Backspacing on 8in vs 9.5 in Wheel - rubber of 245?

[68 SuperRam]

Curious - would the back spacing be the same on a 17*8 wheel compared to a 17*9.5 wheel - both having rubber of 245 - 68 Camaro or would the 9.5 wheel need more backspacing. Looking to increase my rear wheel to a 17*9.5 - disc brake - non rolled lip but run 245 - currently running 16*8 with a 4in BS - and it slightly rubs the fender lip - mono leaf - 2in lower blocks. Was going to go with a 5 in BS to correct the rubbing and go up to a 17in wheel - but was thinking if I go with a 9.5 instead of staying with an 8in wheel - I should increase the BS to either 5 1/4 or 5 1/2.

[baz67]

5.5 on a 9.5. That wheel may be a bit to wide for a 245 tire though.

[MrQuick]

It really depends on how much room you have inside. If your real close with a 8" 5.5bs then you would stay with the same backspace and increase size outward.
A 5" BS would be a safe size but you will have to do some math to get a 9.5 to fit right. Try to leave 3/4" min inside room and 1/2" min outside room.
Things to remember.
A rim is usually a half inch larger than advertised due to lip. ie: 9.5=10" wide.
Try to stay near the rim / tire requirments.
Some tire facts in my experience:
A 245 on a 17x8" rim has a total width of 9 5/8"
A 285 on a 17x9.5" has a total width of 11 3/16"
A 245 on a 17x9.5" has a toal width of 10 13/16"
A larger tire will have a larger bulge. Average for 285 on a 9.5 is 3/8" bulge.
A 245 on a 9.5 will have a 1/8" bulge.
A 275 on a 9.5 has a 5/16" bulge.
You can use the tire that is on the car to figure how much larger you can go. Might as well go with the larger tread section tire.

[68 SuperRam]

Thanks for the all of the info - I have a 4in BS on my 16*8 now with some rubbing on the rear qtr lip - if I stay with a 8in wheel but go to a 17 on a 245 - should I get a 5in or 5 1/4 BS - just a guide - before I take the wheel off to measure.

Thanks again.

[inSANe DIEGO]

If you are intending on keeping the tire size the same, evey bit of b.s. added will equal that much space added between 1/4 lip and tire. check out the stickies in this section for calc formulas, methods and terms. also, the tire plus sizing calculator...invaluable!!! assuming your tire WIDTH stays the same, going from 4" b.s. to 5" - 5.25" should be adequate in creating proper lip clearance. This is IF the rubbing is SLIGHT... Google for CHP article called "Get a Grip." It has some rule of thumb max tire and wheel sizes and corresponding b.s. for 1st gen.

John

[68 SuperRam]

In measuring the tire - against the inner fender - what is the rule of thumb regarding clearance 1/2 in or 3/4 in from the side of the tire to the inner wheel well?

[MrQuick]

I like to keep around an inch but 3/4" is my minimum and will work. The problem is when you enter a driveway, angle up a curb , or one wheel over a bump, the axle goes up and the wheel rolls inward abit.
Try going with 3/4" outer edge and 1" inner side if possible. You can always modify the inner well if needed. I always say its easier to clearence an inner well than an outer quarter.

I also made a mistake, rims are usually 1" larger than advertised. sorry

[68 SuperRam]

O.k. Thanks - I went home last night and did some measuring - here is what I came up with - with the car on the ground with the tires on:

Drivers side: With a a ruler on the top of the tire and my brother crawled under the car - we came to a distance of 2 1/8 in from the top inner sidewall of the tire to the inner wheel well. We also took the measurement from the top outer sidewall of the tire to the quarter panel inner lip and noticed that the tire sticks out 1 3/8 in past the inner part of the quarter panel lip.

Passenger side had 1 3/4 and 3/4 respectively as above.

Question: Is it common to have such a variance between the two - and as such would this mean that I will need to have separate rear wheels with two separate backspacing - if so - is this common?

What would you recommend that I go with in Backspacing? Am going to need the shave the lip alitlle - go with a 225 instead of the 245?

[68 SuperRam]

Side question - the top of my rear tires are about 3 in from the bottom of the fender lip - no I need to ensure that the entire rubber is tucked under the lip or can some of the tire stick out past the lip - but gets tucked in when going over curbs / driveways etc. What about going over a speed bump? - Will it hit or not as the whole car is moving up and down - just looking for guidance. THANKS alot - I really want to order new wheels for the rear ASAP.

[MrQuick]

going over speed bumps should put the wheels up evenly, unless you low and go over them at an angle.The suspension wil cause the body to go up amost an equal distance.

now your 3 inches from the inside? that should be plenty of room but i can't tell be your description. going over bumps will cause your tire to go up and in abit.