It seems my speedometer is reading 2-3 MPH faster than what I am actually doing. I am driving on my stock 235/45/18s, PSI is correct.

I have always noticed this, but last night while I was passing one of those speed traps that show your speed, I started to question the accuracy of the speedo. Well they put one of those up right by my house on a side street so I did some testing driving past it a few times late at night. I got up to 30 MPH according to my speedo, but the readout said 27 MPH on the display. Everytime it was within 2-3 MPH. And I have tested this with other speed traps in the past with the same result.

I know some speedometers are not dead on, but my question is since the speedo is reading faster, wouldnt this mean my odometer is increasing more than it should? Or are they not connected to eachother?

IIRC there was a class action lawsuit against Nissan on the 3rd generation Altimas about their speedos reading too high, which resulted in mileage to rack up alot faster than it should. Should I bring this up to the dealer and have it recalibrated?

 

Check against a good GPS before you blame your speedometer. Radar suffers from "cosine error." Unless you are driving directly at the radar, the indicated speed will be lower, calculated as {true speed} * cosine {angle}. Your reading of 27 would mean a true 30 if the cosine were 0.9, which would be true if you drove past the display at a 26 degree angle.

See also http://www.copradar.com/preview/chapt4/ch4d1.html.

 

now that i think about it I was driving at it on somewhat of an angle. Thanks for the info. I'll have to borrow my dad's Garmin one of these days and do some comparsions.

 

im running on 235/35 19...

problem is i dont know how reliable is the speed on my garmin indicates..

i tried doing 180km/h and the gps clocked 169km/h...

last i tried a top speed run...where i should be running at 250km/h it was clocked on my garmin at 231km/h

i dont know but it seems like a problem to me...

 

I think you can get a fairly accurate calculation if you drive a few miles on a highway with mile markers. I've timed my G on a 3-mile stretch I drive to and from work. Timed it 3 times at 60mph and came up with 3 minutes each time. So dead nuts on.

BTW, I didn't know about the Nissan one, but there was a class action suit against Honda. They wound up awarding a 5% allowance on leases and warranties on all '04-'06 Accords and Odysseys (could have been more models and more years, I don't remember exactly). I had an '06 Accord and noticed immediately it was off (and I had a lease), so the lawsuit was appeciated by me.

 

My speedometer consistantly reads 3-4mph higher than my GPS at highway speeds.

 

If you are locked onto three or four satellites, the GPS will be far better than any speedometer.

 

well if our speedometers are truely reading higher than they should, doesnt that mean we are putting on miles faster than normal? Its probably not very much but over time it could add up.

 

I'm on 235/45R18s and I've checked the accuracy with a couple of my $3K GPS units and the speedometer is within 1 to 1.5mph. Don't forget that some different makes of tires may be shorter or taller than other tires of the same size. Also, as tires wear, they get shorter. It doesn't sound like a big deal, a worn tire could be over .6" shorter than a new one. That will impact speedometer accuracy.

 

^^^ Very true. Check your tires outside diameter specs from the manufacturer's site. On my previous car, I replaced worn tires with new ones (same model and size) and I no longer scraped going down my driveway. It makes a minimal, but noticeable difference.

 

I have found my speedo to be pretty much dead on with my garmin nuvi gps unit.

 

My previous car was well off, which I found shortly after I purchased it for going 82 in a 75 zone (my cruise was set at 75). Got a warning for that.

My current vehicles, a 97 F150 and an 04 G35, both are close on. Within 1mpg (which is nearly about as accurate as you can get with our gauges).