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SR77
Dodge Dakota
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7/30/2002
17:51:34
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 Subject: Wheel and Tire Shopping Lesson
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Can anyone give me a Lesson on everything you need to know when wheel and tire shopping. Stuff like Plus Sizing, offset, backspacing ect.. I have been looking at post and pictures on here and I found 2 trucks with the same stock setup and 1 has 295/45/18 tires and the other 275/55/18 and they both like the same height. I don't understand how that is when the diameters of the tires are 2 inches off. Plus i want to prevent myself from spending all this money and not getting the right fit, size the first time. thanks
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kota on 20s
GenIII
 
7/30/2002
19:03:15
| RE: Wheel and Tire Shopping Lesson
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Message:
295= width of tread from bead to bead.
45= aspect ratio. (its the percentage of the tread makes up the side wall. in this case 45% of 295mm is the side wall height)
18= rim size.
thing to remember is, since the AR is a percentage, the numbers play off of eachother. so if you have a 245/45, the side wall will be concederably lower even though it is still a 45 series tire.
that is why the 2 tires listed above are about the same height.
the basic rule of plus sizeing is to increase the width by 10mm (like by going from a 245 to a 255), decreasing the aspect ratio by 5 (going from a 55 to a 50) and adding 1" to the rim size
if you want to do a +3 size, mulitply everything by 3.
to confuse you a bit more, if you JUST lower the aspect ratio, and keep the width the same, the tire will be wider. this is because it takes less material to make the side wall. (exapmle: a 205/40R will be wider than a 205/65)
let me know if i need to clear things up
Eric, 98 5.2 5speed, K&N gen II, 50mm TB, 2bbl M-1, 1.7RR's, JBA ciramic headers, carsound cat, dumped gibson, and a lot more
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Bob W
GenIII
7/30/2002
21:04:24
|  RE: Wheel and Tire Shopping Lesson
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In Your last example I don't think the tire will be wider, it will still be 205 mm (8.07)inches just the tread portion will be wider.
Gibson headers, Gibson Cat-back, Edelbrock IAS shocks, K & N drop in, 180 stat, Jet stage II chip, ARE Lid
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kota on 20s
GenIII
7/30/2002
21:07:55
| RE: Wheel and Tire Shopping Lesson
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Message:
yes that is right. when the tire is at the factory and it is layed out, it is 205mm. then when the sidewalls are formed, the tread area is wider on the 40 series compaired to the 65 series
sorry about that
Eric, 98 5.2 5speed, K&N gen II, 50mm TB, 2bbl M-1, 1.7RR's, JBA ciramic headers, carsound cat, dumped gibson, and a lot more
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Bob W
GenIII
7/30/2002
21:08:55
|  RE: Wheel and Tire Shopping Lesson
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SR77, Check out this site.
http://www.miata.net/garage/tirecalc.html
Gibson headers, Gibson Cat-back, Edelbrock IAS shocks, K & N drop in, 180 stat, Jet stage II chip, ARE Lid
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kota on 20s
GenIII
7/30/2002
21:11:55
| RE: Wheel and Tire Shopping Lesson
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Message:
here is another version of that, but it also shows how the gear ratio and speedo is affected by going to a differant size tire
http://www.dakota-truck.net/TIRECALC/tirecalc.html
Eric, 98 5.2 5speed, K&N gen II, 50mm TB, 2bbl M-1, 1.7RR's, JBA ciramic headers, carsound cat, dumped gibson, and a lot more
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SR77
Dodge Dakota
JOIN HERE
7/30/2002
22:26:32
| RE: Wheel and Tire Shopping Lesson
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Message:
Thanks for the help. That is some good info. The reason I used that tire size example is because I looked on tirerack for the overall diameter of the tires and the 295/45/18 was almost 2" smaller then the 275/55/18, but they look exactly the same on both trucks with the same suspension. I have no idea why. Also I have one other question, why do some rim companies measure by offset, and others by backspacing. Are these interchangable and can measure any rim by either of these measurements. Its hard trying to measure the backspacing/offset with the rim width hoping you don't go too wide after you spent all that money. thanks again
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Bob W
GenIII
7/31/2002
22:55:05
|  RE: Wheel and Tire Shopping Lesson
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Message:
the backspacing is easy to measure with the wheel off the truck (backside of rim to mount surface of wheel. offset is calculated.
Gibson headers, Gibson Cat-back, Edelbrock IAS shocks, K & N drop in, 180 stat, Jet stage II chip, ARE Lid
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Demon-Xanth
Dodge Dakota
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8/01/2002
09:15:21
| RE: Wheel and Tire Shopping Lesson
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There's another form of measuring tires that's usually only used for the offroad and larger tires.
example: 33x12.50-16.5. Meaning: 33" tall, 12.5" wide, 16.5" rim.
Personally I like that way as it lets you know what the diameter is right off the bat.
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FredDQC
Dodge Dakota
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8/01/2002
10:26:33
|  RE: Wheel and Tire Shopping Lesson
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Demon you confused me there! I was just yesterday at pepBoys and was asking the supposed know it all about tires guy and he was telling me that the 33 meant the with and the 12.5 the height since I was looking for ground clearance more than width.So know that we are here, any of you know what size of tire will be as close to the 31X10.5's width but a lot taller? and also do I have to change to stronger shocks when putting a bigger tire? Because I have heard the many stories in this forum about people noticing a change in confort riding in bigger tires.
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CANTONDAK
Dodge Dakota
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8/01/2002
11:35:55
| RE: Wheel and Tire Shopping Lesson
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FREDDQC : 15" SIZES USE THE NOMINAL TIRE SIZE FOR THE DESCRIPTION. 31 (DIAMETER) X 10.5 (SECTION WIDTH) X 15 (RIM SIZE). 16" USE THE RATIO METHOD TO DESCRIBE THE TIRES. BFG MAKES A 33X10.5X15. SOUND LIKE WHAT YOU ARE ASKING FOR. GO TO BFG'S WEB SIGHT AND THEY HAVE A CHART THAT SHOWS EVERYTHING. SHOULD CLEAR THINGS UP FOR YOU.
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