Speedometer

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

Reply to
boris
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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).

Reply to
John Varela

Reply to
Lena

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

Reply to
boris

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
Reply to
Blair Baucom

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

Reply to
Yousuf Khan

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

Reply to
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

Reply to
boris

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

Reply to
YKhan

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

Actually,

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

Reply to
boris

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

Reply to
Yousuf Khan

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