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Old 03-28-2009, 04:05 PM   #1
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offset confusion

I'm getting confused here with width of wheels and offsets.

which wheel/offset combo has a better chance of inner rubbing?
15x7.5 wheel with 40+mm offset
15x7.5 wheel with 45+mm offset?


...so if a wheel is 6.5 inches wide with an offset of 45+ ....when it moves to 7.5 inches wide with an offset of 40+, did the -5mm difference in offset just offset the 1.5 inch increase in wheel width? or did the two changes both increase the risk of rubbing even more?

In other words, you know how a 195/60/15 tire is the same height as a 205/50/16 tire? The increase of 10 just offset the decrease of 1 inch. Is that the same as how offset and wheel width works?


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Old 03-30-2009, 11:49 AM   #2
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The second one will be in a little further than the first.
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Old 03-30-2009, 01:02 PM   #3
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the 45 is your better choice unless you like the tire at the fenders edge. Rim width has nothing really to do with offset so to speak. offset is measured by laying something flat across the back of the wheel and measuring between that and the wheels hub (flat surface that sits against brake).so a 45mm offset compared to a 40 offset, sets the back of the rim in 5mm more toward the shock / strut. factory rims are 45-49mm offset (svt is 49).
The rim width is just that, the width of the rim and does not change in correlation to the offset. offset distance changes the distance from the back of the rim to the hub.

Not to throw a wrench in and confuse anymore,
what most places today call offset is actually the backspacing.
In actuality, offset is a measurement from the centerline of the rim to the surface and can either be positive of negative.
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Old 03-30-2009, 01:31 PM   #4
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Quote:
Originally Posted by joe23 View Post
I'm getting confused here with width of wheels and offsets.

which wheel/offset combo has a better chance of inner rubbing?
15x7.5 wheel with 40+mm offset
15x7.5 wheel with 45+mm offset?


...so if a wheel is 6.5 inches wide with an offset of 45+ ....when it moves to 7.5 inches wide with an offset of 40+, did the -5mm difference in offset just offset the 1.5 inch increase in wheel width? or did the two changes both increase the risk of rubbing even more?

In other words, you know how a 195/60/15 tire is the same height as a 205/50/16 tire? The increase of 10 just offset the decrease of 1 inch. Is that the same as how offset and wheel width works?
One inch is equal to 25.4 mm, 1.5" would be 38.1 mm. The increased rim width in this case (7.5") along with the reduced offset (40mm) will move the tire much closer to the fender lip. I don't think a 195/60 tire will fit on a 7.5 inch wide rim (if that's what you were thinking about)without stretching the hell out of it and concaving the tread
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Old 03-30-2009, 04:39 PM   #5
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Here's a great calculator I use to compare rims with different offsets and widths:
http://www.1010tires.com/WheelOffsetCalculator.asp
^^^It will calculate inner clearance difference as well as outer position.
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Old 03-30-2009, 08:07 PM   #6
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superb explanations...thanks....so will 205/50/15s fit nicely on a 7.5 inch wide wheel?
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Old 03-31-2009, 10:00 AM   #7
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To get the best answer you need to check the tire manufacturers specifications. They indicate the wheel rim widths that a specific tire will fit on. The one tire I could find in that size was a BFG g force. The info on Tirerack's site says the "maximum" rim width is 7.5 inches for that tire. The downside I see with this tire size is that it has an overall diameter which is 6/10ths of an inch smaller than the preferred diameter of 24.1 inches for a focus. A 205/55-15 is closer at 23.9 inches.
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Old 03-31-2009, 12:41 PM   #8
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is there any serious problem if i go with 205/50/15 on the focus or is it just preference thing to remain as close to stock dimensions and look?
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Old 03-31-2009, 05:53 PM   #9
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Here's a few things that will be impacted going with a smaller diameter tire. Some good some bad.

1. Your speed will read faster than your actually going.
2. Your final gear ratio will be lower and you'll be turning more RPM's at any given speed than with a stock size tire.
3. Acceleration will improve but with a corresponding loss of top end speed.
4. Actual/real MPG will be less as a result of 2 and 3 above.
5. All other things being equal, a smaller diameter tire wears faster than a bigger tire due to increased tire rotation.
6. Your odo will rack up more miles (which actually isn't occurring as odo is calibrated for the larger tire).
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Old 03-31-2009, 07:26 PM   #10
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Is there any reason in particular you're looking at a 205? That's the stock tire size for a 6" wide wheel on the Focus. 7.5" would be better suited to having a 215 or 225 (be careful with that) on it IMO.
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