I dont think I really get the 3% rule.
does the computer measure rolling difference from the front to back (overall diameter front has to be 3% smaller than overall diameter rear).

Most that have asked about this want to run too small of a rear tire. Like 245/40/18 front and 275/35/18 rear, with this you end up with a bigger diameter in the front and upset vdc.

so is this right? the stock 225/45/18 and 245/45/18 have a 2.7% diameter difference from eachother (meaning the 245/45 are 2.7% bigger than the 225/45 fronts). so they are within the 3% rule.

now..

what if you run 255/35/18 front and 285/35/18 rear? (i know that 245/40 and 275/40 is the right size or even 255/40 and 285/40)
thats 3.2% diff right? so does it mean I cant run this set up without vdc acting up?

I think I get it but I want to be 100% positive b4 I buy anything.
oh and...
I dont want 19s
I dont want to run 40 series sidewalls.
my car has vdc
please dont answer unless you know you are 100% correct.

thanks!

 

I assume that the wheel sensors are measuring wheel RPM. The computer is looking at the difference.

 

If you don't want 40's , try a 295/35 rear with a 275/35 front. Or you could do the 285/35 rear , but the proper front would be a 265/35 for best results.

 

The final rule of thumb is use a tire size calculator (1010tires.com) and get tire sizes that are as close a match to the stock tires. Example: the best fit for the 224/45-18 is a 255/40-18 and the best for the 245/45-18 is a 275/40-18. I to was confused by the whole VDC issues but better save than sorry. I would err on the side of caution and go with equal diameters for the F/R's. You do see configuration with a 245/40-18 F and 275/35-18 R. These two tire sizes are almost identical in diameter and if you have a base model without the VDC you might be OK.

 

did you read my post? i already know all of this and i even said my car has vdc and that i dont want to do 40.

 

Yes, you are keeping the front to rear proportion nearly the same so you will have no problems. A smaller diameter will only mean your speedometer will be off.

 

You ask a good question that you present very well. I notice you are not receiving many responses. That is probably because of your statement: “… please don’t answer unless you know you are 100% correct.”

“100% correct” is a concept and not a reality. The only persons who can answer your question are those with the exact tire sizes you mention and a VDC-equipped Z. That field narrows even more when you consider that VDC has evolved from 2003 models to its current configuration (making VDC another variable in the equation affecting your question and your insistence on absolute accuracy).

Since I do not know if I am 100% correct, I probably should not answer; but I cannot resist.

My answer is that you should not go with the tire sizes you mention (255/35/18 front and 285/35/18 rear). I say this because the rears are too short in overall-diameter and that is something that is problematic with VDC (more about this later). I see where you are going with this concept (down-sizing both front and rear by the same percentage), but that may not work as expected.

Here are some “absolutes” I’ve discovered first-hand from experience with tire sizing and VDC:

• The front tires can never have a greater overall diameter than the rears. This causes immediate VDC-failure. I believe this is well-documented.
  
• The rear tires on VDC-equipped cars should be within stock specification (<3% difference).
  
• You can go “all-square” as long as the rears are within stock specification for overall diameter, and the fronts are not taller than the rears.

So… answering your question… I would not mount the tire sizes you mention on 18" wheels with a VDC-equipped car. If you want to go with a 35-profile, get 19” wheels.

--Spike

 

I disagree with Spike. By keeping the ratio proportional front and rear, the computer will see no difference in rotation from front to rear (which the VDC is measuring). It will only effect the speedometer reading in the computer. So it will act only as if the entire car were accelerating faster (or slower if both tire diameters were increased) The car does not know the diameter of the tire therefore it will not have an effect on the VDC because the 3% ratio is maintained.

 

I think that your argument is valid. In fact that is what you should expect. However, it does not play-out this way (at least on the early VDC-equipped Z’s).

I have a 2003 Performance model and this VDC-equipped car “reads” the rear wheel rotation dynamics and expects a stock-sized tire. I did try 245/40-18” all-square (and that would be a slightly less overall-diameter on the rear) and experienced poor results. And, that was testing with various sized front tires.

I can tell you for certain that on a 2003 VDC-equipped Z you should not install an undersized rear tire no matter what size front tire you have.

I have no experience with VDC-equipped Z’s past 2003, so possibly there is a difference here.

--Spike
__________________________________________
EDIT: I did additional research and found that Nissan’s VDC is based in rotational speed (and thus connected to overall-diameter). That means you must mount a rear tire with the stock overall-diameter. If you mount a renegade size on the rear, VDC might not work as expected.

 

would i have problems with VDC running 245/30/19 in the front and 295/30/19's in the rear???

 

Yup... It will work with VDC. Your fronts are a little short (245/35-19 is better), but the rears are right. You are good to go.

--Spike

 

Correct. Wheels sensors do not “see” tire height or circumference. The VDC/TCS/ABS computer looks at wheel RPM. If the difference is within an allowable range, the computer takes no action. If the difference is out of range, the computer acts to correct the problem.

 

^^ Dughhh... That is obvious

How else would it work?

--Spike

 

Well if you understand how this works how can you make the argument that the car is going to know that the tire diameter is smaller in the rear if the 3% difference is maintained with smaller diameter tires rear AND front?

 

yeah thats what i was thinking becasue if that is how it works then it wouldnt matter if its smaller as long as you keep the 3% diff from the front to rear.

 

I'm a little short handed with cash right now and not really looking to get a new set for the front..how much will it affect VDC considering its just a little bit short??? i was thinking of getting 265/35/19's or 265/30/19's when it wears out..but yea it'd be great if u could let me knw if my front tires will affect VDC kuz then i would probably get them replaced a little sooner

 

^^ 245/30-19" front is short (overall diameter is outside specifications for VDC), but you probably can get away with this size if your rears are correctly sized (and I believe they are from reading your post).

If you are short on cash these days (who isn't with the horrible investment bank mess we've seen in the past two weeks), run your setup conservatively to see how it works. I'm guessing, but would bet it runs OK.

--Spike

 

^ thankz a lot..i'll see in about i week when i get my car back..waited for 2 months for my car..the whole exterior got redon

 

Can you show me this so called specification for the VDC??????

The only specification on this entire site that I have read is the FRONT TO REAR RATIO. Not just the front. Not just the rear. BOTH FRONT AND REAR ALWAYS NEED TO BE PART OF VDC DISCUSSION.

Like I have tried stating before, if you decrease the front EQUAL to the rear the VDC will see ZERO effect from the size changes. Contrary to what you might believe there is no little man in your car that is measuring the individual tire sizes

Might be a little harsh but I think you are missing this point

 

Your 245/295-30 combination will work fine.

The accepted variance front to rear is 0-6% and your combination is 4.5%