Anybody ever check their speedo via GPS?

Joe Bob · Joe Bob's Garage

Just did mine....I expected it to read high....boy was I surprised when it found I'm 10% LOW.

I'm running 245/45/16s on my SC and that's a bit shorter than stock. I'm actually doing 77 while seeing an indicated 70.

Up to 30 mph is nuts on....after that........

On my 356 replica I have an adjustable electronic VDO. Fun having NEW and accurate gauges.

Rot 911 · 09-12-2007, 05:31 AM

I have a handheld Garmin Etrex. Checked the speedos in all my cars with it. My Bugeye Sprite speedo didn't work so I just used the gps for a speedo.

My '86 was 10% confident. I had it recalibrated by North Hollywood Speedometer when they reworked my speedo and tach a few years back. Now it is 100% accurate at all speeds.

RickM · RickM's Garage

While I never checked my 911 I have checked my MB SL, '02 Explorer and '99 CRV. All speedos report a higher-than-GPS speed by between 5% & 10%.

Used a Garmin StreetPilot.

nut11 · 09-12-2007, 05:38 AM

I always thought my speedo read high by at least 5 mph. So, if I'm reading 80 mph, I should actually be doing 75. Jeepers, now I may be doing 85. Must check...

87 blk coupe

tonythetarga · 09-12-2007, 09:35 AM

If this is typical, (showing higher speeds than normal) then is my odometer possibly showing 10% higher milage? That would be significant and is it possible/legal to correct back?

ljowdy · 09-12-2007, 09:46 AM

It's NEVER legal to turn back your odometer.

I checked my 1973. GPS says 65 and speedometer says 70.

I'm running 195/65/15's which should read lower than stock 185/70/15's but it doesn't. I've just learned to subtract 5 MPH from the speedo.

msucro · msucro's Garage

I checked my gf's 2001 Boxster S... read about 4 mph high anywhere from 50-85.

Marcus

madmmac · madmmac's Garage

Gps checked on my 911 with standard tires/wheels is exactly 5mph slower than the speedometer reading at all speeds.

4runner is right on at all speeds.

nut11 · 09-12-2007, 11:21 AM

Ok, thx. 5 mph slower than reading. I think that's the norm for at least 84 through 89 Carreras.

87 blk coupe

TechnoViking · TechnoViking's Garage

I checked my my SC with a GPS and, running 245/45-16 tires, the spedometer runs 5-7 miles high depending on speed. Seems to get a little more accurate at higher speeds (like over 55) and a little less accurate at lower speeds.

dvkk · 09-12-2007, 03:18 PM

What do you think the accuracy of your GPS is?

SCWDP911 · SCWDP911's Garage

Mine runs slower than indicated - about 5 mph all the up the dial once past 25. Checked with a Garmin. Stock tire sizes...

Porsche84 · Porsche84's Garage

My 2002 Jeep Grand Cherokee speedo reads 1 mph high at 70mph), my 86 Chevy truck is dead nuts (at 65 mph), my wife's 328i BMW is about 2 mph high at 70. I have not checked my Porsche because it is only driven on the track

ljowdy · 09-12-2007, 04:01 PM

I've checked the hand held GPS that I used to check the Porsche against another expensive street address finder GPS I have and, both read MPH the same.

On a stretch of road between here and Sonoma, I checked the street address finder GPS unit against mile marker signs at 60 miles per hour and it took 1 minute so I know that unit was correct and as such, the other should also be correct.

ausgezeichnet · ausgezeichnet's Garage

I've noticed a delay in my GPS altitude readings as I am driving a highway speeds. It probably is the same for speed so keep your speed steady when measuring.

msterling · msterling's Garage

GPS unit altitude readings are relatively inaccurate. Within 100 feet is unlikey with mine. It actually uses barometric pressure to "guess". The speed readings are updated much more quickly and precisely using GPS signals. Still, an average over a period of time is a better indicator.

Hugh R · 09-12-2007, 05:14 PM

NH Speedo told me all the 84 to 89 speedos from porsche are off by 5-7 mph at speed. The speedo does not dictate odometer readings.

morganb · morganb's Garage

Hugh,

Over or under? My 88 (89 model year, 16 inch std wheels) speedo shows 107 kmh when travelling at 100 KMH

RickM · RickM's Garage

Quote:

Originally Posted by dvkk

What do you think the accuracy of your GPS is?

According to Garmin (with WAAS) 0.11mph accuracy.

Previous technology is not as accurate.

Won · 09-12-2007, 08:56 PM

Quote:

Originally Posted by tonythetarga

If this is typical, (showing higher speeds than normal) then is my odometer possibly showing 10% higher milage? That would be significant and is it possible/legal to correct back?

It was discussed before and apparently the speedo reading and odometer reading are independent of each other. Someone here tested them with mile markers - at 60 mph, in 1 minute, one would have driven slightly less than 1 mile, according to both the markers and the odometer.