01torchred

I bought my 2001 Torch Red C5 new it only had 3 miles on speedo. It is still 100% bone stock with barely 50,000 miles on it. I do all maintenance on it myself.. In 2020 I wanted to go to a much larger tire wheel package. I went to 18 front 19 rears with front tire sizes of 255/40 ZR 18 Michelin and rear to 285/40 ZR 19 Michelin. I immediately noticed a power drop and believe that going so large of a tire I may have affected the trans and rear ratios ? I have a Predator programmer that I bought shortly after I bought the car in 2001 for simple tuning etc. It has the option to program tire sizes but apparently is so old of a programmer that it just states there is an error. I called Predator they no longer make my programmer , I asked about doing a programmer update and they said it would not be worth it that I should purchase a new programmer.... So For anyone who knows if going to such a large tire can change ratios and if reprogramming tire sizes will help to get the power back I would greatly appreciate a little tip or two.. My main question is what is the best ( or ) advised best programmer to get for my LS1 engine that would include the tire reprogram option, And possibly be used on other vehicles to do some tuning etc. I also own a 2017 Chevy Silverado LT 5.3 auto and would be interested in using the same programmer ( if possible ) to use on it . Any advise at all would be appreciated.. I began self teaching and building my own street race cars back in the good old days when there was nothing but carbs, and alternators on engine. Then I was a dealer certified heavy/medium duty diesel technician for almost 40 years.. There was nothing I could not do on tractor trailers to medium duty trucks I specialized in everything.. But did not work on many gas engines throughout the many years of being a diesel tech. Thank you all

tblu92

You need to use either HP Tuners or EFILIVE software---Most handheld type tuning boxes can fix speedo corrections but NOT shift point corrections----Depending on how much taller your rear tires are from stock will affect the up and most importantly your downshift points
ALSO You cannot alter the tire height split between the front heights compared to the rear heights much more than what the STOCK split is Normally about a 1 inch difference----If you alter this too much more or too less than 1 inch Your ABS braking system will not work as designed And your Traction control system may put your car into some sort of "reduced power mode" and you can risk your life on a high speed stop !!!!!! Send me a private message to discuss---------

RonSSNova

Stick shift or auto?
Stock rear tires are 26” tall. How tall are the new ones?
And lastly, how much heavier are they than the stock wheels?

01torchred

It is an automatic. And at this time you have stumped me with the weight and how tall the new wheels and tires are. I just looked up my tires online the rear Michelin 285/40 zr 19 is 27.98 in. diameter the front Michelin 255/40 zr 18 is 26.03 in. diameter. As far as weight difference ? no clue. I did want to at least let you know I did read this and thank you for your attempt to help me out. Looking at the specs on your Vette & Nova mine would be considered to be a dog I'm sure. It is exactly the way I bought it new in 2001 with 3 miles on it and it is still ( bone stock ). Thank you again

01torchred

Thank you for the info. I just looked up my tires online the rear Michelin 285/40 zr 19 is 27.98 in. diameter. The front Michelin 255/40 zr 18 is 26.03 in. diameter.

tblu92

2 inch split is too much !!! It's your safety your risk with a 2 inch split On a high speed panic stop your ABS braking system will Not work as designed----Looks good Yes----but depends on the use of your car if you putt putt around town no big deal ---- but if you race or get racy be careful on a wet road braking or on a high speed stop---- the tires will SKID

RonSSNova

That’s a big change in rear tire size. Essentially kills the gear ratio.
A4 has two possible ratios. 2.73 or 3.15. If it’s a 2.73 it will be on its lips.
The factory wheels are pretty light. A lot of the aftermarket wheels are heavy. Plus you have a bigger tire, which is also heavier. It does make a difference.

A good tuning shop can fixe the speedo calibration and shift points. They will need to be good with Vettes.

I can’t speak to the Diablo stuff.

Tron Z

@RonSSNova is on the right track, but please allow me to restate:
Installing a much taller wheel/tire package on your car has a couple different effects. As you have already noticed, throwing off the speedo calibration is one of them. This can be fixed in the PCM tune. But, the PCM tune can not compensate for, nor get back the power/responsiveness of the car, because you have effectively changed the mechanical final drive ratio of your driveline. If would be no different than removing the existing ring and pinion gears from the differential and swapping them out for the tall gears in grandma's grocery getter. While a higher stall torque converter may help, the real fix is to compensate with a different ring and pinion gear ratio.

Krusty84

If those are Cray Machined Spiders, you added quite a bit of unsprung weight to your vehicle. About 6-8 lbs per each front wheel/tire and about 12-13 lbs per each rear wheel/tire. The effective gear change between a 275/40/18 and a 285/40/19 is only about 5% (26.7" tall to 28" tall), but I suspect that the combination of the effective gear change and unsprung weight adds up to about 15-20 hp reduction when using the seat of the pants dyno.

Over the years, I have seen plenty of people lower their car's performance with different wheel/tire packages. They look nice, but how much do they weigh and where exactly is that added weight. If the added weight is on the outer edges of the wheel/tire, which it is in your scenario, it can make big difference in how the car performs and handles.

Missileboy

the general rule is to keep the diameter of the tire change to 3% or less to the new setup. You will have speedo issues (usually ~1-2mph up to 70 or so) but it should not throw off the ECU too much to cause problems. More than 3%, the speed sensors and shift tables and such get out of sync and it can cause performance issue and weird driving/shifting operation. you can have a 2 inch difference front to rear, but it needs to be accounted for in your tuning to compensate.

I planned all my changes together, new 19/18 wheel set up, 3.42 rear end swap and the ECS SC upgrade so that all was accounted for in my tune when ECS was done. 3.42 in the A4 IS AWESOME!