wheel size and backspacing question

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wheel size and backspacing question

Postby 1977astre » Thu Apr 27, 2017 3:48 am

I own a 1977 pontiac astre. I was wondering if a 15x7 with 3" backspacing will fit inside the wheel well or would a 3.5" backspacing work better. I seen people posting that they are using over 4" backspacing but from what I measured 4" would be hitting the back wall. unless I am doing it wrong. wheels will be areo racing 55 series 15x7 with 195/50r15 possible 205/50r15 on rear. if I can get away with 15x8 in the rear. I want it as close to the fender as possible but since the car is going to be lowered they need to fit just inside the lip
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Re: wheel size and backspacing question

Postby spencerforhire » Thu Apr 27, 2017 6:20 am

If you're trying to get the car low, then start with the 3.5" backspace and if they rub on the inside, a 1/4" spacer can be used to adjust. Easier to start with too much backspace than too little.
If you get the offset just right, you should be able to get even a 225/50/15 up inside the fender, especially if you roll back or cut off the inner lip of the opening.
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Re: wheel size and backspacing question

Postby cosvega76 » Thu Apr 27, 2017 8:26 am

I had used a 225/50-15 tire on my Vega in the past, but used a 15x7" Centerline wheel with a 4-7/8" backspacing and a 1/2" spacer - making it about a 4-1/2" BS. I currently run a 215/40-17 tire with a 17x7" wheel with a 4-1/2" BS. Anything less and you're limited to the diameter of tire you can run, or you have to jack it up to the sky. I like mine low.


Chuck
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Re: wheel size and backspacing question

Postby 1977astre » Thu Apr 27, 2017 1:29 pm

cosvega76 wrote:I had used a 225/50-15 tire on my Vega in the past, but used a 15x7" Centerline wheel with a 4-7/8" backspacing and a 1/2" spacer - making it about a 4-1/2" BS. I currently run a 215/40-17 tire with a 17x7" wheel with a 4-1/2" BS. Anything less and you're limited to the diameter of tire you can run, or you have to jack it up to the sky. I like mine low.


Chuck


how are u fitting 4 1/2 bs. that would rub the inside wall with mine my drum is 3" another 1-1/4" is the inside wall so 4-1/2" bs would put the rim on the wall. unless I'm doing it all wrong. my stock 13" have 3-1/4" bs unless I measured wrong
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Re: wheel size and backspacing question

Postby Monza Harry » Thu Apr 27, 2017 6:38 pm

1977astre wrote: My stock 13" have 3-1/4" bs unless I measured wrong
'77 is that the 5" Vega wheel or the "Ralley II" [aka the "GT" wheel]? I get 3 1/2" with the Ralley II, never measured a 5" wheel. I have to ask how are these giant "Back Spaced" wheels fitting as well, when I had [1982] P205/60R-13 Dunlap GT Qualifiers on my '76 I had to use spacers to stop the rubbing, Those tires were WAY BIGGER than my current 205's, tires were measured edge of tread to edge of tread, not bulge to bulge, I suspect this was left over from the Bias-Ply days to make upgrading easier(?). So either way your measurements are at least with-in reason of mine! I wouldn't surmise them to all be the same from that era, close was "good enough" , :lol: Harry
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Re: wheel size and backspacing question

Postby hammerdown7 » Thu Apr 27, 2017 9:41 pm

The Cosworth Vega original equipment wheel and tire combination was a 13X6 wheel with a backspace of 3 7/8 inches, the tire was a BR70-13 which had a section width of 7.60 inches. These dimensions should help in finding today's size tires and wheels. The edge of the tire had approximately 3/4 inch clearance to the inner fender brackets. Hope this helps.

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Re: wheel size and backspacing question

Postby cosvega76 » Fri Apr 28, 2017 12:15 pm

1977astre wrote:
cosvega76 wrote:I had used a 225/50-15 tire on my Vega in the past, but used a 15x7" Centerline wheel with a 4-7/8" backspacing and a 1/2" spacer - making it about a 4-1/2" BS. I currently run a 215/40-17 tire with a 17x7" wheel with a 4-1/2" BS. Anything less and you're limited to the diameter of tire you can run, or you have to jack it up to the sky. I like mine low.


Chuck


how are u fitting 4 1/2 bs. that would rub the inside wall with mine my drum is 3" another 1-1/4" is the inside wall so 4-1/2" bs would put the rim on the wall. unless I'm doing it all wrong. my stock 13" have 3-1/4" bs unless I measured wrong


My mistake - I thought you were talking about the front wheels. For the rear, you'll need a 3 1/2" to 4" backspacing, depending on the size of the tire and width of the wheel. My car has flared wheel openings so I'm running a 17x8 with a 3 1/2" BS. There isn't a lot of room with the stock wheel opening, so my best suggestion is to mock it up and measure carefully. I did a lot of measuring before I decided on wheel and tire size with 17s.

Chuck


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Re: wheel size and backspacing question

Postby hammerdown7 » Fri Apr 28, 2017 12:46 pm

Here is the thread on my 15X8 wheels, viewtopic.php?f=1&t=31893

This mostly explains what I did to use wheels with a 4 on 100mm mounting but does include info with 225/50-15 tires.

Dick
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Re: wheel size and backspacing question

Postby 1977astre » Sat Apr 29, 2017 10:06 am

cosvega76 wrote:
1977astre wrote:
cosvega76 wrote:I had used a 225/50-15 tire on my Vega in the past, but used a 15x7" Centerline wheel with a 4-7/8" backspacing and a 1/2" spacer - making it about a 4-1/2" BS. I currently run a 215/40-17 tire with a 17x7" wheel with a 4-1/2" BS. Anything less and you're limited to the diameter of tire you can run, or you have to jack it up to the sky. I like mine low.


Chuck


how are u fitting 4 1/2 bs. that would rub the inside wall with mine my drum is 3" another 1-1/4" is the inside wall so 4-1/2" bs would put the rim on the wall. unless I'm doing it all wrong. my stock 13" have 3-1/4" bs unless I measured wrong


My mistake - I thought you were talking about the front wheels. For the rear, you'll need a 3 1/2" to 4" backspacing, depending on the size of the tire and width of the wheel. My car has flared wheel openings so I'm running a 17x8 with a 3 1/2" BS. There isn't a lot of room with the stock wheel opening, so my best suggestion is to mock it up and measure carefully. I did a lot of measuring before I decided on wheel and tire size with 17s.

Chuck


Chuck


ok well the rims that came on the car are 13x5 with a 3" bs. I live 2 blocks from basset racing wheels hq. they kindly did the measurement for me 4" bs will not work on the rear of my astre without the rim hitting the inside wall. my car must be weird
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Re: wheel size and backspacing question

Postby Monza Harry » Sat Apr 29, 2017 12:00 pm

1977astre wrote: ok well the rims that came on the car are 13x5 with a 3" bs. I live 2 blocks from basset racing wheels hq. they kindly did the measurement for me 4" bs will not work on the rear of my astre without the rim hitting the inside wall. my car must be weird

I don't think yours is too weird as that seems in line with the five I have owned [all Hatch back 2nd gens: Monza/Skyhawk] Front or back My Dunlap's needed spacers with the Rally II's [GT Wheels] on all four corners! With I might add all stock suspension, save shocks and Sway bars. Harry
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