Hey everyone,
I currently have C7 replica wheels on my 94. 18x8.5 in the front and 19x10 in the back. The website says that the fronts have an offset of 56 and rears have an offset of 79 though I needed to use spacers (definitely in the front but I believe in the rear too) to push them out further.

Either way, I'm thinking about getting new wheels and I'd prefer not to use spacers in the future if possible. Does anyone know what offsets I'd have to look for to get proper placement on 18x8.5s in the front and 19x10s in the rear on a 1994 base?

 

Why do you prefer or what interests you regarding the 'stagger' you mention? Just because the '94 base was produced with a 'stagger' there's no argument for retention? An argument for 'stagger' from an OEM is drivability(easy steer)? I'd rethink this and maybe choose a 4 X something and 10 or 10.5 interests many.

 

I don't really have a preference either way. I know that I definitely do want to keep 18s in the front and 19s in the rear as I prefer the look. As far as width goes, I enjoyed the cheaper rubber in the front but it's not mandatory. I can always put 18x10 in the front but I still need to know what offset to look for.

 

I'm not a fan of 19" on a C4 so I might avoid this entirely BUT I understand the math and I could accomplish it but maybe first of all you determine the tire sizes you'd actually want to use and make sure you can match front/rear to a brand you like and can afford.. A 19" that most closely matches the 285/40 RPM is I believe a 295/30 and OE it's mounted on a 10.5 wheel likely(suggested rim width is likely 10 - 11), for the front maybe a 275/35-18, that matches RPM very close to the 275/40-17 and a prefered wheel width is likely 9.5 (suggested 9 - 11 maybe). Matching rear RPM is important to maintain speedometer/odometer accuracy.

Wheels I'd think something either +50 or +56 should work on either end, you could 'tinker' there with various branding and make decisions again on what you like and can afford.

Tire sizing and branding first I believe is important!


****@MatthewMiller should likely stop by and maybe offer up something more for you maybe even a suggestion to do something with a front closer to an 8.5" wheel spec.

 

I agree with everything WVZR wrote. The offset recommendations don't really change with wheel diameter, and they barely change (they don't really have to change at all) with width. The stock 8.5" and 9.5" wheel offset was 56, so it's safe to stick with that range for those widths. As you go wider, the offset becomes more critical. The widest practical rim width on a non-ZR1 C4 is 11", and at that width you really need 50mm offset. You really won't want any less offset than that on any wheel. If you get a wheel with too much offset, then spacers subtract from it to get a net offset. For example, the 19x10 you found with 79mm offset (sounds like C6 sizing) could be brought down to a net 54mm offset with a 1" (25mm) spacer.

Spacers or adapters are both safe to use if you do them properly. Mostly that means having long enough studs to ensure adequate lug nut thread capture for plain spacers, and having short enough studs so as not to hit the back of the wheel and keep it from seating for the adapters.

Please make sure you get the proper 5x120.65mm (5 x4-3/4") bolt pattern. Do not get 5x120mm, as they are not the same and that is not a safe fitment. You may have trouble finding a 19" wheel with the proper bolt pattern and proper offset. You may have to get a wheel with too much offset and then use spacers. But look around. I agree with WVZR that a C4 aesthetically looks best with 16-18" wheel diameters and that 19s are a little too big to look proportionate on a car as small as a C4, and also I prefer no stagger in the diameter...but that's a matter of preference rather than function. As long as you keep tire sizes in the right range, they will function the same. Shoot for tire diameters in the range of 25.6-26.0" on both ends and you'll be good shape.

On a front 18x8.5, You could try a 275/35/18, but that would be a rather "pinched" fit for that narrow of a rim. A 245/40/18 or maybe 255/40/18 would be better on that rim width. A 275 really wants at least 9" rim width for best fit and performance, and can go on wheels much wider than that. For a 19x10 rear, 285/30/19 and 295/30/19 are both good sizes to use.

 

Thanks, all for the help. I'm somewhat surprised at the negative sentiment surrounding 19s but like I said, I think it looks great. The idea is to upgrade to C5Z/C6Z rotors and calipers in the future to fill up the empty space but we'll see.

I'm currently running 285/35R19 in the rear and 255/40R18 in the front. I wouldn't mind going 295/30R19 in the future depending on rubber costs. Either way, I appreciate the help and for someone else doing the math on the offsets. While I currently use good spacers and haven't had an issue, if I'm going to buy new wheels in the near future, I figured I'd see if I can get away with buying the correct offset to not need spacers.

Thanks again. ​​​​​

 

I agree that 19's are too big, and here is why: The rear of the C4 is rather compact...minimalist, IMO -which I love. The smaller wheels are proportional to the size of the rear of the car. The rear of newer cars are huge; tall, big slabs of rear 1/4 panel acreage...they NEED a giant wheel to be proportional to that acres of sheet metal and paint that surrounds the wheel opening and still, the wheel can look on the small side (see Gen II CTS-V coupe, for example Also. look at the rear of the BMW X////M ).

When you start putting 19's and 20's on C4's (and other '80's and earlier cars) it looks like a covered wagon wheel....it is out of proportion, and it looks like someone is trying to make the car, something that it's not. That's one perspective.

EDIT: And C5/6 rear rotors aren't going to help the rear cavern appearance at all; the C6 rears are ABOUT the same size as the C4 rears.

 

To each their own! It's a beauty of our Corvette community.

 

Yep!

 

FWIW, a lot of people clearly agree with your opinion because lots of people have 19s and staggered diameters. To each his own on that.

Tire diameter is another matter, because it's not just a matter of aesthetics but also a matter of chassis dynamics. That 285/35/19 you're running is 26.9" in diameter. Even if it fits without rubbing, it raises the ride height and therefore the rear CG, and it also raises the rear roll center. The effect is to reduce rear grip, which is not a good idea even if you aren't trying to track or autocross the car. Especially for a car and driver that only see street duty, you don't want the rear to lose grip first or be unpredictable, and that's what this taller tire does. This is not to mention the negative effect it has on overall effective gearing, which is not dangerous but does make the car a little slower. So unlike the wheel diameter, this is not just an aesthetic matter but an important matter of function. I would strongly recommend staying in the range of 25.6"-26.0" tire diameter front and rear on a C4.

 

You forgot about increasing unsprung weight and rotating inertia.

 

After moseying around the internet to find options, I think I'll probably revert back to 18s in the front and rear. MMR M755's specifically, most likely 18x9.5 in the front and either 18x11 or 18x12 in the rear to get at least 315s back there.

I must admit, I'm not great at wheel/tire math so I appreciate the advice from everyone. The MMR M755's have an offset of +50 in the front and +55 in the rear (with 18x11, +50 with 18x12). What size tires would you suggest to get them as close to flush with the fenders as possible while keeping the diameter that you recommended?

 

Those wheels will look outstanding on a C4! They are a replica of the C7 ZR1 wheel.

18x9.5 with 55mm offset in front is just fine. You can do 275/35/18 up there for a 25.6" tire diameter.

In back, 18x12 is going to poke out of the wheel well. 11" width is the practical limit for any C4 except a ZR1. With an 18x11, you are going to want 50mm offset, and it's pretty critical. If they come with 55mm offset, you will want to run 5mm spacers, I think. You could try them without, but check very carefully to make sure they don't rub anything on the inside. You can run 315/30/18 tires on those easily, and they would also have 25.6" diameter. The only difficulty with this is that most of the tires in that size are targeted toward competition use. The Michelin Pilot Sport PS2 is a great summer tire but it costs $540 each. However, the Pirelli P Zero Rosso is on clearance at Tire Rack right now for $383, which is a great price. Problem is, they don't come in the 275/35/18 size for the front. The next best bet is probably the Falken 615K+, which was a competition tire but is relatively cheap and will be the longest lasting one for street use.

Last option: run 18x11 and 315/30/18 front and rear and get the Pirellis. They will fit at 50mm net offset (55mm minus a 5mm spacer), and they will badass. Here's a decent pic of that exact size combo on my 96:

 

Does the fronts being 18x9.5 +50 make a difference as far as tire size goes? As for the rears, I wouldn't mind a 5mm spacer to fit the 18x11 +55. I also don't mind paying a premium for Michelin tires. I don't track the car and I only put a couple of thousand miles on her per year.

 

You won't want to put 315s on a 9.5" rim width. It can physically be done, but they won't work well and won't even look good. 275 is normally the widest section width that works well on 9.5" wheels. The 275/35/18 is a perfect match in diameter for the 315/30/18 you want to run in back. If you want 315s up front, you really should use 18x11s in front (like I was using in my pic). You'll probably need 5mm spacers up there to run those MRRs with 55mm offset, too (the wheels in my pic are all 18x11 with 50mm offset).

As for tire choices, if you ended up running 18x11s and 315/30/18 front and rear, I would snap up those Pirelli closeouts. For a car that is really only going to see street duty, they should be great. There's no need to spend so much more on the Michelins. If you end up sticking with 18x9.5 and 275/35/18 up front and you don't generally drive the car in rain, then I'd recommend the Falken 615K+. It's cheap, very grippy in the dry (more grip than either the Michelin or Pirelli), and will probably still age out before it wears out with only 2000mi of street driving each year. It just can't be driven in snow/ice and won't be great in rain (but can still be driven in rain by taking it easy, if needed).

 

Matt,
I'm looking for rims for my '90 ZR-1 track-only car...
I've got C7 Z07 calipers with GiroDisc steel rotors...so caliper clearance is going to be tight. CCW said virtually anything C7 Z07/ZR1 diameter fitment will work, it's just getting the offsets right.

I'm going to be running slicks on it, and will buy some rain tires so I'm not screwed if the weather turns nasty for a track weekend. CotA is expensive enough without pissing away your weekend fee because it's raining.

I did slip my '19 Grand Sport tires/rims in the back to see what kind of issues there would be, and the first item I need to address is the hex-bolts on the trailing links. They take 3/4" or more out of the inside clearance, so I'll need to order some button-head bolts to replace the stock ones first before I go any further.

I know that many of the C4 ZR-1 guys have either widened their OEM rims to 12" and put 335's on the back.

 

On a ZR1 with its wider fenders, there should be zero issues running 12" rim width and 335-width tires. The C7 12" wheels are going to require spacers to fit properly. With 59mm offset, they will have 8.82" of backspacing, whereas 8.00" is the limit. You need a 12" wheel with 37mm offset, or else you need 22mm spacers (7/8" would work, maybe even 1") to push the C7 wheels out more. If those are 20x12, you're also going to be screwed on tire sizes, because even the 325/35/20 that comes on the C7 is 26.4" in diameter, which is a bit much for a C4. You might be able to make that work without rubbing, but the stock C4 ZR1 tire is 25.7" tall (only 653mm if we're looking at slicks). Are there even slicks available for 20s? How tall are they? I can't imagine 19s wouldn't fit, and I'd even be surprised if some 18" barrels won't fit over those brakes.

 

I think you misunderstood (or I wasn't very clear, more likely)...I'm only running the C7 Z07 rotors and calipers...not the rims. Anyone who makes an 18" or 19" rim that clears the stock C7 Grand Sport Z07 should work. It's just getting the backspace right to fit and clear the trailing links on the inside. There are some guys in the C4 ZR-1 group running 12" rear rims, so maybe they'll chime in eventually.

 

Okay, gotcha. I still stand on what I wrote: 8" of backspacing is the limit on 88-96 C4s, and this should include ZR1s. I don't think anything in their suspension geometry was different - they just had wider rear fenders to accommodate more rim width on the outside. I think a 12" wheel will fit within the rear fenders, but the most offset you can use while keeping 8" backspacing on that rim width is 38mm. You can take any wheel with more offset and add spacers to subtract offset. For example, if you find a 12" wheel with 50mm offset, then a 1/2" (12.5mm) spacer gets you 37.5mm net offset, which is right on the limit on the inside.