KJL

I would like to run slightly larger diameter tires on my 96 collector. It has the z51 wheels which are the 275s all around. I would like to run the non z51 stock tire sizes instead. I believe they are 285 40 rear and 255 45 front. They are about .5” larger in diameter. My rims are all 17x 9.5s. Will these tires fit? I don’t see why not but the suspension my be set up lower for the z51 option. The car is just so damn low to the ground. My front spoiler is getting wrecked.

bjankuski

They will fit.

WVZR-1

255/45 isn't going to do the 9.5 width BUT I'd concentrate on correcting the 'trim height'. I've a friend with a similar issue and he actually trimmed the lower deflector to get to where he was/is happy with the issues.

Measure the actual trim heights using the FSM specifications and measuring points.

QCVette

The base size tires will fit on all four 9.5" wheels. If you look at sources like Tire Rack, 255/45/17 size tire shows it has a recommended wheel size that includes 9.5". The 285/40/17 tires came stock on the base cars on 9.5" wheels. In short, they will fit. The fronts will be at their limits and some will look a little stretched to fit the rim with no sidewall bulge. There are variations in widths between different brands/models of tires in the same size, so a narrower tire will stretch more than a wider tire. I ran 255/45/17 tires on 9.5" rims (stock C5 Z06 wheels) and I thought they were OK. See the picture below.



Another option is to run the 285/40/17 tires at all four corners. I have done this and it works fine.

Good luck.

WVZR-1

Fit? I'll maintain the thought that it's a 'stretch' that many/most aren't happy with. The OP mentions .5 diameter and I also doubted that specification would improve what the OP mentions his issue is. It's a waste of $$$ in an attempt. A change from 'summer' air to 'winter' air will likely accomplish as much!

Once you halve the diameter difference what's accomplished? Nothing likely!

The OP has some very nice C4's - I'd think it much wiser to change or correct something that will 'satisfy' long term.

KJL

Thanks all for the feed back. I like the trim idea but I also need a new set of tires anyway. I am down to the indicators on a set of very old tires. I figured if I need to buy a new set, way not go a little bit bigger in diameter. I also like the 285 all around suggestion. Perhaps that and a mild trim on the spoiler will stop those embarrassing road scrapes every time i go over a speed bump or attempt a steep driveway etc. I bottom out on relatively low speed bumps on the transmission even when taking it slow. The road directional road spikes that are often used in pay parking lots rake across the bottom of the car. Surprisingly they do not seem to do any damage. At one time I considered a quick automatic car wash.....that turned out to be a bad idea. That did gouge my floor pan.

WVZR-1

If the 'trim height' is corrected OR modified it's a done deal. Tire diameter you've mentioned I doubt is a solution. If trim height is equal both sides and the spring is undamaged I'd think modification of it is a longer term solution. The transmission scrapes etc hint there's trim issues. Needing tires soon I don't believe a valid reason for an attempt.

Use the information in the FSM section 3 to check trim heights I'd think an important first step.

Being at the 'wear indicators' of your current tires NEW correct sizing is going to increase to nearly the entire 'tread depth' of new to your tire radius concerns.

KJL

After looking at the FSM at the recommended trim height it appears my car is about 1" too low. Given the tires are worn that would put me closer to maybe .75". The only thing I can think is the springs are getting tired but the car seems to be sitting OK. I will stick my head under the car and see that all shims etc. are where they are supposed to be.

WVZR-1

This eBay offering is actually a later C4 fabrication and using the images you can see how the pads on the end of the springs are configured. If your spring is in good condition and trim height equal 'side to side' I don't know why a fabrication of sorts to stuff into the control arms to change the trim height. 10 -12 years ago an acquaintance did this and he was quite happy with his results. I've not seen him in probably 8 years but using 'zoom' on the eBay offering should allow you to understand options. Tires ain't a solution!! Disregard the eBay 'title' it is a '96 spring!

https://www.ebay.com/itm/1988-1996-C...UAAOSwSBFgAgXq

MatthewMiller

I agree with WVZR-1 that tires aren't the solution. You'll gain 1/4" of ride height from going to the base model tires. That's nothing - as you note, it's less than the difference between new or worn street tires of typical tread depth. Something is strange on your car: it really does appear to sit quite low up front, but appears to still be at stock ride height in back. I think the Z51 cars sit a touch lower, but not by much. As advised, check your front spring's condition, including the pads where it sits on the control arms.

Also, if you've never replaced the shocks, that could help. In fact, if the front shocks have lost their gas pressure that actually will cause a bit of ride height loss. You can remove them and compress them to see if the rebound back on their own. If they don't, they've lost their gas pressure and it's time to replace them.

If for some reason you decide you absolutely must get taller tires, the 255/45/17 will fit fine on the 9.5" wheels: that is within the rim width recommendations for all the tires in that size that I've seen (the 285s are good for up to 11" wheels!). You also get a lot more choices in that size than for 285/40/17. And frankly, the 285s look like balloons on the 9.5" wheels. But by going with taller tires you're giving up some handling/braking ability and making the overall gear ratio taller. It also won't look as good. It's not a coincidence that 275/40/17 was Chevy's choice for the "handling package" option.

KJL

agree....looking at the pic it does seem low in the front. The front wheel seems tucked up into the fender well more than most. I know the previous owner changed out the front shocks because he said the ride was too stiff. He gave the original shocks to me. The ride is very harsh now so I cant imagine what it must have like before. It seriously feels like the car has no suspension at all. It is quite unpleasant to drive actually on any road that is not completely smooth. A small pot hole that would go un-noticed in most cars feels like a crater in the car. I understand it has the performance suspension but damn! I will check it this weekend weather permitting.

MatthewMiller

Makes me wonder what shocks are on it. Also, it might be close to the bumpstops up front. You really need to go completely through the suspension to fully check it out for travel and proper function.

KJL

Going to be hard to check some of that with car on the ground. Ultimately may need to replace the spring. May start with changing to original shocks just to see if it corrects the ride height. I will also see what shock is used and post that info soon. Thanks for the help guys! I have been working on these cars myself for years and never occurred to me there was a suspension issue. Just assumed these cars were supposed to be this low in the front.

WVZR-1

Make a block that matches the height dimension currently now to the rocker @ (J) from the trim height section of the FSM, then you could just insert the block to gauge your improvement. Inspect the spring for damage and if all seems well I wouldn't be concerned with maybe creating spacer/shim like I mentioned.

I don't believe I ever mentioned tires being a 'solution'. I did mention I thought changing tire specifications/sizing was a 'poor choice' and I still do believe that foolish. I compared thoughts on sizing but I don't believe I ever mentioned tires being a 'solution'.

I also believe conversations discussing 255/45 on 9.5 wheels is foolish.

KJL

I believe what Matthew stated was likely a typo as he goes on to say "That is nothing". Implying he agrees tires are not the solution.

I did check out the front end yesterday and everything looks fine. The spring has the spacers between the spring and the a-frame, space between bumpers seems OK.....car never has bottomed out on the bumpers. I could not get a name off of the shocks.... they say made in USA and are black. I know they sell lowering kits that involves the center spring mount but I am assuming mine is stock. I am not sure I could tell the difference between OEM and aftermarket.

All this said perhaps the spring has just relaxed a bit over the last 24 years. There appears to be some extra thread ...about 3/4" at the center mount bolts that may allow an extra shim. Perhaps that, new shocks and new tires will improve things.
I still may go with the 285's...... not to concerned about losing a little torque. the car is not exactly a powerhouse to begin with. If I can figure out how to shim it up, I may stick with the stock size.

MatthewMiller

Yes, that was a typo of course.

WVZR-1

Yes I see he's done the correction 3 days later!!! I don't 'proof read'. Good catch on your part!
So your thoughts are maybe adding shims (#45 this image) to your current arrangement? I had thought of shimming perhaps in the 'control arm spring pocket'.




Here's a thread where it's been 'mentioned' previously:


https://www.corvetteforum.com/forums...post1574310366

The part # of the shim is correct for all C4 years. Maybe someone has one laying around they could provide you with dimensions. I don't believe I'd pay the eBay $$ for them.

KJL

Yes, Originally I thought about adding shims to where the spring mounts to the cross member but now I see there is a LOT more to it then that. I do need to get the car up on some ramps and look at that location to make sure those mounting points look OK and not lowered by someone else. The spring pocket ends seem to be the best, easiest and least invasive option. The attached diagram is for 84-87, mine is a 96 so it is a bit different

WVZR-1

My image is from a GM SPO Parts Catalog that is very generic for '91 - '96 and only shows the spring pad #39 that was unused through '91. The post I linked to was a comment by an LT1 owner, but tagged on to an early '84 - '87 discussion.

Reach out to @PLRX and ask him how involved the install might be. He's been here for a very long time. As long as me but few have that '20 year tag you've been blessed with.

KJL

Any suggestions regarding shim material and thickness? I was thinking aluminum, lifting the spring with a jack from under the a-arm and slipping it under the spring with maybe a little RTV to keep it in place.