Re: Ping Dave or Dad

I am totally scratching my head. I found a tire diameter calculator, and if I put in 275 - 40 - 18" I end up with a diameter of 26.661417322834644, which means my ratio is 2.116. The tire diameter website is

That doesn't get me even close to 2.73 or 3.15. What on earth am I doing wrong? (2,000 rpm = 75 and 1,500 rpm = 55). Should I have been in 3rd gear and not 4th gear (overdrive)?

Third is straight through. Fourth (OD) is 0.70 on the idavette site. My owner's manual doesn't have the transmission ratios.

-- PJ

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OK, thanks. Lemme try it again. So I take the inverse of the .7 and that's what my 3rd gear would have been doing; then use the ratio program again? If I do that, I get an inverse of 1.4 and when I multiply that by the 1,500 rpm I get 2,100 rpm. the tire is 26.7 inches and the speed is 55 mph. I get a ratio of 3.04. If I was doing an extra 75 rpm's (2,175) I am right at 3.15 ratio. I must have that, no? I am much closer to that than to 2.73.

One other thing, doing that website it appears that having the 3.15 ratio means that top speed on the car is 147 mph instead of 174 mph. Is that correct too? Or did Chevrolet do something with 4th gear so you don't lose all of your high speed capability?

Fourth gear will not pull up to the 174, I can tell you that with the

3:15 it will go to nearly 150 in third. Right at 6,000 RPM, I say nearly because I didn't have HUD and didn't spent much time looking at the dash. A very slight dip in the road made my decision to shut it down. I had traveled that stretch a few time to get the feel of the road before I open it up and it didn't seem that bad at normal speeds. The Corvette suckeddown tight as your speed increases but when you bottom out the frame it can get scary real quick.

I did a search on this group and found an old post that might be of interest to you.

------------------------------------------------------------------------------- Interesting mix,

Gear Ratios LS1-2/M6 LS6-7/M6 LS2/M6 LS1-2/A4 LS2/A6

1st Gear 2.66:1 2.97:1 2.97:1 3.06:1 4.02:1 2nd Gear 1.78:1 2.07:1 2.07:1 1.63:1 2.36:1 3rd Gear 1.30:1 1.43:1 1.43:1 1.00:1 1.53:1 4th Gear 1.00:1 1.00:1 1.00:1 .70:1 1.15:1 5th Gear 0.74:1 0.84:1 - .85:1 6th Gear 0.50:1 0.56:1 - .67:1 Reverse 2.90:1 3.28:1 3.28:1 2.29:1 3.03:1 Std Z06 Z51 Auto Auto Rear gear for the LS1-2 A4 is 2.73 optional 3.15 LS1 M6 is 3.42

LS6 M6 is 3.42

LS2 A6 is 2.56 no options yet

LS7 M6 is 3.42

I would be extremely curious to see top speeds by gear depending on rear end ratio. So far, I have only taken this car to about 130. That felt extremely fast; considering that it was not a limited access highway. I hope the car can do 150, but 170 would be even better! I was at the Chevy dealer the other day, and they gave me the big brochure for the 2006 C6. It said the car could do 198. Can you imagine that?

That 198 would be the C6 ZO6, the C6 coupe 6sp manual will only do about

186 as it is about 105 hp shy of the ZO6.

Tire calculators are neat toys, but if you want accuracy, you have two options - measure it or use the manufacturer's data. Each manufacturer makes different sizes, even if they are all the same numbers, i.e.,

255/60-16 is not the same for everyone.

The most accurate measurement is to mark the tire and the ground, roll the tire one revolution, then mark the ground where the tire mark hits. Better to do two or three and then divide, so that you take some of the error out. Remember C = pi * D.

Remember you want loaded rolling diameter, so it has to be on the car and rolling on the ground. You can't just wrap a measuring tape around the tire.

I have found this axle calculator close enough plus it lists several other axles. There is also a list with transmission ratios, which need to be figured it, especially with an OD.

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