[mikeygar]

Was able to do several tests on open highway yesterday on my stock 08 STI.

Cluster 240 kph/149 mph = GPS 224 kph/139 mph = 7% off
Cluster 260 kph/161 mph = GPS 240 kph/149 mph = 7% off

On flat road, max was 240 kph/149 mph on cluster, was able to do 260 kph/161 mph downhill.

7% variance was more or less consistent at speeds over 200 kph. Anybody else get the same results? Was hoping it would stay steady at 255kph stock, but speed wouldn't rise past 240 on flat road at all

[sleepsindirt]

gnrly when im goin that fast i need to keep my eyes on rush hour traffic not my speedoj/k

[altaic]

245/40R18 or 235/45R17 tires (both are available stock, claims the FSM)? Is it always off by about 7%, even at low speeds? And what are your tire pressures?

Edit: I was curious, so I checked if there was a procedure to calibrate the odo/speedo. Well, it appears that the speedo sensor sends 4 pulses per tire rev. Our stock tires have a diameter of 25.71" and a circumference of 80.77" and therefore should have roughly 807 revs per mile, which equates to 3228 pulses per mile. The thing is, it clearly states there should be 4104 pulses per mile. That's a huge discrepancy. We'd have to run 20.25" diameter tires to make the FSM accurate.

If I assume your tires are indeed making 3228 pulses per mile, the 7% discrepancy you are seeing would seem to indicate 3000 pulses per mile is what the odo/speedo is actually calibrated for. Anyhow, I couldn't find a way to calibrate the number of pulses per mile. Kind of stupid.

[solovus]

Originally Posted by sleepsindirt

gnrly when im goin that fast i need to keep my eyes on rush hour traffic not my speedoj/k

Yeah...and thats without using the abacus to make the kph=>mph conversion!

[DrPhyzx]

This is pretty weird, as my 08 STI is far more accurate than any other car I have ever owned: within 1%. Even at those speeds, I'd be really surprised by 7% tire slip.

[mikeygar]

Originally Posted by altaic

245/40R18 or 235/45R17 tires (both are available stock, claims the FSM)? Is it always off by about 7%, even at low speeds? And what are your tire pressures?

Haven't tried the difference at low speeds yet, but I will do so when I get the chance and update this thread. I'm running on the stock Dunlop SP600s on 18" rims, not sure about the sizing, will check tomorrow. Tire pressures were at 35 psi all around, due to heat and the distance to where I tested, I can assume it went up a few psi during the trip

Originally Posted by DrPhyzx

This is pretty weird, as my 08 STI is far more accurate than any other car I have ever owned: within 1%. Even at those speeds, I'd be really surprised by 7% tire slip.

was actually more surprised by the consistency of the 7% margin at 240 and 260 kph. 1% is good, that means your limiter won't interfere until you're actually at 155mph!

[ake109]

Hmm, what the OP wrote makes sense.

AFAIK, just about every production spec car has speedos which overread anywhere from 6-9% most of the time.

But 240 is pretty far from the factory claims of 255.

[DrPhyzx]

Originally Posted by ake109

Hmm, what the OP wrote makes sense.

AFAIK, just about every production spec car has speedos which overread anywhere from 6-9% most of the time.

But 240 is pretty far from the factory claims of 255.

That certainly used to be the case, but there seems to be a trend toward more accurate speedometers based on what I've been hearing: not sure why. Lots of other owners have reported 2008 STI speedometers that are spot-on.

[BlueSubaru]

My speedo my nuvi 660 and my apv2 all say the same up to 120 mph...

[method]

Even modern consumer grade GPS isn't all that accurate, +/-15m or so, and that doesn't include the 1% additional inaccuracy due to time discrepancies.

[DrPhyzx]

Originally Posted by method

Even modern consumer grade GPS isn't all that accurate, +/-15m or so, and that doesn't include the 1% additional inaccuracy due to time discrepancies.

"Modern GPS" uses doppler shift rather than position/time to compute speed, and the precision is very good... typically 0.1mph where satellite coverage is good.

[UturnSTi]

Tested at 55mph and 80mph with a calibrated radar... Was spot on.

[method]

Originally Posted by DrPhyzx

"Modern GPS" uses doppler shift rather than position/time to compute speed, and the precision is very good... typically 0.1mph where satellite coverage is good.

Modern GPS units (consumer grade) use a combination of both doppler and pos/time delta algorithims. Yes the doppler method is more accurate, I'm not so sure 0.1mph accurate though. Closer to 0.5mph, and per axis (at least this is the performance spec. Garmin uses.)

I'm not saying there isn't discrepancies between the speedo and your actual velocity. I'm just saying gps error can account for some of it.

[Digitalfiend]

I know my '05 speedo was off by 4-5km/h at around 100km/h. I always attributed this to the JDM car having 235/45/17 and us getting 225/45/17. My '05 ECU used to read lower and the, seemingly, correct number while the speedo was always off by 3-5km/h. With my '08 I've found the speedo to dead accurate, at least up to 100km/h (haven't really verified anything past that) and in line with the ECU.