thinking of going to 285-30/18's on all four from factory C5 sizes.

 

YES when changing tire heights you must correct your speedo However on a C5 when making a gear swap you do not have to make a correction to the speedo BUT you Must make a "shift ponit correction" in order for it to shift properly at the right MPH ON AN AUTO TRANS
If you only add a small amount of tire height you will not need to make a shift ponit correction on an auto trans However if you add over or subtract over 2" of height then again you should make a shift point correction as well

 

Most of the handheld tuners can adjust for tire size.

 

circumference difference is less than 10%. 84" vs 78". I wouldn't do it unless it was free and easy.

 

I've never heard of changing the shift points. You may well be right. However, what I HAVE heard is you don't need a speedo correction IF YOU HAVE AN A4 C5. From what I've heard, you MUST make a speedo correction if you have the M6 T56 trans. Only the autos self correct on the speedo. This is what I've heard from 2 or 3 sources, one if whom worked at Wheel to Wheel Powertrain before they closed down......

 

Well the stock rear tire height on a C5 is 26.90 However the ACTUAL tire height is really only 26" which is a very small tire height Today when people run 20" rear rims the tire heights may be 29" which would be a gain of 12%----That is huge---Not a big change in the P/T 1-2 shift however when you get to the 2-3 shift and the 3-4 shift 12% is a lot Typically it should shift STOCK from 3-4 while normal driving about 39 to 47 MPH But while adding a +12% tire size in the rear those average normal shift speeds should be 12% higher--- Like from 44 to 53 MPH otherwise it will short shift
When going to WOT again it can cause faults--something they call the "spilt" the difference between the WOT shift RPM and the SHIFT at WOT SPEEDS There must be at least 600-800 of an RPM SPLIT between these two parameters The more modded your engine is the HIGHER the split has to be---If the split is too close the trans will not shift at it's designated programmed RPM and will hit the safety rev limiter--This is confusing stuff-
Good thing about EFILIVE is that they give you you these WOT shift speed splits--- If your trans will not shift at WOT and hits the safety rev limiter then you will see what I am talking about On a stock engine you may not hit the safety rev limiter only trying it out will tell But I will certainly tell you it will short shift at P/T at very low RPM's unless you correct the shift points

 

The speed sensor is on the differential carrier beside the ring gear. So, the PCM gets the rotating speed of the axle. The speedometer is driven directly by a PWM signal from the PCM.

This means a tire diameter change will require an adjustment to correct the speedometer. A gear change does not need a correction to correct the speedometer.

Most of an auto shifting is decided via %TPS and speed.

The acceleration rate of the car changes when you change the gears or make a significant change in the tire diameter (or tire inertia for that matter). There is always a delay time from when the conditions are met to shift until the shift is completed. So, for easy numbers say the 1-2 shift takes 1/2 second. Also say the engine in stock form accelerated 300rpm every 1/2 sec. So, the PCM needs to start shifting at 5700rpm if you wanted the shift complete at 6000rpm. Now, change gears so the engine accelerates 500rpm every 1/2 second. The PCM now needs to start shifting at 5500rpm to complete the shift by 6000rpm. The change in the engine acceleration rate is why just fixing the speedometer or speed in the PCM isn't enough for an auto car.

Generally speaking, small tire changes don't affect the shifting since both the cars acceleration and the speed signal error are small, But, if you change the tires far enough you do get the effect described above.