John H

On my previous car, I would switch between tire sizes for the purpose of changing shift points/gearing at different tracks. On that car, the tire circumference was about 4 inches different which made an immediately noticeable difference. 80.5 inches versus 84.2 inches

However, currently I am looking at at difference of 1.4 inches in circumference between potential tire sizes. 78.1 inches versus 79.5 inches

to make it more complicated, I am going from a 17 at 80.1 inches I have always run on this car and looking at going to 18's at 78.1 inches for a 2 inch difference.

Query, will it really make any noticeable difference with gearing? I suspect it will be negligible but would like to hear from folks with real world experience.

M758

diameter is what makes a difference not circumfrence. (yes I know PI()*D = circumfrenece)

I know that on a 944 1" diameter makes noticable difference. 16" 225/50 vs 15" 225/50's

Chaos

At 1.4 it will make .1175% more torque at 1,100 ft above sealevel.

Potomac-Greg

Best move I've made for track performance is going 225/45/15 for my 944. It's MUCH smaller in diameter than the stock size, but it puts all five gears in play on all tracks. Downside is I'm racking up miles on the odometer at an abnormal pace too (a small price to pay). That's almost 2" difference in total diameter vs. stock. Calculator says it's about an 8% speedo error.

I don't have any ABS or traction control to worry about, but I've heard you can create a problem with those systems (although logically I cannot see how as long as diameter is the same at all four corners).

jpk

It's not that complicated. If your original circumfrence is 80.1 inches and the new one is 78.1, the ratio is 78.1/80.1 or 97.5%.
If you were running, say, 3200 rpm in top gear at 80 mph, you would now be running 3200/0.975 or 3282 engine rpm to travel that same 80 mph.
The diameter of the wheel (17 or 18 inch) makes absolutely no difference. The only measurement that maters is the overall circumfrence, or how far the tire rolls in one revolution.

John H

when I look a the Hoosier tire chart, the difference in circumference between the two tires you mention is 2.8 inches. 16=77.6 and the 15=74.8.

I don't think diameter matters. My Pista dash wants to know the tire circumference to calculate MPH.

Potomac-Greg

Diameter x Horsepower
---------------------------- = RL Debate of the Week
Circumference x Torque

John H

Spoken like a true engineer (I'm guessing)

It doesn't seem like you'd really notice it at that rpm but if you doubled it to 6000 rpm the new number would be 6153 which could make a difference I suppose in causing one to shift slightly earlier. Realistically on my car, I don't think I would notice a 100 rpm difference in the power band. I was hoping to make third gear usable at a lower speed. Not sure the MPH difference under this set of facts will make it much of a consideration.

John H

that's what got me ta thinkin'. I haven't been able to race this year so I am jonesing really bad...Too much time to think.

U4EEAH

HUH??? Change in C or D , same effect. Circumference will always be Pi x D.

2-3% is not much difference John, but 100 RPM higher coming out of a corner or up a hill can count

North Coast Cab

JH,
I have all the gearing charts and can layout the difference in tire for you. Frnakly that change is pretty minimum. A lot of 17's seem to be the largest OD @ 25+ with 18's slightly smaller around 24.7". To get a really small tire OD you need to go down to 15's @ 23". E-mail if you want me to send you the info.

sig_a

Stock 3.2 liter makes most HP and torque between roughly 5000 to 6000 rpm. Dropping from 25 inch diameter on 17 inch wheels to 23 inch diameter on 15 inch makes a huge difference. Lower CG reduces load shift thus less chassis roll. Unsprung weight is reduced about 12 pounds. Turning radius is reduced. Braking performance feels as if it's much better.

This helps evaluate the performance gain: http://www.miata.net/garage/tirecalc.html

This helps predict power and torque values: http://www.kenson.nu/ultima/930T.htm

This is the guy setting the E class records in TX on 15x8 225/45 R6; 15x9 275/35 R6 Hoosiers: www.buckley-racing.com/index.htm[/url]

Hope this helps.

mark kibort

Not really, you would have to have 102.5% greater engine speed to achieve the same 80mph.
Yes, the diameter of the wheel is really not an issue, its the diameter of the over all rolling surface. (or curcumference)


I think the diameter can dictate the circumference, but i dont know......



usually the change is pretty small . I regularly go between a 1" taller tire. I try and drive it the same rpm, so that it forces me to brake later at a faster speed. in the end, the lap time difference is less than a rounding error.

mark kibort

This would depend on the track you are visiting. gearing or tire diamater doesnt buy you power and power is what it takes to accelerate.
acceleration = power/(mass x velocity)

So, I do buy that the smaller diameter tire, lowers the car and that is all good, as you just showed an example of dropping the car 1", which can be like widening the car by near 2". (as far as G loading capability, all other things being equal)

but the gearing aspect is a little more dependant on the ranges of speeds used, which would be determined by your cars characteristics (hp, weight, handling, driver, etc) optimal gearing for one track, maybe totally wrong for another. a gain in one speed range will be a loss in another, because gearing doesnt increase hp or rear wheel torque, it only optimizes it for a particular speed range.

mark kibort

so, to answer your question directly. no you 1.4" circumferece diffeence will not be noticable. what will be is the pressures you run the different tires at.

-M