18x10 backspacing question

[Bad70sbchevy]

Hi all, was looking on getting some 18" rims on my 1970 c-10. Currently I'm in the process of converting it over to disk brakes front and back, swapping to a 5x4.75 bolt pattern, and a 2.5" drop spindle with 4" drop coil springs in the rear. With that said I'm looking at running the following tire combo:

18x8 with 275/40r18 with 4.5" backspacing fronts
18x10 with 295/40r18 with 5.75" rear

I think the front combo will work, but how about the rear? It is a fleetside, but was curious if I had to run a custom backspacing (looking at torq thrust IIs) to get them to fit or not as I'm currently running 15x10 all around with 4" backspacing.

Axle is the stock 12in btw.

[SCOTI]

Originally Posted by Bad70sbchevy

Hi all, was looking on getting some 18" rims on my 1970 c-10. Currently I'm in the process of converting it over to disk brakes front and back, swapping to a 5x4.75 bolt pattern, and a 2.5" drop spindle with 4" drop coil springs in the rear. With that said I'm looking at running the following tire combo:

18x8 with 275/40r18 with 4.5" backspacing fronts
18x10 with 295/40r18 with 5.75" rear

I think the front combo will work, but how about the rear? It is a fleetside, but was curious if I had to run a custom backspacing (looking at torq thrust IIs) to get them to fit or not as I'm currently running 15x10 all around with 4" backspacing.

Axle is the stock 12in btw.

I like my tires within the fenderwell lip & able to clear. Some people like a 'flush' look. Each requires a different back spacing.

The 12bolt housings had 2 different lengths in 1970 (depending on the build date). There is a possible .750" difference per side depending on the housing. I would verify which one you have. With a mild drop, the wheels in the rear will likely be just under the fender lip opening w/the wider housing. My 68 had a more aggressive drop & I did 10" wheels w/6"BS (narrower housing) so it wouldn't rub.

On the front.... The disc brake swaps usually push the track width out .500 - .750" per side vs the original drum set-up. I would want 5 - 5.25" back spacing on my front wheels w/the disc swap. You'll prob be fine w/the mild drop but it's something to keep in mind.

Also... Food for thought. It's a personal thing but I like the widths as consistent as possible front vs rear. I don't like a 'stretched' tire on one end & a 'pinched' tire on the other.

Your proposed 295/40-18 on a 10" wheel yields 29.5mm of section width per inch of wheel width for the rubber. That same 29.5mm amount per inch of wheel width 8" wide fronts would be 236mm section width for front to rear consistency. A 235 or 245/45-18 would get you close in the height but more consistent w/the front to rear tire visually.

[SkidmoreGarage]

SCOTI pretty much nailed it. I have an 18x10 5.5" BS on the rear of my truck and I had to use a spacer to get the track width I wanted with the narrower housing. This was mainly due to the front track width/offset. There is a lot of room under the truck bed, so clearance isn't usually the issue.

I would only add that you may want measure your existing front and rear wheel mounting widths to verify what you have. A lot of things can change on an older truck. I personally prefer the wheel lips to be in the same plane and the tire sidewalls to have a similar amount of bulge (what SCOTI is referring to with the tire section to rim width). Some folks like the old drag look with rear wheels tucked in and huge sidewall bulge.

A test fit of the actual wheel is worth a thousand words, I used Summit's and Jegs' generous return policy to try a couple wheels before I committed. I called around locally and they wouldn't even take an unused return without a huge restocking fee.

[Bad70sbchevy]

Thanks SCOTI and SkidmoreGarage. I did some measuring on my truck and on the rear axle, from the factory drum brake mounting face to the other drum brake mounting face, it measures 64". Now I ordered some axles from Moser and they measured 31" to the outer flange (they asked for a year and not a measurement). Curious if those were the correct axles...

Good call on the section widths, I'm not too much of a fan of the stretched look either so I might play around with the tire size up front.

Main thing is I'm just trying to make sure the rears fit within the fender wells (and from yall it sounds like it should). I think if I bump up to an 18x9 I'll get at 5in backspace.

[SCOTI]

Originally Posted by Bad70sbchevy

Thanks SCOTI and SkidmoreGarage. I did some measuring on my truck and on the rear axle, from the factory drum brake mounting face to the other drum brake mounting face, it measures 64". Now I ordered some axles from Moser and they measured 31" to the outer flange (they asked for a year and not a measurement). Curious if those were the correct axles...

Good call on the section widths, I'm not too much of a fan of the stretched look either so I might play around with the tire size up front.

Main thing is I'm just trying to make sure the rears fit within the fender wells (and from yall it sounds like it should). I think if I bump up to an 18x9 I'll get at 5in backspace.

The drum brake 67-70's look good w/the old factory 6-lug 4x4 Ralleys on the front & rear. They are 15x8 w/4"BS. So some 9" wheels w/5"BS out back would maintain that spacing. The fronts would get pushed out thus usually wanting additional back-spacing.

The issue might be finding wheels @ a budget friendly price-point that have the back-spacing/offset desired. It's harder than it seems for anything beyond the Average Joe.