20 sizing help
 

Looking at doing new brakes and 5 lug conversion on my 70 c1500 so now I am looking at new wheels.

Right now it has a 2.5/4 (i think) lowering kit but looking to go to a 3/5ish kit with either a ridetech streetgrip or airbags cannot decide. But for now if I plan for that drop I figure I can figure out the wheels.

I would really like to run in front 245/45/r20 & 295/40r20 in the rears as I would like to get as much rubber in the rear without tubbing as I should have close to 500hp but plan on a supercharger in the future so 600-650hp should be realistic easy.

I am looking at the detroit steel wheel d town smoothie. With the tire sires I want to run I figure I have to go 20x8 in front and 20x11 in the rear. What should I run as back spacing and also can I run a 20x11 in the rear without tubs? A little nervous of the rears.

Re: 20 sizing help
 

I don't remember what backspacing I ordered, but this is a 2/4" static drop with 20x8.5 in front with 245/45-20, and 20x10 rear with 295/40-20. No mods at all.

Attachment 2439329

Attachment 2439330

Re: 20 sizing help
 

Couple things you might want to know before you move forward with it. (Not assuming you dont know, but just trying to help)

Speaking from the 67 model year on up through early 1970 the rear axle housing was 1.5" narrower than in the later years. Im not sure if this is the reason but my assumption would be in 71 when they went to disc brakes up front they had to adjust the track width in the back to match up. Makes sense to me.. but this is definitely something you want to verify before you buy your wheels. Different trucks can fit differently depending on the model year. 1970 is the odd ball.

I can see the people in the production plant just using up what stock they had before they transitioned to the later axle housing. 1970 was this 'transition year' so the early build trucks got the more narrow axle housing. Later builds got the wider one. How do you know which one you have?

The early housing (63-early 70) all 6 lug. The axles are 30.5" long measuring from the end of the splines to the mounting surface of the wheel. So if you have the narrow housing, 6 lug for example. You want to make sure you get the 30.5" long 5 lug axles doing your conversion.

The later axles (late 70 - 72) the axles are 31 5/16" long. Splines to mounting surface. ** can be either, 5 lug or 6 lug. Some 1970s had the wide housing and still 6 lug. If you have the wide housing, you'll want the 31 5/16" axles when you go to 5 lug.

Considering the wide housing is 1.5" wider than the early one (.75 on either side) this makes a big difference in your wheel backspacing.

Here's how you know / how to check.

Laying underneath the back of your truck. Have a buddy / wife hold one end of a tape measure against the brake backing plate on one side. Measure across to the other side at the same point. So "plate to plate" the early axle will be 55.5" long. The later axle is 57" using this same method. This is just how I do it..

But - the wider axle housing can mean your wheels either fit, or they don't.

Fleetside truck, early housing. I use a 20x10 with 5" backspacing. 295/40r20. Or 295/45r20. *Adjustable trackbar will be required*

Fleetside, late housing 20x10, 5.75" backspacing (this is to accommodate the rear axle housing length difference)

If you get really low (5 /7 ) I'd go 6" backspace on the late housing.

The early housings are ideal, and preferred (just my opinion) if you decide to build one, 4 wheel disc, etc. Your wheels will look wider from the outside, they'll have .75" more "lip" on the outside because that added axle width doesn't require you to sit the wheel that much further under the truck.

If you have questions let us know man, happy to help. Just saying I wouldn't order wheels until you know what rear housing you plan to use. 1970 is the questionable year because they can be either width.

Re: 20 sizing help
 

^^ just to note, I was assuming fleetside. If your truck is a stepside these measurements will not work. Nothing wrong with a step side, just saying the fenders are a little tighter on a step.

Re: 20 sizing help
 

When I do wheels and tires on a hot rod project I always bolt up the wheels before mounting the tires checking for clearance. Then if everything looks good I have the tires mounted. The nice part about the newer tires is that their short side walls is that the tires are just a bit wider than the rim in many cases.
But being the paranoid guy that I am I always have the tire shop spin the wheels on their balancing machine before mounting the tires looking for wheel run out.

Re: 20 sizing help
 

My 20x10 rear wheels rubbed inside wheel wells. Went with 4” wider tubs to accommodate for the narrow axle.

Re: 20 sizing help
 

thank you great info here ....with CRS Im not sure what axel /rear year I swapped in....ill take you up on your help when I get to wheels and tires

Re: 20 sizing help
 

Thank you for this info! This helps a lot. I will measure the axle tonight.

I do have 6 lug currently with 71/72 discs on the front. I plan on switching to 5 lug, knowing the axle difference helps.

Detroit Steel Wheels only makes a 20x8, 20x9, or 20x11 so I am imagine either way I am stuck with 20x9 as it sounds like 20x11 will rub correct? I was hoping to get a 275/40r20 in the rear as I have 245/45r20s planned for the front.

Re: 20 sizing help
 

A 275/40-20 on a 9" wheel will fit out back. Backspacing will depend on axle width. If it's the later/wider housing, it needs additional backspacing for the wheel.

Re: 20 sizing help
 

Oh awesome! I was afraid it was too wide for a 9". I would really like to run a 295 but I get a 20x11 is probably too wide without tubbing.

Re: 20 sizing help
 

Is your truck a Fleetside, or Stepside ? Making sure, is it trailing arm rear suspension, or Leaf spring ?

Re: 20 sizing help
 

x2..... This does make a difference on solid/accurate recommendations.

A 275mm tire is technically 10.8" in width (1.20 per inch of wheel width). Personally, I'd prefer a 10" wide wheel @ the minimum but starting with late model 17" wheels they've used 275's on 9" wheels from the factory. For reference, the 275mm on a 9" wheel is no different than the 245mm on an 8" wheel (9.64"/1.20 per inch of width).