DunskisC3

Update and more info further down this post!

I read quite a few posts and received some help in a past post considering wheel options for my 75. I bought a set of chrome C5 wheels, and wanted to do a sense check before ordering adapters / spacers. I believe they are 1998 wheels with 8.5" on the front and 9.5" width on the rear (based on 1998 corvette spec sheet)

From the many posts I read to properly mount and run these wheels without issues, or off set TA's the following spacers are needed. Please let me know if I am mistaken. If you have the same set up please let me know what worked for you (any pics would also be a huge help)
As always thanks for the help (in advance).
Front Adapter / Spacer needed - 2.5"
Rear Adapter / Spacer needed - 3.0"

Here are pics and specs on the tires.
Front - P245 / 45 ZR17
Rear - P275 / 40 ZR18

Oldguard 7

The only way to know for sure is to measure the backspace on the new wheels and get the appropriate adapters. Note I said ADAPTERS not spacers. Make sure the adapters you purchase are made of quaility material and are hub centric. IMO spacers are not safe as adapters.

DunskisC3

Thanks, I have hear them called both spacers and adapters, but have noticed quite a few posts on quality. Not looking to spare a couple dollars and give up safety.

I will research older posts to learn how to measure the backspacing.

DunskisC3

I found another post on wheels that had a link which was very helpful in leaning how to measure wheels. It is: https://rsracing.com/tech-wheel.html#backspace

Below are the pics with stats for my original rallys, and the new wheels that are pictured in the beginning of this post. The new wheels have the GM stamp on the front but the widths seem to be different than the 1998 C5 spec sheet I found online. My measurements were taken with a thinner piece of wood and tape measure but should be within +/- 1/8" max. **update: For the overall width measurements in the diagrams below I measured from outisde flange to outside flange for overall width. From further discussions to measure width I should have taken inboard seat to outboard seat. Had I done that my measurements would have been the 8.5" Front & 9.5" Rear width which would match the 1998 C5 spec sheet. My measurement for the original rallys would also have been the correct 8" width.

I would like to defer to, and ask further advice from, those of you who have done this before and / or are certain you know how to calculate what adapters are needed front / back for my 1975 C3. (Keeping in mind I have standard T/A's and really dont want to go to off set if I dont have to). It looks like I would need 3" adapters on the front and 3.5" on the rear to maintain the same spacing as my originals????
Thanks again!!

Fast81

IIRC most people have been successful with 2.5" on all 4 corners...

vettehardt

You want your total back spacing (wheel and adapter) to be between 4" (original) and 4.5" Look on the back side of your new wheels. There should be an offset measurement cast into one of the spokes. It will be in millimeters.

Now you have a 9.5" wheel so at 0" offset, the wheel would have a back spacing of 9.5"/2= 4.75". Then you add in your offset from above after converting to inches. Yours should say +63mm on the 18" rear wheels. That equals ". So your total offset for the wheel is 4.75+2.48=7.23" To get a 4" back space you will need to subtract 4" from that 7.23-4=3.23" or 4.5" BS is 7.23-4.5= 2.73 That would be the with of adapter required. So a 2.75 up to a 3.25 would work for you on the rear.

Do the same thing with the fronts. The offset should be 56mm or 2.20". I come up with a 2" or 2.5" adapter.

If I were to use those wheels, I would probably run the 2.5" up front and 3" rear to get things to look right. I like the wheels to stick out further in the rear than the fronts.

I am planning on getting a set of 4 of the 17X8.5" in the near future for autocross use. I will still use the 3" adapters i currently have on my car to run 16X9.5" wheels. The extra with of the adapter helps to push the tires out further, therefore giving me a little extra track with on the car.

DunskisC3

Thanks Vettehardt!! That was just what I was looking for.
Appreciate the help

DunskisC3

Just ordered Adapters from Motorsport Tech. A bit more pricy than I expected (about twice as much as spacers).
They have some positive reviews here regarding quality. They build on demand so it looks like it will be two weeks until they ship.
Thanks for all the feedback.

Fast81

Wouldn't the 3" rears push the tire outside the body ?

thebruce

Any chance you'll share with us what you paid for the adapters? I have been keeping my eye out for a set of C5 wheels to do exactly what you are doing.

bluegtp

I got my adapters off ebay (others here have too)....no issues with them. Car drives straight and handles well, doesn't shake or anything. My fronts are hubcentric and the rears are not.

I have 3" in front and 2.5" in rear. moved ebrake cable and shortened fiberglass spring.

I did replace all of my wheels studs prior to running the larger adapters.

DunskisC3

The price for ebay spacers was about $45 and up depending on thickness. From what I found those were not hub & wheel centric which is important to me since I want to drive aggressively without worries.

I paid $360 for all four adapters from Motorsport tech (90 each). That did seem a bit high. Since I always have a bit of buyers remorse I also called EZ Accessory. They dont carry the 3" centric's, and I tried one other who didnt get back to me.

My pricing breakdown still makes sense. Pricing decent tires for my original 15" rallys I only found Mickey Thompsons (H - speed rating) which would be just north of $800 installed (I wanted a good speed rating so the firehawks for 15" wheels and the like were not good options for me).
The wheels and tires on this post cost me $300 off craigslist without center caps (wheels with okay thread are Z rated runflats and list at $1500 per set). Reproduction "corrrect" center caps from ebay cost $76 (shipping included). The adapters were $360.

My total investment is $736 for the setup in this post (not counting all my time in research )
From my point of view I saved around $100 from the price of the Mickeys alone, have wider tires front back, like the look of the larger chrome C5 wheels, and have top of the line tires to play with.

The only part that seemed a bit high priced where the adapters. Hopefully the quality was worth the price, and the adapters ship on time (another post remarked on delayed shipping from Motorsport tech). The owner assured me I would get them shipped as promised so I should be good.

Once I have all the parts I will take before and after pics and attach to this post. Hopefully there is enough info on this post for others like me to get all they need in one spot.

DunskisC3

The adapters came in almost a week earlier from Motorsport tech than expected and in perfect shape (although I did not know lug nuts for the new wheels were not included extra $35) I used 2.5" front and 3" rear adapters. Pricy but mounting them I see how the adapters being hub and wheel centric really helps to hold the adapters to the center of the wheels and rotor hubs.

I did have to move the emergency brake but did keep the original rear trailing arms without issue, and I did not notice any rub on the fronts.

Here are the pics of the car with the old rallys and the new 1998 C5 wheels. The car had not been cleaned nor where either wheels cleaned / polished but it does give a good visual transformation of the car when using C5 wheels.

Hope this post is useful. Let me know if you have any questions.

DunskisC3

Here is the car from the rear. The first is with the rallys (225 width) and the second is the new C5s (285 width). Neither pic is the best but hopefully it gives some idea of the difference.

CheezMoe

Looks Great! Well Done!

KevinK

Replace your wheel studs! The stockers are short and thin. You dramatically increased the grip...Which increases the strain on the studs. Get a set of arp studs before you snap a fees of your stocks, and have to wobble to the side andwait for a tow.

V8Ranger

Do you have any pictures of the adapters?

Fast81

WOW - those look great.
I'm adding stock 99 alum WW's to FAST81 in about 3 weeks.
What brand tires are those ??

blckslvr79

Great transformation!

I likey!

DunskisC3

Sorry for the delayed response, I had to pull the wheel to take the pics. To answer the questions and comments:
- The tires are Firestone Firehawk Z rated Run flats.
- Thanks for the advice on the studs. I didn't think of that. After I put the suspension parts on, and before I add the HP's I will replace them.
- The pics of the adapters are below.
Let me know if there are any more questions.
Have a good one!!
Tim