89transam

I just got some cragar ss 15's and I was wondering what size tires would keep them the same Height as stock?

255/60/15
255/65/15
255/70/15

The 60's correct?


Nate

Doomsday Danny

Hi I'm new here and I'm getting a 86 Trans am today. It currently has the firebird alum rims on it but there's a guy that i know that is selling a set of 15' cragar rims. the exact sizes are 2-15x10, 2-15x8. I think these rims will look really aggressive once i put them on. I would also like to know what tire sizes i should get.

thanks.

89transam

anyone?

ATOMonkey

Ok Here's how you figure it. The first number is section width in mm (255 mm) the second number is the aspect ratio, which is basically the percentage of the section with and describes the sidewall height ( .7 x 255 = 178.5) The last number is wheel diameter in inches ( 15 ). So the standard tire wheel size is 245 50 16. 245 x .5 x 2 (sidewall twice) = 245. There are 25.4 mm in one inch, so 245/ 25.4 = 9.65 inches. 9.65 + 16 = 25.65 Now we work backwards

25.65 - 15 = 10.65

10.65 * 25.4 = 270.51

270.51 / 2 = 135.255

135.255 / 255 = .533

135.255 / 245 = .552

135.255 / 270 = .500

135.255 / 225 = .600

So somewhere between a 50 and 60 series 255 section width mounted on a 15 in wheel. So, which way do you go? A 60 series will make the car think it is running slower than actual, so you'll get more tickets. A 50 series will make the car think it's going faster, and you also get a little extra torque mult. If you get a 270/50/R15 you keep the same height, or a 225/60/R15 (stock specs for a 15 in. wheel).

89transam

What is torque mult?

ATOMonkey

The tire is shorter than what you have now. The result is similar to a bigger gear number. You multiply your torque output to the ground.

traumatech

hey atomonkey, if you happen to check this post again,

could you double check my math here--

I saw a set of tires on a thirdgen that I really liked height/aspect wise. The size was a 245/45R17. I would like to run 295's, so if I go with a 17 tire, I'll need about a 295/40R17, and it would be just a hair taller. That seems a little odd, but here's my math.

245 * .45 * 2 = 220.5 total sidewall height

220.5 / 2 / 295 = .37, so either a 35 or 40 series, right?

If anybody else wants to explain this better or agree, I'd sure appreciate it. I'm trying to find a good mix between performance and avoiding the thin walled rice look. Thanks.

tomster03

AUhhh i have the Old cragger SS on my babey they are the deaper dish and i will tell you what... THEY SUCK... i cant go no wider than 205 60 15 on the front becuase it will rub if i do down and hight i can go wider BUT that just looks stupid...
I got some Big meat on the back tho and well its good for racing in stright lines i guess i blan on buying Elite Barb rims 16 one the opisite dish from what mine are and stuffing 275 55 16 on there

TomP

More on "torq mult"

Shorter tire/wheel height? Faster 0-60, smaller top speed, more rpm's in gear. Speedometer reads faster than actual speed- ex, you're doing 60 mph, speedo says 65.

Taller tire/wheel height? Slower 0-60, faster top speed, less RPM's in gear. Speedometer is slower than actual speed- ex, you're doing 60 mph, speedo says 55... bad for speeding tickets!

logikal6785

speeding ticket prone or not ... gotta <3 the 17's =P

jawzforlife1

or just go here

http://www.eunos.com/tirecalc/

it will show you a comperision pic of the 2 sizes of tires, comperisian data, and tell you how much you speedo will be off with diff size tires.

By using this I found out my new 17" ZR1 wheels and tires will couse my speedo to read 5 mph higher than the car is really going.

255/60/15 is amost the same as stock 225/60/16

traumatech

sweet site jawz, answered all my questions. Thanks.

jawzforlife1

no problem, I got that out of an old TRUCKIN' mag. It help out when i was buying my new wheels and tires.

gtabadboy

While we are on the topic, will 315/35 R17 fit on the back of a stock trans am? I am interested in these BFG street slicks and some new 17" rims.