ir_fuel

Anyone happen to know how much the speedometer in a GT3 lies compared to GPS speed?

ADias

Probably 2MPH faster than reality.

ir_fuel

Over the entire range?
Even at 180mph?

911rox

Can try it tomorrow for you but wont be doing 300kmh where i'm going as I will be without a license or pulled from a heavily treed forest...

Clifton

Coming back from a mountain drive July 4th weekend, I ran past a hidden officer while I was running about 15 over. Around my parts the leave you be for 10 or less. He pegged me with radar (Ka band), looked at me as I got closer, but did not persue.

That got me thinking - So I pulled out my GPS and started tracking. At speeds above 70, my speedo was reading 3-4 fast.

Macster

Can vary depending upon the wheel/tire option. My experience with 3 different Porsches (Boxster, Cayman S, and Turbo) is 5mph, dead on, 2mph. These errors are for speeds less than Vmax. I have had very few occasions to be able to test speedo errors above the posted limit but my brief experience is the error can get a bit worse -- the speedo reads even more optimistic -- as speeds get quite high. This is why I always take postings of max speed obtained based speedo displays with grain of salt.

A few other car brands based on my experience and direct observation delivered similar variations.

What I do is characterize my car's speedo error vs. GPS or OBDII speed and then take that into consideration when driving. For instance I know my Boxster's speed is 5mph optimistic so in a 65mph zone I'll have the speedo at 70mph. I have enough confidence in the speed to even pass slower moving LEO vehicles as long as there's no other reason not to.

The Turbo's speedo is 2mph optimistic so I put speedo on 72mph and know the car's traveling 70mph.

BTW, early on I tested my Boxster's odometer and found in a test drive of 200 miles an error of just 0.2 mile.

Sincerely,

Macster.

Leigh2

On my 997TT speedo is 3% high as near as I can calculate

TB993tt

The photo below was taken on an airfield day - the GPS maximum was 194mph.

4porsh

Back in the late 90s i was testing some motorcycles for a mag and we were doing top speed runs with a radar gun. I did three runs with my girlfriends 97 C4s and the digital on board read out showed 178mph but the gun read 160mph every time.

fc-racer

With my Aim system, I get GPS speed. I get ~5% difference across the range. At Mosport, the speedo shows 253km/h but the gps shows 240km/h.

NJ-GT

I measured both my 996 and 997, they were off by 3.5% on stock tires.

If you run a shorter rear tire it gets worse.

ADias

I buy that. 3% optimist seems about right.

MJones

3MPH indicated slow
68 = 65 MPH
73 = 70 MPH

Verified via GPS
Have an iPhone?
Theres a cool application for just that
SPEEDBOX

911rox

Gave it a crack today. I got the following using a gps phone:

Speedo- GPS

60km/h - 57km/h
105km/h - 101km/h
150km/h - 145km/h ....

Bill_C4S

Rox - 150 in Victoria ? ..... Is that still hand over your licence for 5 months territory.

Or have they toned the intolerance of any speeding down?

Naturally, I assume you were taking the front straight at Phillip island....