ZERO

hey guys,

i know several of you have probably switched out your tires and wheel for whatever reason and i had a couple of questions about it. i know when i watched 2005red get his car tuned, he had to have them input the size of his tires when they were dynoing his car.

i understand this is because your computer is controlling your RPMs and your MPH, so because he changed the size of his wheels and the diamater of his tires, to get a true reading on your speedometer the computer needs to be adjusted for the change.

since changing your tires and wheels doesn't void your warranty, does the adjustment that is necessary due to this change cause any problems? you obviously have to get a retune, otherwise you don't know how fast your actually going after you've changed your wheels/tires. does ford consider this an "acceptable" reason to get a retune?

any help you can offer would be appreciated. i was thinking about doing larger wheels and tires in 2005, and was curious.

thanks guys.

ZERO

*crickets*

n-e-one?

1 COBRA

The size of the tires, which in turn have a certain amount of turns per mile unique to them, is what determines if a different calibration is required. TireRack has a chart which provides such info. In other words, changing the wheel size does not necessarily mean the computer settings need to be changed, specially if the new tire has similar amount of turns per mile.

Here is an example for the '99 - '04 Mustangs:

245 45ZR 17, 275 40ZR 17, and 275 35ZR 18, are close enough not have to alter the stock settings.

ZERO

good to know. i haven't looked at tire rack recently, but i'll see if i can find that chart you mentioned. i was thinking of going to 275/45 R18 from the stock tire size. it seems like you gain an inch in diameter, but loose a little by reducing the sidewall height, may not be enough by going to 45 though.

nynvolt

You should keep the weight and overall diameter as close as possible to stock. So to plus size you need to change your side wall hight. If you go too much above or below your stock diameter, it will not only affect your speedo but may in some cases affect other things such as abs, suspension ect.

I have done business with www.thetirerack.com before and they are fantastic, they will give you the correct offset and wheel options, and will email you if you order something that may not have a perfect fit.

Also a useful tool, it's from a miata site but works for any size tire. http://www.miata.net/garage/tirecalc.html

A great way to check your tire size options. I'd be very weary of buying rim/tire combo's without knowing your offset (or offset adjustment for plus sizing) and the proper tire diameters. Keeping it +/- 1% I think is acceptable. If you go with a heavier rim than stock you will lose some to the wheel hp because of extra unsprung weight. Going to a larger diameter will also cause a lose in hp to the wheels, the speed channel RSX-S lost like 7 hp to the wheels just converting to larger rotors.

Keep all that in mind, I'm no expert so make sure your getting the right product for your car. Do your homework before buying.

GarageLogic

Nothing has to be changed as long as the overall wheel/tire diameter remains the same as stock. If you go up a size in wheel, say from a 17 to an 18, then your new tires will need a smaller sidewall. For instance, the '05 GT runs a 235/55-17 tire. 235 is the width of the tire in millimeters, the percentage of the width in a sidewall and the 17 fit the rim size. Using the following formula, here is what you get:

235mm*.55=129.25mm
divided by 25.4 (mm-to-inches conversion factor)=5.088 inches
multiply this number by 2 and add the rim diamter and you get an overall diameter of 27.17"

If you were to go to an 18 inch rim you would need a 255/45-18 tire to achieve the same overall diameter.

255mm*.45=114.75mm
divided by 25.4 (mm-to-inches conversion factor)=4.158 inches
multiply this number by 2 and add the rim diamter and you get an overall diameter of 27.03"

Use the same math to arrive at a correct diameter using 19 or 20 inch tires. Just remember, the bigger the rim, the smaller the sidewall. A smaller sidewall may give better handling, but ride quality will suffer. Also, a low profile tire does not lend itself to drag racing.

ZERO

thanks GL...

i was thinking about 275/45's so they actually come in really close. .15 of an inch really isn't anything. it's a little more than an eighth of an inch...more like a seventh of an inch. lol.

now my only problem is, i can't find any wheel that i like. i've tried tirerack and discount tire so far. i'll keep lookin' though.

GarageLogic

I went with the Boze Stixx in an 18x9 in front and 18x10 in the rear. I will run 255/45-18 and 275/40-18 tires, respectively.
A link to the wheel here

Hopefully, I will have them by this weekend and I can post some pics of them on the car along with the Eibach Pro-Kit that is sitting in my garage. B)

ZERO

looks nice, can't wait to see the pics.

bishop

Here is a caculation tool on Discount Tire Direct..

http://www.discounttiredirect.com/direct/b...nfoTireMath.jsp

ZERO

thanks for all the help guys.

BillP

What brand and model tires are you going to run on that combo?

DozerzPony

How much are your Boze wheels without tires?

GarageLogic

BF Goodrich GForce KDW2 255/45-18 front and 285/40-18 rear. Wheels were $500 each.