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+1y
will tell you now. An 8.5 with +40 or +42 (come on, seriously, 2 mm is like a 16th of an inch) will NEVER TUCK with drop spindles and 2wd sheetmetal. It will also BARELY tuck without drop spindles and with 2wd sheetmetal, and only in the front, the back will chew up fenders.
in order to know offset you have to know width. The two go hand in hand, as I'll explain. Without knowing width, you could run ito trouble just basing your wheel buying on offset alone. For example, IF (hypothetically, not having anything to do with mazdas or any specific truck) IF a 7 inch wide wheel with +20 barely tucked, an 8.5 inch wide wheel with +20 will stick out almost an inch. Same offset, different fit. Here is how to figure it:
Figuring backspace from offset and vice vera is EASY. You need to know width.
step one: add an inch to the width. A 7 inch wide wheel is actually 8 inches wide lip to lip because of the half inch bead on either side. adding an inch allows you to know the exact centerline.
step two: divide by two. This is the centerline of the wheel. Any offset quote is given FROM THIS IMAGINARY LINE. Thats what offset means, "the wheel mounting surface is OFFSET from the center by + or - XX mm. A + offset means the mounting surface is offset to the front side of the wheel (FWD cars are high + offset) a - offset means the mounting surface is offset to the inside of the rim (you have seen wheels that really stick out, thats - offset)
step three: convert the mm to inches. 25.4 mm = 1 inch, so divide your offset number by 25.4. If you want an easy way, go to google.com and type in "XX mm in inches" (where XX is your offset number) and it will convert for you.
step four: If it was + offset, add the offset in inches you got in step three to the wheel centerline measurement you got in step two. If it was negative offset, subtract the offset from the centerline. This is your backspace.
Recap: Add an inch to the stated width. Divide this number by two and record. Convert mm offset to inches. Add (or subtract) this number from half the width. = backspace.
SO lets use what we know: +42 on an 8.5 inch wide wheel.
8.5 + 1 = 9.5
9.5 / 2 = 4.75
42mm / 25.4 mm = 1.65
1.65 + 4.75 = 6.4 inches of backspace.
to go the other direction, converting backspace to offset, is easy too.
add an inch to the width
divide by two
sutract this result from the stated backspace
convert the result to mm by multiplying by 25.4mm
So, 6.4 backspace on 8.5
8.5 + 1 = 9.5
9.5 / 2 = 4.75
6.4 - 4.75 = 1.65
1.65 * 25.4 = 42mm offset.
Back to mazdas. No way will a +42 fit with 2wd sheetmetal unless you do as said and narrow the upper arm. It absolutely will not fit the back, ever, unless you shorten the axle or go to a courier rear. The guy who has them also neglected to mention he is running an RX7 rear end, which will camber in like the front. For 2wd sheetmetal, 7 inches of backspace on a 8.5 inch wide wheel would be
8.5 + 1 = 9.5
9.5 / 2 = 4.75
7- 4.75 = 2.25
2.25 * 25.4 = 57mm offset. Which is moot, because they dont make anything that high offset in 6 lug.
Switch to five lug. the toyota rear end will give you some extra tuck in the back, and you can buy 20x7 with +40 offset in a 5 on 4.5 pattern, which will tuck great on a maz.
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Russ-D
Posts: 1823
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View other posts by this member. Posted - 2 June 2006 9:8
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oh, and adding 4wd sheetemtal will give you about a 3/4 inch backspace cushion.
SO if you needed +39 on a 7.5 (I know this fits, I had it on a previous truck)
7.5 + 1 = 8.5
8.5 / 2 = 4.25
39mm / 25.4 = 1.54
4.25 + 1.54 = 5.79 or roughly 5.75 backspace
having 4wd sheetmetal will give you .75, so you need 5 inches of backspace to tuck a 7.5 wide wheel.
7.5 + 1 = 8.5
8.5 / 2 = 4.25
5 - 4.25 = .75
.75 * 25.4 = 19.04 or roughly 20mm offset for a 7.5 wide wheel.
To use this number to figure up what you would need on another width wheel, find the FRONT SPACE for the wheel you know fits. This frontspace CANT change and still tuck no matter the width. To find frontspace, you subtract the backspace from the total width.
7.5 + 1 = 8.5
8.5 - 5 = 3.5 fronspace
For an 8.5, you would need
8.5 + 1 = 9.5
9.5 - the frontspace you know fits 3.5 = 6 inches of backspace.
now figure out offset
9.5 / 2 = 4.75
6 - 4.75 is 1.25
we already know that 1.25 * 25.4 = 32mm offset.
So, knowing that a +39 7.5wide wheel fits with 2wd sheetmetal, we figured out what offset we needed to fit a7.5 inch wide wheel (20mm) and an 8.5 wide wheel (32mm)with 4wd sheetmetal.
dont thank me thank russ d hes the mazda god on here...