec/sec and plug them into the various acceleration/speed and time/distance formulas. If you have an appropriate location you could also mark off distances (like 200 or 300 feet) and time the segment using various starting speeds. Try not to have segments that are so short (in time) that "operator variance" with clicking the stop watch becomes a significant part of the times.

The easiest way to convert between mph and ft/sec is to remember that 60 mph is 88 ft/sec.

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Dick Rasmussen

FS 50 2018 Mustang GT

Kevin,

Even though I tend to use the rev limiter a lot, I don't think you "lose" time on up shifts (we don't have the high speed drag that actually slows F1 and other winged cars during shifts). You just don't gain all the speed that you think that you might by shifting. Keep in mind that if you accelerate from 48 to 55 that your average speed for that period is actually about 51.5.

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Dick Rasmussen

FS 50 2018 Mustang GT

I agree with Dick. It _does_ take over 1/2 second to complete a shift but during that time the car is coasting at the speed it reached when you shifted, not magically losing time back at 0 MPH. Shifting doesn't cost much time _on_course_ at all, maybe 0.1 sec max for a well executed shift at the redline.

Like Aaron says, downshifts should happen under braking so unless the heel/toe coordination makes you drive rough (it does for me!) there should be zero time lost for a downshift. I've got an old e-mail from Steve Hoelscher somewhere that breaks all of this down. FYI: Steve said he shifted something like 18 times on Topeka N in 2002!

--Kevin H.

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2003 WRX (again!)