Speedometer not accurate?
 

Anyone notice a difference between the car speedometer and GPS speed?

Since the installation of my Kenwood with Garmin GPS I noticed a 3 MPH difference between the speedo and the GPS.

Car speedo reflects 73 MPH and GPS reflect 70 MPH.:eek:
Car speed at 65 MPH, the GPS reflects 62.

Kind of curious if anyone else notice a difference?

I verified the proper tire spec's are accurate, and I know a GPS can be subject to speculation..
I also used the GPS on my iPhone and it reflects that same speed as the Garmin GPS..

T

Yup, many of have noticed a 2-3 mph difference. The needle is always a bit higher.

Which is good for speeding tickets, or moe accurately avoiding them by accident.

I have 2 boxsters and a C 4 and they all read from 2 to 5 MPH more than the gps.

No matter what car I am in and no matter what GPS I use, I get the exact same result.

GPS shows 2.5 mph lower than the speedometer.

I tried this test with a 2000 Honda Accord, 1996 Mazda Protege, 2003 Chevy Tahoe, 2012 Ford Focus, 2008 Toyota Rav4, 2001 Nissan Altima, and my 2000 Porsche Boxster. Same results in all of them.

I used my Garmin in all of them.

I tried TomTom in 3 of the cars mentioned above. Same result. GPS is 2.5mph slower.

My speedo always reads 2-3mph faster than my GPS.

Unfortunately, I take this into account and go 2-3mph faster!

Wildly inaccurate speedo - and I hate it. I'm always doing the math in my head, trying to figure out at what indicated speed am I pushing the limit of what Johnny Law will tolerate. I think it's just inexcusable - my Toyota is dead nuts to the GPS as is my Explorer. Yet my most expensive car can't get the most basic instrument correct. It's total BS. Makes you wonder if the tach is wrong too.

Yes, all of the speedos read Higher than actual depending on your tire diameter/size.
~3 mph faster.

It helps keep your speed down a bit and many times I have been glad it has as I cruise by an unseen CHP a little on the high side.
(Radar Detector ordered before its too late)

Your allowed 10% over actual by most police unless they are feeling ornery or are just plain bored.

They are generally not amused when you mention that due to the rotation of the earth you are already traveling ~300mph plus or minus your speed depending on direction.

If they offer to write you up for 300mph, accept, so that when they show up in court and cannot demonstrate to the Judge how they added or subtracted your cars speed vector from the 300, it will get thrown out. :D
Plus the judge will want a demo in your 300 mph Porsche
and you can frame the ticket and put it up in the garage as a conversation piece. :cheers:

in Europe, manufacturers must not allow a speedometer to under-read and the maximum permissable overread is by a calculation (this was discussed on a professional pilot forum i use).

by law in the UK we are allowed 110% +4

yes, i know it's europe, but the cars are designed and built to be sold across continents

for those interested in the legalities it's here: http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:1975:196:0001:0005:EN:PDF

honda got drawn up on the over-read in US, here: Odo Uh-Oh: Honda extending warranties on 6 million cars

You'd lose the entire court room if you tried to go into centripetal force and angular mechanics

I imagine 110% and +4 can only really truly happen at one speed. You can have your speedo read 110% faster, or you can have it read 4 MPH faster. But only at one specific crossover point, will both be true. Am I thinking about this correctly?

Also, Honda's odometers did (maybe still do) read 2-4% higher. However, their speedometers, at least all the ones I've driven, have been spot-on. I've had five Hondas now.

I have 2 Acuras that are dead on with the GPS, but the Boxster reads 2.5 to 3 mph over at all speeds. I figure if I drive 5 over the speed limit by the speedo, I am really only a couple mph over.

When my 986 was on the OEM 16" wheels and tires that came with the car, the speedometer read 5 MPH too fast. I switched to the OEM 17" wheels and tires that came on my wife's Cayman and my speedometer is right on now.

With stock 17" I am +4 mph off from actual.

at 30... 33 (110%) + 4 = 37

at 40... 44 (110%) + 4 = 48

at 50... 55 (110%) + 4 = 59

at 60... 66 (110%) + 4 = 70

at 70... 77 (110%) + 4 = 81

My Porsche speedometer over reads by close to 10%, at higher speeds the accuracy reduces, but the error seems linear. IE at 30 MPH indicated my GPS speed is 27 MPH and at 75 speedo indicated, the GPS speed is 70 MPH

Generally, the police will leave you alone on the UK M-way's at 80 MPH, so at 85MPH (speedo) i stay under the 10% error margin.

speeds high than that, no comment ;)

Ah, okay. I misunderstood. I thought you were saying your speedo reads 110% of actual and +4 of actual at the same time throughout the entire range. I was tired last night when I read that... ;)

Those are pretty big margins that the cops will let you get by with. Here, it's really up to the cop's disgression. He can write you a ticket for doing 1 MPH over. Or, he can let you go by at more than 20 MPH over. Just depends on who the cop is and what his mood is at that moment.

And, you're the first Boxster owner that I've read that has a speedo that's linear. All others report the same ~3-4 MPH higher reading throughout the entire speedo range. I wonder why yours is different? Stock wheel sizes?

Going 73 as read from the digital readout in the dash, i'm going 69 - 70 by radar.

Based on the readings from my GPS, mine always varies by 5% at all speeds. At an indicated 80 the GPS reads 76, and at an indicated 50 the GPS reads 47-48, etc, etc. I've tracked this over many trips and it never changes. I just live with it. As was posted above, it's apparently a law in Europe and the fines for having the speedo read slow are, from what was discussed in a previous thread, pretty substantial. It's not that big a deal to me. If you can figure out what the factor is to correct for the error (assuming it stays constant, which it does on mine) it becomes pretty automatic when on the road.

yep 18" as per the pics.. though i was tired when i wrote that too.

Does this effect odometer reading as well?

It probably does affect odometer readings over a rather long period of time, much like over- or undersizing your wheel and tire combination from stock diameter will throw off your speedometer because the whole assembly is going to be spinning faster or slower than it's supposed to.

Also, over time you have to assume that any measuring device is going to get out of tolerance and need a recalibration. We're talking cars that are up to 15 years old...it's certainly a possibility.

My car seems to read a little on the fast side, too, but it's not by much. I notice maybe up to 200rpm difference between what speed the car is indicating and what rpm it should be spinning at that speed. It's most evident to me in 3rd gear around 45mph. I should be right around 3000 rpms, and I'm just ever so slightly past the 3k hash mark on the tach.

Use that nifty little equation to calculate what your speed should be at whatever rpms, and you'll know how fast you should be going at those engine speeds (in a 97-99 2.5, at least).

(RPMS/1000) X 5 X Gear = mph

Example: (3000/1000) X 5 X 3 = 45mph

(4000/1000) X 5 X 5 = 100mph

I'm sure there are ways to do this for the 2.7s with the different ratios and the 3.2s with the 6 speeds, but I don't think the numbers to compute with will turn out nice and round like they happen to for the 2.5 setup. It's pretty accurate.

Yes. The distance from my fuel station and home is 6.6 miles. When I get home in the Boxster on the OEM 16's, it reads 6.8.

All my German cars have read slower than actual.

2005 Boxster 2.5 mph off
2006 Passat 2.0
2010 A6 Just got it, but appears to be in the 2 mph range
2001 SLK 2.0

All figured at 60 mph