Hey Guys,

This is my first post so sorry if I am in the wrong place!

I bought a set of c7 19/20 replicas from a forum member (Awesome look!). I had the stock gumby 18/19 set on the car before the buy. When I got them put on, I noticed a very noticeable change in acceleration. The gear change seems sluggish and the take off is poor. It also takes a heavier foot to really get her to open up.

I know the added weight and diameter will slow me down, but wasn't expecting such a drastic change! Has anyone else faced this problem? Are there any modest mods that will bring my car back up to speed?

I have heard changing the gears to a 3.42 would do it. Mind you I am very green in everything mod, so I am open to suggestions. I don't see me driving any faster than 180-185 in the car ever, so am willing to sacrifice some top speed for better 0-60, 60-120.

My budget with labor is about 1200 ish. I can go up or down depending on the gain to be had, but not by much. I understand labor is a large variable. I am in the Atlanta GA area if anyone has suggestions!

Thanks in advance guys!

 

I don't know this for a fact but it almost sounds as if something's off with the stagger of the fr. to rear wheels which is causing some traction control to come on and limit acceleration. I wouldn't think that the added size/weight of wheels and tires should make such a noticeable difference. Someone correct me if I'm wrong.

 

18/19 to 19/20 is easy.
Just go from 45 fronts/35 rears, to 35 front and 30 rears instead.
This reduces the OD of the larger rim/tires to get them back to very close of the smaller rim/tires OD's to begin with.

Also, make sure that the rear tires are 1.08" larger OD than the front tires. If the OD ratio sizes are not sized front to back correctly, then the car will freak out since the ECM thinks that the front tires are not rotating the same speed as the rears.

Lastly, with the different OD size tires on the car, make sure too have the speedo recalibrate (AC delco speedo shop will do this cheaply if you don't have a tune on the car and can change the needed ECM programing instead in the tune).
Note, having an AC Delco speedo shop do the ratio change in the programing will not void the warranty, while having an aftermarket tune will.

 

Thanks for the replies! I have the tire sizes you recommend already on the wheels. 19s up front with 35 and 20s on back with 30. I have noticed my speedo being a little off, but will the tune fix my actual acceleratuon or just fix the speedo display? Not 100% sure what you mean by 1.08 though. Sorry.

 

It is a ratio. Our vettes do not allow for the front tires to be significantly smaller (or larger) than the rear tires. There is no way to "tune it out" either. They have to be within the correct "ratio".

The front to rear tire diameter ratio is tied into numerous systems in the vette (for handling purposes), and because it is a safety related issue, GM made it practically impossible to edit features relating to it.

Sooooo, if your front to rear tire ratio is incorrect, your only option is to replace them to get closer to the same diameters......

 

Has to do w. diameter, actual tire diameter. Compare your orig. tires' diam. frt. to rear; you'll get a ratio. Do the same w. your new tires; the ratios should differ as little as possible fr. original. Supposed same size tire but different makes can sometimes have slightly diff. diam.

 

The first post referred to a 1.08 INCH allowable rear to front difference before AH/TC hell is unleashed. Your post refers to a ratio. I need to decide on a wheel tire combo for my 2012 GS nearing completion that can hook 800 RWHP at the strip but not trigger all the bad computer intervention. I have been searching for some time for the definitive answer as to the MAXIMUM allowable rear to front diameter in terms of absolute measurement or ratio. There is much conflicting information on the subject. Any help is appreciated.

BIGMAC

 

Being that the front tires need to be very close to 1.08" diameter smaller than the diameter of the rear tires.


If you don't know the OD of the tires, then just put a string around the front tire and make a mark on the string, then put it around the rear tire to make a mark, and make sure that the mark on the string is close to 1.08" longer for the rear tires.

Or, if you are sure that the rim widths are the correct size for the tire widths and not ballooning the tires on the rims, you can use this. Just plug the front tire in at the original tire, and the back as the new tire size to get the difference is tire OD sizes.

http://www.sizemytires.com/calculators/compare

The computer reading the 4 wheel hubs sensors is set up to read the hubs close to the 4.24% OD difference from the front to back tires, and when the front to back tires sizes OD sizes are the wrong ( needed to get the needed difference ratio of the back tires being 4.25% larger OD), then the computer does not see the front and back tires moving at the same distance/speeds,and first tries to correct the problem, and if it can not, put the car into limp mode instead.



As for the speedo being off, its a quick fix in the computer programing, and again, if you have the car tuned (already paid to the $100 for the tuner to use of the software to tune the car in the first place), your tuner can make the change in the tune.

If you don't have the car tuned, then its cheaper to just have a AC Delco speedo shop make the changer in the programing instead.



Really, what size tires did you put on the car?

Doing the math on a say a 285/35/19 to a 285/30/20 for a base model rear tires, has the 20" tires smaller in OD that original 19" tires, and this alone has made your gearing lower already.

Same goes for 325/30/19, to 325/25/20 for the Z06 back tires.
Note, Z runs 35/30 front and back, and with the 1" increase in rim size, you go to 30/25 instead, to try to hold the same tire OD sizes


Hence Z06,
Front: 275/35/18
Rear:325/30/19

While for a ZR1 with 1" larger rims front and back to clear the carbon fiber rotors,
Front: 285/30/19
Rear: 335/25/20

 

Ok got it. But with your suggested tire size on this size wheel, am I still missing some piece of information that would allow us to rule this possibility out? I can physically go measure the wheels if it will solve this. Also, if this is going to stick, I'm still looking for ways to boost accel within my budget. I've heard tune more than once but that's about it

 

Thanks Dano. I think I have the 285-35 285-30 setup. I will triple check when I get home. If this is the case, your suggestion is for me to move up on the rear to 325? If so this can be done pretty quickly

 

The computer is looking for the rear to be 1.08"/ 4.24% larger OD than the front tires. There is wiggle room in these numbers for tire wear, but the faster you go, the smaller the wiggle room before the computer freaks out and sees the hubs not moving at the correct speeds against each other front to back (sees it as either the back tires spinning/the front of tires locking up/ or the opposite depending on the tire sises, tries to correct the problem and when it can not since is a tire size problem, the car goes into limp mode).

As for the strip (over road course corner handling), your going to be working in the opposite rear rim direction as the op, being you want smaller rear rims with higher/softer side wall tires instead for launch grip.

Once you figure out the rear tires, then the easy part is just finding a front rim/tire size for the needed match (read front rim can be larger that the rear).

At 800HP, and the drive line that will hold up for a solid hook/brutal launch out of the hole, and set up to keep you out of the rail on launch, I would be looking for 15" rear rims with bead locks so you can run the race slicks as low pressure, and not just slip them off the rims.

Short off this on a DOT tire, will need to be ginger off the line to not break the tires loose on launch/blow the drive line/put you into the rail, and try to make up time on the far end of the track instead.

Launch of a 10 second corvette

Launch of a 8 second Vega making the same power as the above corvette.

So on that note, here is how hard you are trying to launch for a quick 60 time, so you don't have to make up time on the far end of the strip instead,

 

If base model, then should be 245/35/19 front, and 285/30/20 rears as your 1" inch size up in stock widths.

If the rims are 8.5" front with a +56 offset, rear 10" with +79 offset, then you are safe to 255/35/19 front, and 295/30/20 rears (could go up to a 305's, depending on the tires actual width and it's not going to balloon out on the rim). I bring this up since you can pull it off with super sports since they tend to run on the narrow side, while something like a Kumbo XS tires are slightly wider in the same sizes, and a no go on the 10" rear rims.


As for ZO6/ZR1 wider tire sizes, wider rims and wider fender on these cars to pull off the wider tire sizes.


So lets back this up, and start with if your car is a narrow body base model, or a wide body/fender z06/gs style model.
Next what are the rim widths and off sets front and back.
Lastly, what brand/model tires are you installing?

Hence once it all said and done, you still need to keep the tires inside the wheel wells. Tires sticking past the wheel well and one good bump to crash the tire into the fender, and you will be buying new fenders as you shatter the SMC fender panel.

 

Id like to chime in here as I am about to have this same problem as the original poster here.
I have a Z51 that I am upgrading to big brakes, through research I have found that a C7 Rim will Bolt right up with no needed spacers or stud extensions. Same as the OP here, we have OEM 18x8.5 front and 19x10 in the rear.

So..to his question (I think?) and mine....what is the correct tire size to add the C7 style rim in a 19x8.5 and a 20x10, so we keep the tires inside the wheel well and not loose any performance and not have the computer freak out.?

I think for the OP here, he's just in need of a tire size change as he already has the 19x8.5's up from and 20x10's in the rear as far as rim diameter goes.

 

It is probably the extra weight. Oem wheels are pretty light and i would guess those replicas are 6-8 lbs heavier each.

 

I'm surprised the C7 wheels don't look more aggressive on a C6, especially a base C6. The back wheels are pretty sunk in. Definitely do a little research on offsets and backspacing to get the right fitment next time you change the wheels. I don't mean to sound negative, just trying to help! Good luck with the acceleration issue!

 

Dont know if i am misunderstanding the OP question , if the tire front to rear height is off by more then a certain % you will get a DIC warning like service TC or calculating, I have run a lot of tire combo's and the taller heavier tires will slow the car on take off and in the 1/4 by as much as .50 or more its rotating weight, I have ran 27" tall down to 25.5" with the same fronts with no adverse effects to the system , and the lighter settup always the fastest

 

So that link provided to compare tire sizes is nice...If I look at it correctly the suggested size on the OP's 19/20 setup, if he runs (and what I guess I would run when i do this) is...
245/35/19 front, and 285/30/20

here is what the site says is the diff between the OEM 19's and the new 20's with the suggested size 285/30/20

 

Brians1,

C7 Z51 rims,
Front Wheels: 19" x 8.5" +56mm
Rear Wheels: 20" x 10" +79mm
Which are the same widths and off sets of the C6 base rims, just 1" larger around.
Note, base C7 rims are the same as the base C6 hence,
Front Wheels: 18" x 8.5" +56mm
Rear Wheels: 19" x 10" +79mm
I bring this up, since if you want the 19/20 rims, you want the C7 stingray rims, not the base C7 rims.




So if on a Base C6 model car (narrow fender model) then the stock size up 1", but a lower sidewall profile to keeps the tire OD's close to the stock OD's sizes.

So to match the tires you have on the c6 base model in width and OD, but just 1"larger ID to match the new 1" larger rims,
Front; 245/35/19
Rear: 285/30/20
This puts the tires OD very close to the same OD size as what is on the car in OEM form.

If you want to go slightly wider tires, you are safe with, and will still fit in the wheel wells,
Front: 255/35/19
Rear: 295/30/20.
To add, since these are not the stock C7 stingray tires sizes as the 245/285's, they tend to be cheaper instead. Don't ask, but when a tire manufacturer make a OEM size tire for a specific car model (gets gouged by the car manufacturer to supply them in the first place as OEM), then want a lot more money for that specific size come later consumer buyer time instead (making the difference up in the retail market that the lost getting them OEM to the car manufacturer to begin with).

As for stepping the rear tire up to a 305/30/20, all depends on the tire brand and model. In the Super Sport tires, you can get away with it, while other tires like the Kumbo XS's, they are wider to begin with, and will end up ballooning on the 10" rim (less actual grip to the road since the outside edge of the tread to the ground will be rounded up instead).


Now the huge note, the 1" larger, lower profile tires are not the exact same OD sizes as the smaller OEM 1" rim smaller tires, so the speedometer will be slightly off.

From 285/35/19 (OD 26.86 in), to 285/30/20 (OD 26.74 in) tires

When your speedometer reads mph70 with the new tires, your vehicle will actually be traveling at 69.69 mph, which is 0.44% slower than the original tires.

 

Thanks Dano! - makes sense now on the suggestion that C7 Z51 rims in 19/20's will clear the Z06 BBK for those of us who choose to modify on base/Z51 cars.

Im glad the OP started this thread as I would have been in same boat on why poor performance.....so long story short a C7 rim and tire swap for base and Z51 guys is use the 19/20 C7 Z51 setup and you can bolt on and go with very little diff in speedo and perf and if applicable also clear your BBK setup.

 

Just checked fellas. I have 245-35-19 up front and 285-30-20 rear. So now that we've cleared up that I have the recommended tire size, any other ideas why I'm dragging? No other mods have been done to the car. I was asked about offset. Where can I find those numbers on the car? Also would still love to hear an opinion on the 3.42 setup. Also, any other benefits to resetting the speedo other than it being off by a few clicks on the display?