I have 255/30/19s in the back and the speedometer is as accurate as u can get. I was doing a triple digits recently in a closed controlled environment and my car topped out at 150 mph showing on the speedometer at ~5800 rpm 6th gear, so im like WTF! . Well when i had the car tuned last weekend they said they accidentally set the top speed to 149. Well it was showing 150 on the speedo and the actual speed was 149, that is pretty damn accurate considering the speed. Just wanted to share. Now my top speed is set at 190 lol.

 

I don't understand what you are saying. The RPM limit that is set uses the same signal as the one supplied to the speedometer. Of course they would give the same value. This doesn't mean you are actually going 150mph, it simply means based on the OEM diameter and axle revolutions, the ECU calculates your speed as 150 mph.

 

^+1 Your speedometer uses the same info as you limiter.

 

Ahhh, I see.

 

anyone wanna say IBTD?

 

While on the topic, my speedo seems to be off by 2 mph...at least according to my GPS. Which should I trust? Even on cruise control, the GPS will always say about 2mph higher than what my speedometer says.

I'm in a bone stock 2004 5AT sedan.

 

Ibtb

 

i know on a bone stock car manufacturers have tolerance of +/- 5%. So that is normal to have 2mph difference from the actual speed.

 

Cool, thanks.

 

any more IBTDs?

 

Nope:
Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
215/55-17 4.7in 13.2in 26.3in 82.7in 767 0.0%
255/30-19 3.0in 12.5in 25.0in 78.6in 806 -4.9%

 

Im gonna have my buddy radar me once to see how accurate my speedo really is.

 

Its going to be off by different amounts as your speed increases, ~5%

 

yes, i know. If it shows 35 mph when im going 40 mph then that means it is off by 12.5% so if im going 150 mph on the tach, that means i am really going 131.25 mph.

 

ibtl