[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

*To*: "Robert B." <robbrauns@bb.eulink.net>, "Quattro List" <quattro@audifans.com>*Subject*: Re: Rim and Tire sizes (long)*From*: "Ryan Hoitink" <hoitink@bytehead.com>*Date*: Fri, 20 Aug 1999 18:27:28 -0500*References*: <37BDC479.C3B591B6@bb.eulink.net>*Sender*: owner-quattro@audifans.com

----- Original Message ----- From: Robert B. <robbrauns@bb.eulink.net> To: Quattro List <quattro@audifans.com> Sent: Friday, August 20, 1999 4:11 PM Subject: Rim and Tire sizes (long) > Hello Q-List: > > someone posted off list to me and so I thought I would share this > information on upgrading to bigger rims and tires. Sorry if it's long, > but it is a detailed mathematical formula and explanation on how to > calculate the rolling circumference of any tire and rim combination. > > I think there is a formula somewhere on the web, but you can calculate > rolling circumference pretty easily... > > Firstly, it is not the size of the rim only that makes a difference, but > rather the rolling cicrumference (RC from now on) of the rim PLUS the > tire size. For example, a 175/65HR15 tire has a RC of 61.2". A > 205/45SR17 tire has a RC of 64.8", so these are almost the same RC, but > your speedo will read too low by about 6%. A larger RC means your car > will accelarate slower, but have higher top end speed and lower highway > rpms. Your speedo will read too low if the RC of your new tire/rim combo > is bigger than the RC of the stock tires. Now for the math: > > The formula for the circumference of a circle is: 2TTr, where > TT=pi=3.14169 and r=radius of the circle (rim + tire). > > So, to calculate the RC (how long the tire would be if you cut it and > laid it on flat on the ground), you have to add the diameter (which is > 2x the radius) of the rim PLUS add the diameter of the tire. Since the > 175/65 are in millimeters, you first have to convert the 175/65 to > inches. BUT before you do that, the 175/65 number has to first be > converted to the actual tire height. 175/65 means that the tire (or > tread?) width is 175mm and the height of the tire is 65% of the tread > width. BTW, this ratio of 65% to the tread width is called the aspect > ratio. So, the height of the tire would be 175x65/100=113mm. Now take > this figure and divide by 25.4 to get the height of the tire in inches. > 113.75/25.4=4.47". So the diameter of the rim + tire (called the RC) is: > TTx (15" [rim] + 4.47" [tire]) = 61.2". > > If you have a spreadsheet program, enter values for rim size in A1 > (15"), the tread width in A2 (175) and the aspect ratio in A3 (65) and > then enter this formula in A4 to get the RC: = > (3.14169*(A1+(A2*A3/100)/25.4)) > > Think of rolling circumference this way: for every ONE revolution of the > tire, the car will move forward exactly 61.2". > > The letters, HR, SR, TR, VR are speed ratings (H being the lowest and Z > being the highest) and R simply means a radial tire (as opposed to say, > bias-ply tires). (BTW, are these still made?) > > If you plan on upgrading your rims to a larger diameter, you need to get > a tire with a smaller aspect ratio or else your speedo will be off. A 5% > difference is the most you can change without worrying about your > speedo, since there are variations between tire companies of about 5%. > > HTH. > > Rob > Germany > > Actually, that's the wrong number for pi. Off the top of my head it's about 3.141592653589793238462643383279502884197169399 . I used to get really bored in my calculus class. Ryan

**References**:**Rim and Tire sizes (long)***From:*"Robert B." <robbrauns@bb.eulink.net>

- Prev by Date:
**Re: More projects/seat heater** - Next by Date:
**Re: 5000/200 Windshield replacement... bad experience (looking foradvice)** - Prev by thread:
**Re: Rim and Tire sizes (long)** - Next by thread:
**AUDI USA personel** - Index(es):