Stock wheel question: How wide is the stock '78 16.5 rim, does it have 4" backspace?

[Treozen]

Hello folks -

I recently purchased a 1978 C20 Camper Special (2wD) with the 454, and she needs some things. One of those things, is new tires, and so perfect opportunity for new wheels.

I've been "doing the math" and a lot of reading about what fits, but much of the calculation depends on what is on there now. I know the tire is a 9.5 R16.5 - which is basically equivalent in height and width to a 235/80/16 (Radius 30.6, width 9.5 vs 30.8 and 9.3) So I've been using the 253/80/16 as my baseline. When measuring for clearance changes, I need to know if the stock 16.5 wheel is 6" or 6.5", and does it have 4" backspace? I've read posts from several sites that conflict - some say its a 6" rim, others 6.5 and other sites suggest 7". Backspacing is suggested at 4" , but again, conflicting data.

I could go pull a wheel off, but that requires going outside....and well....its cold out there, plus its dark. Who knows what's lurking around. Does anyone have a definitive stock wheel width and backspace for the 1978 C20 16.5 rim?

Thanks - and although you've clearly seen one before, the obligatory picture of the truck:

 

[Shorty81]

Welcome from Ohio

 

[bucket]

If 9.50's are the original size, the rims should be 6.75" wide, as measured between the beads (not overall width). I measured the backspacing on mine once before, they were in the 3.75-4.00" range iirc.

What are you looking to run? I've tried a number of combinations on my '78 C20 454 Suburban.

 

[Treozen]

If 9.50's are the original size, the rims should be 6.75" wide, as measured between the beads (not overall width). I measured the backspacing on mine once before, they were in the 3.75-4.00" range iirc.

What are you looking to run? I've tried a number of combinations on my '78 C20 454 Suburban.

The 9.5s on 16.5 are what's called for in the glove-box RPO codes, so its original equipment spec.

As far as what I want to run - ( and this is all for show - this is a 2WD and the only "terrain" its likely to see is either black top or the back field of my property) - but, ideally I'd like to see 285/75/16 on a 8"rim, with 4.25 backspace. According to "the math" and comparing the stock specification (granted I assumed a 6.5 inch rim, so I'll re-run everything at 6.75) they should fit. Ive also seen a few examples of this tire on a 2WD, but unfortunately never with an owner that can tell you anything about the fit. That being the case, I've been considering the same tire but in a 265/75/16 - that seems like a more commonly fitted size.

This is the truck that got me thinking about the 285/75/16 - according to the add material its running 16" wheels with 305/70R16 BF Goodrich All Terrain T/A Tires. It doesn't say anything about a lift and I don't see evidence of a lift kit in the chassis pictures. Its a '74 C20 Camper special 454 2WD, so other than year, same truck as I have.

 

[bucket]

I've run 285/75's on factory '99-up aluminum wheels and there was plenty of room. On 16×8's with 4" backspace, they should still fit just fine.

I've also run 12.00x16.5's on a 16.5x9.75" rim, they were close to hitting the fenders, but did indeed clear. I'm currently running 16x8's with 35x12.5" tires that have big shoulder knobs... I did finally have to trim for those.

I'll be traveling home tomorrow and can get some measurements the following day.

 

[Treozen]

I've run 285/75's on factory '99-up aluminum wheels and there was plenty of room. On 16×8's with 4" backspace, they should still fit just fine.....



......I'll be traveling home tomorrow and can get some measurements the following day.

I would definitely appreciate some measurements. After 45 years, its hard to say what has changed or settled vs the factory conditions - after this long, these trucks are almost all unique specimens! I'd really like to get the 285/75 in there if I can. I was thinking about buying at least one and having it mounted on the wheels I want to use, then asking the shop to try it on all four corners to get an idea of any issues - if none, buy three more. If on the other hand it looks dicey, buy four 267/75 instead and I'll have an extra 285/75 tire laying around......could always use a new flower bed ;-)

 

[Treozen]

Well, don't I feel like an idiot......

You must be registered for see images attach

There it is, right from GM......6.75" rim. I'm just not used to older vehicles that have any of the factory data plates or stickers....didn't even occur to me to look.

So there you have it, you there searching for this answer years after this post has been lost to the archives - The stock rims on a 1978 C20 Camper Special with the 9.5 r16.5 tires, use 16 X 6.75 wheels.