Doing almost 90 in my 06 C2 everyone is passing me. I'm beginning to wonder if my speedometer is off? Being out of town and not having access to a portable GPS I have a question to help me resolve this problem. How fast are you going in C2 in 5th gear with a Tiptronic at 3000 rpm?

 

You could have just timed yourself between mile markers to see how fast you were going.
No GPS required!

 

I quess that would tell me at 60 since that would be a mile a minute, but 80? I flunked math.

 

80 mph would be 45 seconds per mile
90 mph would be ~40 seconds per mile
100 mph would be ~36 seconds per mile.

Do you have a tachymetre on your watch?
http://en.wikipedia.org/wiki/Tachymetre

 

...and 180 mph would be 20 seconds per mile.

 

Instead of timing your distance between mile markers. Reset your trip gauge at the start of a mile marker, drive interestate miles and compare the mileage to your trip gauge.

I did this a couple of months ago. I drove 79 interstate miles and my trip gauge showed 83 miles. My speedo is about 5% fast.

 

according to my portable GPS my speedo shows 2mph slower than actual speed

CORRECTION: 2MPH Faster than actual

 

My speedo reads 2 mph faster than actual per my portable GPS.
19's on my car.

 

If you are running 265/40 - 18's in the rear and are doing 3000 rpm in 5th gear and the final drive ratio is 0.83 and 5th gear is 3.56 then you are going about 80 mph.

 

Calculate as follows:
1. 3000 rpm divided by (.83 gear ratio x 3.56 final drive) =1015.3 axle turns per minute.
2. 265/40 x 18 tire. Sidewall height= 265 width x.4 aspect ratio divided by 25.4(mm to inch)=4.173".
18" + 2 x 4.173 = 26.35" tire diameter. 26.35 x pi x1/12 = 6.897 ft tire circumference.
3. 1015.3 X 6.987 divided by 88 = 79.57 mph.

Say 80 mph.

 

Not fast enough

 

Just returned from a weekend in Redwood City and the speed from north of SLO to south of Salinas is amazing! Hit 115 for a brief spell going and 125 for an equally brief period returning. Averaged about 90-95 in that area and I wasn't pulling away from that many cars! Car ran great.