Important - DO NOT put 315's in rears!!!!
#46
03-15-2007, 10:54 PM
Originally Posted by fredttx50
Only cost me $1200 for two freaking tires!!!!! 
But I will say that my car now runs much better. Grips and handles a lot better. Having the correct tire sizes does make a big difference.
I guess bigger/wider isn't better in this case
But I will say that my car now runs much better. Grips and handles a lot better. Having the correct tire sizes does make a big difference.
I guess bigger/wider isn't better in this case
Love,
-p
#47
03-15-2007, 10:57 PM
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Tire math, using a 305/30-19 as an example:
305 is the tread width in millimeters, as Damon already mentioned.
30 is the ratio of tread width to sidewall height.
19 is obvious. If you don't know what that is, please stop reading.
So, multiply 305 x 0.30 (30 percent). This is the height of one sidewall, in millimeters. Multiply that answer by two, for the top portion and bottom portion of the sidewall. Yay.
But wait, you say. The sidewall is in mm and the wheel is in inches. So divide your answer by 25.4 to convert to inches (25.4 mm in an inch). Now add the wheel size. This is your overall diameter in inches.
Or, if you have Excel, plug this into it... =(((A1*(A2/100))*2)/25.4)+A3
where A1 is tread width
A2 is the sidewall ratio
A3 is the wheel diameter.
Then substitute your numbers and calculate to your heart's content.
305 is the tread width in millimeters, as Damon already mentioned.
30 is the ratio of tread width to sidewall height.
19 is obvious. If you don't know what that is, please stop reading.
So, multiply 305 x 0.30 (30 percent). This is the height of one sidewall, in millimeters. Multiply that answer by two, for the top portion and bottom portion of the sidewall. Yay.
But wait, you say. The sidewall is in mm and the wheel is in inches. So divide your answer by 25.4 to convert to inches (25.4 mm in an inch). Now add the wheel size. This is your overall diameter in inches.
Or, if you have Excel, plug this into it... =(((A1*(A2/100))*2)/25.4)+A3
where A1 is tread width
A2 is the sidewall ratio
A3 is the wheel diameter.
Then substitute your numbers and calculate to your heart's content.
#48
03-16-2007, 10:00 AM
Originally Posted by Gramicci101
Tire math, using a 305/30-19 as an example:
305 is the tread width in millimeters, as Damon already mentioned.
30 is the ratio of tread width to sidewall height.
19 is obvious. If you don't know what that is, please stop reading.
So, multiply 305 x 0.30 (30 percent). This is the height of one sidewall, in millimeters. Multiply that answer by two, for the top portion and bottom portion of the sidewall. Yay.
But wait, you say. The sidewall is in mm and the wheel is in inches. So divide your answer by 25.4 to convert to inches (25.4 mm in an inch). Now add the wheel size. This is your overall diameter in inches.
Or, if you have Excel, plug this into it... =(((A1*(A2/100))*2)/25.4)+A3
where A1 is tread width
A2 is the sidewall ratio
A3 is the wheel diameter.
Then substitute your numbers and calculate to your heart's content.
305 is the tread width in millimeters, as Damon already mentioned.
30 is the ratio of tread width to sidewall height.
19 is obvious. If you don't know what that is, please stop reading.
So, multiply 305 x 0.30 (30 percent). This is the height of one sidewall, in millimeters. Multiply that answer by two, for the top portion and bottom portion of the sidewall. Yay.
But wait, you say. The sidewall is in mm and the wheel is in inches. So divide your answer by 25.4 to convert to inches (25.4 mm in an inch). Now add the wheel size. This is your overall diameter in inches.
Or, if you have Excel, plug this into it... =(((A1*(A2/100))*2)/25.4)+A3
where A1 is tread width
A2 is the sidewall ratio
A3 is the wheel diameter.
Then substitute your numbers and calculate to your heart's content.
#49
03-16-2007, 10:05 AM
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Join Date: Nov 2004
Location: Delawhere?
Age: 44
Posts: 3,725
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Yeah, but now you know how and why it works, not just some magical internet thingy you put numbers into.
#50
03-16-2007, 10:38 AM
Although the calculation of tire diameter and width may help in making a decision always refer to the tire manufacturers actual dimensions. These are usually listed on the tire manufacturers websites. Often times auto manufacturers will dictate diameters and widths to the tire manufacturers and they do not always match what the tire should be based on the numbers on the sidewall.
#51
03-16-2007, 11:44 AM
Originally Posted by Gramicci101
Yeah, but now you know how and why it works, not just some magical internet thingy you put numbers into.
#52
03-16-2007, 01:00 PM
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Location: South Bend, IN
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Keep important thing to keep in mind when using these tire calculators is that they don't take into consideration that some makers size their tires differently than the average tire in the same size. Those with Bridgestone S-03, for example know that they run much bigger than average for the size. It's still best to use the sizing charts provided to us by the manufacturers when determining proper size.
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#53
03-17-2007, 01:43 AM
Those asking about the 996... So long as a differential connects the front and rear wheels, changing only the front or the rear diameter will cause problems. This will cause problems on ANY AWD car. However, as others have mentioned, if the ratio of front diameter to rear diameter is kept constant, you should be problem free.
#54
03-17-2007, 02:37 AM
Originally Posted by dr_sharp
..... if the ratio of front diameter to rear diameter is kept constant, you should be problem free.
#55
03-17-2007, 05:24 AM
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ofcourse you will overheat your front dif if you change the diameter of you rear wheel!!!!!!! your car is awd!!!!!!!! you have to change the diameter of the 4 wheels at the same time!!!!!!
where have you change your rears tires? they dont know anything about 4wd cars!!! that was not your fault!!!!
where have you change your rears tires? they dont know anything about 4wd cars!!! that was not your fault!!!!
#56
03-17-2007, 03:27 PM
Originally Posted by Dietz
Ummm, as noted - look at the manufacturers numbers and specifically the circumference or revs per mile and make sure it's within 4% front to back.
#58
04-02-2007, 08:13 PM
There are a ton of guys on here running 315/25/19 on the rear and 235/35/19 on the front of their 996tt without any issues. Are you guys saying that these sizes will give problems on a 996tt as well?
Also, if you are running a 12" wide rear wheel, don't you need at least a 305 rear, and preferably a 315, to avoid the stretched look?
A 295/30/18 stock tire has the following dimensions
78.4 circumfrence
12.5 radius
25.0 diameter
A 315/25/19 tire has the following dimensions
79.2 circumfrence
12.6 radius
25.2 diamter
The 315 will register less than 1 mph slower than the stock.
The stock 225/40/18 fronts have the following dimensions
80.0 circ.
12.7 radius
25.1 diameter
The 235/15/19 non stock tire has the following dimensions
80.0 circ
12.7 radius
25.5 diameter
There should be no issues with running the aforementioned 19" tires on a 996tt.
Also, if you are running a 12" wide rear wheel, don't you need at least a 305 rear, and preferably a 315, to avoid the stretched look?
A 295/30/18 stock tire has the following dimensions
78.4 circumfrence
12.5 radius
25.0 diameter
A 315/25/19 tire has the following dimensions
79.2 circumfrence
12.6 radius
25.2 diamter
The 315 will register less than 1 mph slower than the stock.
The stock 225/40/18 fronts have the following dimensions
80.0 circ.
12.7 radius
25.1 diameter
The 235/15/19 non stock tire has the following dimensions
80.0 circ
12.7 radius
25.5 diameter
There should be no issues with running the aforementioned 19" tires on a 996tt.
Last edited by amerlemans; 04-02-2007 at 08:37 PM.
