patricksd

By chance, does anyone know what size tires came stock on a 2001 Z28?

I am trying to figure out the difference in final gear ratio going from the stock size (which I don't know) to a 275/35ZR18 (Z06 rims).

The reason being, the previous owner had a Hypertech programmer and had the gear ratio listed as a 3.23. The car is an M6, and I know it has 3.42 gears. But with going to a taller wheel and tire combo, I know it would make the gear ratio "feel" less than 3.42.

I want to make sure his 3.23 setting didn't make the odometer run slow.

MrDavid

iTrader: (11)

your stock size should be 245/50/16. Going to an 18" rim and tire is going to throw the speedo off considerably. Yes, your final drive ratio is 3.42. It shouldn't feel different though, probably just throw the speedo off about 7 or 8mph.

patricksd

This is what I've been able to find out. With a stock tire size of 245/50/16 the tire diameter is 25.6 inches. With an 18" rim and 275/35/18 the tire diameter is still 25.6 inches. From my understanding, the effective gear ratio would stay the same with the 18" rim (and smaller profile tire). I'm sure the rotational mass of the wheel affects the drag somewhat, but my concern here is the odometer.

As I mentioned, he had a Hypertech programmer with the gear ratio set at 3.23. I am not sure how often he ran the programmer, but it looks like setting the gear ratio at 3.23 would make the odometer read lower than what it actually is.

Does that sound about right?

He gave me a folder with all his service receipts, and the mileage flows pretty well. On average, he put 1,000 miles on the vehicle per month. The only time it sat was when he moved to Illinois for a year, and in 14 months only put 4,000 miles on it. He told me he didn't drive it much that year because of the roads, gravel, etc.

I know, in all reality, I will never know if he intentionally set the gear ratio lower to lower the odometer. But he purchased the programmer in 2004 with 30,000 miles on the car. A little over two years later, the car had 53,000 miles on it which seems to coincide with the 1,000 miles per month.

MrDavid

iTrader: (11)

If this guy you bought the car from was that concerned about mileage, he could have spent $50 and bought a gauge cluster off of E-bay with X amount of miles on it and had it in the car 5 minutes later. I don't see where a different gear ratio would affect the odometer, nor have I ever heard anyone else mention that it would. I don't know why he would do something like that though. Hypertech Power Programmer is a piece of crap, I used to have one. Do yourself a huge favor and get your car tuned with HP tuners or another reputable tuning software and you can fix tire size, gear ratios and just about anything you can think of.

slowjoe99

iTrader: (7)

the size of the wheel doesn't matter at all. it's the height of the tire that matters. you can get an 18" tire that is the same height as the 245/50/16 tires are now. and the 7 or 8 miles an hour difference is at what speed? lol the faster you go the more mph off it will be.

if you had the same height of tire you would not have and problems. the height of a 245/50/16 is 25.645669 inches tall. the first number (245) is how many mm wide the tire is from where it bow out on each side of the sidewall. the footprint may be a lot narrower then the overall width of the tire. the second number is the height of the sidewall. it's a percentage (50%) of the width compared to the height, meaning your tire is 50% as tall, on each sidewall, as it is wide. the third number is obviously the wheel size. performance tires, compared to all season tires, usually have a wider tread for better traction - even if they are the exact same size. a 235-75-16 is a lot taller and not near as wide, on the tread, as a 235-55-16. even though they are both 235 mm wide. the reason for this is because the sidewall(aspect ratio) is a lot taller on the 75 series and has more room to bow out. when the tire is inflated, just like a balloon, it wants to take a round shape. the taller the sidewall the more room the shape has to bow out - the narrower the footprint is.

to determine the height of a tire in inches use this formula.

width (245) divided by 25.4 (amount of mm in an inch) = 9.6456692

that number times the aspect ratio (50) = 4.8228346
*hint* remember 50% is equal to 1/2 and in decimal form is 0.50, so multiply 9.6456692 by 0.5 and get 4.8228346

times that number by 2 because there are a top and bottom sidewall on a tire, duh! and get this = 9.6456692
in this case it's the same as the top number

then just add your wheel size (16) = 25.645669

here are a few other examples:

275-40-17

275 divided by 25.4 = 10.826771

multiplied by 40% (0.4) = 4.3307084

times 2 ( cause two sidewalls) = 8.6614168

add wheel size (17) = 25.661416

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225-60-16 (common luxury type car tire size)

225/25.4 = 8.8582677

times 0.6 = 5.3149606

times 2 = 10.629921

plus 16 = 26.629921

as you can see these two tire sizes are almost the same height but a lot differnece in width. the height of a tire determines how much your speedo will change cause it changes the distance needed for a revolution. the taller the tire the greather the distance needed for it to make one full roll. the taller the tire you have the higher the speedo will read. it may say your only moving 60 but you will actually be going faster, cause the car still thinks that shorter tire is still on.

with all that crap said... if you get a tire that close to 25.645669 inches tall, you will not have to adjust anything for the speedo.

I know I typed a lot but hey... i was BORED!!!

slowjoe99

iTrader: (7)

i just noticed that the two example tire sizes i used are not almost the exact same height. they are almost exactly and inch different! lol whoops

patricksd

Thanks for all the information guys! I agree, I would never buy Hypertech simply to recalibrate the speedo or odometer. I think he did it because of the 160 stat.

I found the stock tire online and the aftermarket one I have know. The revs per mile are 811 for the stock one and 804 for the aftermarket (the wider tire making it turn slower).

If I assume he ran the programmer from the time he bought it (2004 with 30,000 miles on the car) until I bought it, it would have meant a 5.8% difference in the speed of the vehicle. This would translate to about 750 miles difference for every 12,000 miles transferred. I bought the car with 54,000 miles, so that would work out to a potential 1,500 mile difference in odometer reading. Not a big deal I guess.

But what would prevent somebody from buying a new vehicle, spending $400 on the Hypercrap, putting the gear ratio super low, and driving around for 60,000 miles. There could be a significant difference between displayed and actual miles traveled.

slowjoe99

iTrader: (7)

wider doesn't matter. it's how tall the tire is that matters...