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  How does rear tire size affect 1/4 mile times?

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Author Topic:   How does rear tire size affect 1/4 mile times?
70coupe
Gearhead

Posts: 382
From: Toronto,Ontario,Canada
Registered: Aug 2000

posted 05-06-2003 11:43 PM     Click Here to See the Profile for 70coupe     Edit/Delete Message   Reply w/Quote
I run a 26x10x15 slick on my mustang with 4:11's in the rear. I was wondering how 28" tires would affect my 1/4 mile times and mph?
Thanks,
Brian

------------------
351c 4v clsd heads
60'1.619
1/8 [email protected]
1/4 [email protected]

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steve'66
Gearhead

Posts: 7226
From: Sonoma,CA,USA
Registered: Mar 2000

posted 05-06-2003 11:49 PM     Click Here to See the Profile for steve'66   Click Here to Email steve'66     Edit/Delete Message   Reply w/Quote
Brian,

It'll be like swapping to 3.89 gears. It depends on your converter and where your torque curve is. It may be more consistant and slower, or.....?

SteveW

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74merc
Gearhead

Posts: 763
From: Demopolis AL
Registered: Jun 99

posted 05-07-2003 12:00 AM     Click Here to See the Profile for 74merc   Click Here to Email 74merc     Edit/Delete Message   Reply w/Quote
I've heard of going the opposite route... friend of mine ran some tiny little 13 inchers on a Pinto for an effective 4.55:1 ratio...

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jsracingbbf
Gearhead

Posts: 2160
From: Batesville,MS. , U.S.A.
Registered: Mar 2002

posted 05-07-2003 01:12 AM     Click Here to See the Profile for jsracingbbf   Click Here to Email jsracingbbf     Edit/Delete Message   Reply w/Quote
Brian, IF you are switching to get better traction becuase your not getting it now, then your 60' times may go up! but ordinarily a taller tire will MPH better but hurt your 60' ET.
Personally, I am a firm believer in run the biggest tire you can then gear for it. AND size convertor for it. I know there are alot of 10.5 inch tire guys that don't belive in this and they do run some impressive times.....Can you try them first?

------------------
Jerry
69 Mustang Pro ET Drag
70 Mustang retired former footbrake car

"Whiskey for my men, beer for my Horses" -- Toby Keith

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Toronado3800
Gearhead

Posts: 364
From: St. Louis, MO
Registered: Jul 2001

posted 05-07-2003 07:00 PM     Click Here to See the Profile for Toronado3800     Edit/Delete Message   Reply w/Quote
JS, just like you say, I was always told to imagine how small the contact patch of a ten inch long pencil is be before putting smaller tires on my car to affect my effective gearing.

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n2oMike
Gearhead

Posts: 1805
From: Spencer, WV
Registered: Jan 2001

posted 05-07-2003 08:25 PM     Click Here to See the Profile for n2oMike   Click Here to Email n2oMike     Edit/Delete Message   Reply w/Quote
If your car is traction limited (spins too much) the taller tire might help, if it doesn't kill the gear ratio too much.

Two full inches changes the gearing a LOT.

mph = [rpm x tire diameter]/[336 x overall gear ratio]

solving for rpm...

rpm = [mph x 336 x overall gear ratio]/tire diameter

*Overall gear ratio is found by multiplying the transmission gear ratio and rearend gear ratio together. High gear (except overdrive) is normally 1.00 in the vast majority of transmissions.

Here's an example (not accounting for converter slippage)

rpm using 26" slicks
rpm = [110 x 336 x 4.11]/26
rpm = 5842 rpm
---------------------
rpm using 28" slicks
rpm = [110 x 336 x 4.11]/28
rpm = 5425 rpm

You'll lose around 420 rpm (at your current speed) through the lights by going from a 26" to a 28" slick as long as the mph stays the same.

If your're not traction limited, I'd keep the shorter tire. It will give your more gear, and keep the back end of the car lower, which aids load transfer. (Weight transfer is an invalid term. Both ends of the car weigh the same, regardless of how it launches.)

Good Luck!

------------------
Mike Burch
66 mustang real street
302 4-speed 289 heads
10.63 @ 129.3
http://www.geocities.com/carbedstangs/cmml_mburch.html
http://www.fortunecity.com/silverstone/healey/367
http://www.mustangworks.com/cgi-bin/moi-display.cgi?220

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