Just got the latest issue of HR with a writeup on the 66 convertiable they have had for awhile. Here's my question, they put 17 X 9 with 5 3/4" backspacing on the rear axle. Tires are 275/40R17. I get lost in the terms backspacing and offset, even though one of the members posted a nice diagram showing the difference. I have a set of waffle? style wheels (think GNX) that are 15 X 8 X 4" backspacing, I think. These look like they will either rub the body or stick out past the fender lines, like the old style with air shocks. I have found a place that has these wheels and they said they could give me any backspacing I want. Should I go with a 5" backspacing and 0 offset with 15" wheels? Wheel company will not take returns, so I've got to be right the first time. Thanks
Hey Gary, From the tirerack website, backspacing is defined as a measurement of "the distance from the mounting plate to the back edge of the wheel". Offset is the measurement of "the backside of the mounting plate to the center line of the wheel" http://blog.tirerack.com/blog/hunters-ramblings/what-is-wheel-offset-and-backspace Therefore, the way I read that: If the wheel is 9" wide, and the backspacing is 5 3/4", then there is 3 1/4" to the front edge of the wheel. If the wheel is 8" wide, and the backspacing is 4", then there is 4" to the front edge of the wheel. I have a set of 17x8 wheels with 0" offset, meaning 4" backspacing, and they fit in the wheel well of my '67. (Only about 1/4" wiggle room though, and I only tested one side.) The attached pic is with a 225/50-17 on my car with the fender skirt attached (you don't have that problem). Got rust? Hope this helps.
Gary, Do yourself a big favor, completely forget the term offset, and focus on back spacing. That's all you need. Back spacing centers the wheel in the wheel well. You don't need to even mention offset when ordering a wheel, so forget the term. It's very easy to measure back spacing. Place the wheel on the floor, outside edge down. Measure from the wheel lug mounting surface to the inside edge of the wheel. I am familiar with back spacing requirements for the 70-72 skylark/GS, not the 60's cars. The wheel diameter doesn't affect back spacing requirements, but wheel width does. So you need to decide what width you want to run first. When you get up in the higher width wheels, you need to realize that cars will differ even if they are the same year, make, and model. Sometimes, the body is shifted a bit on the frame, so there is a small difference from side to side. If you want to run a 10" wide wheel, you need to measure your own car to see how much room you have on each side. I recommend staying with a wheel that is 8" wide. You should be able to get a recommendation on wheel and back spacing from someone on V8 with the same year car. Like I said, forget offset, it's irrelevant to what you are trying to do.
Thanks for the input and I will never again mention offset! The wheels I now have are a true 4" backspace measured with a straight edge across the rim lip measured from the bolt hole. So I'm thing a 4 3/4 or 5" will work and keep the tires out of the body. My goal is to run 225/60 15 up front with 245/60 15 rears. Also looking to keep a minimum of 27" tire to a max of 29" tire. This is all because I want to go to front disc brakes and my 14" wheels won't fit.
For the 70-72, an 8" wide wheel wants 4 1/2" BS. I know the 68-69 want 5" BS on that same 8" wide wheel. I think the 64-67 are like that also, so I wouldn't go less than 5". I would take your original wheels and measure the back spacing, and go from there. Measure how much room you have on the inside, and outside of the wheel.
I have Corvette rally wheels on my 65 Skylark- I am able to fit 255/60-15 tires in the back just fine (stock Vette rally wheels have 4" of backspace). I got some wheel vintiques rally wheels with 5" backspacing and am able to run a 9" wide MT slick in the back (28X9x15)- I am 99% sure a 275 will fit as well. My fenders are still stock- with a 5 3/4" backspace you can stuff a 295 tire in there from what I gather on the Chevelle forums. Hope this helps- let me know if you want any pics (although I currently have 245-60/15 tires on the rally rims due to dry rot on the 255s that came on the car). I can mount up the slicks too if you want to see them. Need to replace the intake gasket tonight/tomorrow so I can get to the track on Sunday so the slicks need to go back on anyway. ~Kevin
Sigh, here we go again..... The necessary backspacing to center the tire in the wheelwell varies depending on the width of the wheel. However the necessary offset remains constant, even if you change wheel widths. I prefer the offset spec. A stock 15x7 Buick rally wheel has 4-3/8" backspace and 3-5/8" front space.... or a 3/8" offset. Note that adding front and backspace gives 8" on a 7" wheel. 255/60-15's are a tight fit on a '66 using a 15x7 Buick rally wheel.... they're tight on the outside at the fender lip. I can fit 275's on my blue car which has a 1" narrower 12 bolt rear. Tires are nicely centered in the wheelwell. So..... For a stock rear in a '66, ideal offset is about a half inch more than a stock 15x7 rally.....3/8" + 1/2" = 7/8" to 1 inch. If you want to go by the back space spec: 15x7 wheel (measures 8" wide)..... 5" backspace, 3 inch frontspace. 15x8 wheel.....5-1/2" backspace, 3-1/2" frontspace. 15x9 wheel.....6" backspace, 4" frontspace There are tools to measure how much offset or backspace is ideal for a car: http://www.summitracing.com/search/Product-Line/Percys-WheelRite-Wheel-and-Tire-Fitment-Tools/ Or make your own: http://www.homemadetools.net/tire-fitment-tool
My 255/60-15 BFGs on an 8" rim with 4" backspacing is a tight fit- no rubbing on even the biggest bump but I cannot fit my fingers between the wheel well and tires. Probably about 3/8" clearance to the sidewall. Granted, I did not build this car so the rear end may not be stock- may be a different rear end that is a bit more narrow than the stock Buick? I only know what fits on my car. What is the WMS to WMS width on the stock Buick rear end, that way I can figure out if my axle is stock or been narrowed? I do have tubular upper and lower trailing arms and the uppers are adjustable for pinion angle so it would not suprise me at all if my axle were modified or not stock. I would like to know- Thanks! ~Kevin
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Oddly enough, I have 14" Buick Chrome wheels, newer models, on my 1971 with disc brakes. I thought it would rub but it doesn't.
I had Wheel Vintiques whip me up a set of 15 x 10in wheels with 6.125in backspacing for my 67 GS. I rolled my fender lip and I had to relocate my lower shock mount to clear the offset on the inside. I started with 295 BFGs, but the widest tire I could fit were 255/60 R15s. I landed up with approx. 1/2 clearance to the wheel lip and about the same to the frame at the closest point at the front. Thanks,
