Speedometer

[boris]

Hi,

It seems, speed reading in my car is ~3 miles/hr less than actual (I
compared it with speed reading by Garmin GPS). To calculate MPG I use
gallons reading at gas station and total miles count since last fiil-up - is
this number (MPG) also incorrect?

Thanks,
Boris

 

[John Varela]

It seems, speed reading in my car is ~3 miles/hr less than actual (I
compared it with speed reading by Garmin GPS). To calculate MPG I use
gallons reading at gas station and total miles count since last fiil-up - is
this number (MPG) also incorrect?

It's probably off, but not necessarily off by the same amount or
even in the same direction as the speedometer. Find a stretch of
highway--any Interstate will do--that has milestones and check your
odometer against 100 miles (or however many miles you can manage).

 

[Lena]

Your speedometer is measuring speed along the path you are driving while the
GPS is taking samples of your position and calculating a average speed
using a series of straight line distances. Plus GPS's aren't overly
accurate- I think approx 10 meters.

 

[boris]

Your speedometer is measuring speed along the path you are driving while
the
GPS is taking samples of your position and calculating a average speed
using a series of straight line distances. Plus GPS's aren't overly
accurate- I think approx 10 meters.

I see. But I also noticed smthg funny with equipment that shows your speed
(these devices are installed at some streets - when you approach such a
device, its displays your car speed - if your speed is more than what's
allowed, the display begins to blink) - the above devices always show my car
speed as 2-3 mph less than what my car speedometer shows.
Actually, it comes handy: I can drive 75 mph (on freeway) and cops won't
stop me, because my *real* speed is 72-73 mph.
I was just wondering how/if incorrect mph reading correlate with incorrect
total milage reading.

Boris

Boris

 

[Blair Baucom]

All three of my GPS units are always same as indicated by radar, +/- 0 mph.
I would expect there would be a difference due to elevation, because going
up & down hills is further than straight line travel, but have not observed
this real world. With cruise on, they lock with my speedometer. Indicated 68
mph on GPS, 72 mph on speedometer, 30 mph on GPS, 32 on speedometer. My GPS
normally indicates accuracy of 18 feet, around 5 meters. I don't think 18
feet in one hour travel for 60 miles creates a huge speed error.

After doing quite a bit more reading on these speedometer accuracy issues,
it seems that what was stated earlier is correct, most countries allow -0%
and +5% error to keep from under reporting actual speed. I am still not
positive on odometer readings, as many have reported these to be accurate
despite the speedometer being 5% optimistic. I read on one site, the
parameters to calibrate speed and distance are separate entries in the
software. Odometers are a much more sensitive issue due to warranty issues
and resale involving state laws here in the U.S.

I am still quite surprised at how many cars I check, normally one to two a
week, (rentals, late model) are reading exactly with my GPS. My personal
car, 2005 Forester XS, being the worst I have checked. I always set my
cruise control by my GPS, normally 3 over the speed limit, and I note what
the cars speedometer indicates. I normally drive 600 to 1000 mile a week, so
I get plenty of time to observe the readings, and as an added bonus, I get
to hit the road construction radars here in Illinois to get 3 readings, and
I have never seen a variation between a radar and any of my 3 GPS units.


I found this at SAE.org:

SAE Standards for Works In Progress

Not available for purchase at this time.

Document Number: J678
Project Number:
Project Initiation: November 2008
Revision Number: A


Title: Speedometers and Tachometers-Automotive

Issuing Committee:
Odometer And Speedometer Standards

Scope:
This SAE Recommended Practice applies to speedometers, odometers, and
speedometer drives typical of passenger vehicles, buses, and trucks used for
personal or commercial purposes. The method of determining wheel revolutions
per unit distance and overall system design variation are applicable to
passenger cars only. Comparable recommendations for trucks and buses are
under development. The data of tachometers is applicable to vehicular use,
as previously described, and also to stationary and marine engines and
special vehicles.

Blair Baucom
2005 Forester XS

Your speedometer is measuring speed along the path you are driving while
the
GPS is taking samples of your position and calculating a average speed
using a series of straight line distances. Plus GPS's aren't overly
accurate- I think approx 10 meters.

 

[Yousuf Khan]

Your speedometer is measuring speed along the path you are driving while the
GPS is taking samples of your position and calculating a average speed
using a series of straight line distances. Plus GPS's aren't overly
accurate- I think approx 10 meters.

The 10 meter distance accuracy limitation won't affect the speed
readings' accuracy. The GPS can track you within 10 meters, but if
you're moving, the speed accuracy penalty cancels out because you'll be
moving much more than 10 meters during your travels. So the GPS speed is
actually highly accurate.

Yousuf Khan

 

[Yousuf Khan]

Hi,

It seems, speed reading in my car is ~3 miles/hr less than actual (I
compared it with speed reading by Garmin GPS). To calculate MPG I use
gallons reading at gas station and total miles count since last fiil-up
- is this number (MPG) also incorrect?

That's interesting, I've measured my own speedometer accuracy this way,
and I found mine was within 1 km/h of the GPS reading. Are you using
after-market low-profile rims and tires?

Yousuf Khan

 

[boris]

That's interesting, I've measured my own speedometer accuracy this way,
and I found mine was within 1 km/h of the GPS reading. Are you using
after-market low-profile rims and tires?

Yousuf Khan

I changed tires couple times - it's an old car: 2001 Forester. But I never
changed rims and use regular AS tires (Goodyear ComforTread).

Boris

 

[YKhan]

I changed tires couple times - it's an old car: 2001 Forester. But I never
changed rims and use regular AS tires (Goodyear ComforTread).

Hmm, mine's an even slightly older 2000 OBW. My understanding is that
these speedometers are completely electronic, so I don't know how in
the world they would adjust the timing of it, unless they hook it up
to a computer reprogrammer somehow.. In the olden days when the
speedos were electromagnetic, you could probably adjust some sort of
tensioner or something.

Yousuf Khan

 

[boris]

I changed tires couple times - it's an old car: 2001 Forester. But I never
changed rims and use regular AS tires (Goodyear ComforTread).

Hmm, mine's an even slightly older 2000 OBW. My understanding is that
these speedometers are completely electronic, so I don't know how in
the world they would adjust the timing of it, unless they hook it up
to a computer reprogrammer somehow.. In the olden days when the
speedos were electromagnetic, you could probably adjust some sort of
tensioner or something.

Yousuf KhanActually,

Incorrect speedometer readings aren't a problem for me: I know is off by
~3mph and adjust my speed accordingly. I wanted to find out what's my *real*
gas mileage - whether odometer readings are also incorrect. I get ~20-22/mpg
in city driving.

Boris

 

[Yousuf Khan]

Incorrect speedometer readings aren't a problem for me: I know is off by
~3mph and adjust my speed accordingly. I wanted to find out what's my
*real* gas mileage - whether odometer readings are also incorrect. I get
~20-22/mpg in city driving.

That's about how much I'm getting myself in mostly city driving.
Sometimes it's a little less even if the city driving component goes up.

Yousuf Khan