Speedometer vs. AVG MPH computer calculation

davidbus159 · Tue Jan 31, 2012 2:40 am

In my former 2010 Kizashi (SLS with 18 inch wheels), I would get on the freeway and set my cruise control on 76 mph, then I would re-set the AVG MPH calculator. After driving for awhile, the AVG MPH reading would always settle around 72mph. I can accept that this is not an exact science, but I can't help but wonder which number is closer to being correct... especially whenever I see a cop!

I'm really HOPING that the speedometer is reading a little "high."

Any thoughts or experience with this?

Tue Jan 31, 2012 3:26 pm

Keep in mind that "avg" means its taking all the speeds that you traveled at and taking the number right in middle. So thats why in day to day traveling you typically see a number in the 40's even though you have been on the highway doing 80. Unfortuneatly that end result means nothing to you and me. But it is used in caluculating the mpg's (which also produces a number that is not "real"). In your circumstance, reseting the avg mph while doing around 76mph is giving the computer numbers that are relatively the same. Even thought you really aren't doing 76 exactly all the time. The car is still going up and down somewhat. Hence the 72mph avg you are getting. Personally I don't care for the method of caluculating Suzuki chose for the center display. It almost seems like they don't want you to see the reality, but the "avg's". Anyway hope this helps you somewhat.

sj85 · Tue Jan 31, 2012 3:34 pm

Just my 2 cents

I have an OBDII Bluetooth scanner that I use with Torque in my phone. Looking at the data that the car's speed sensor is providing with the GPS speed, car's digital speed sensor is more accurate than the speedometer.

Now, as nytq explained above, average is average, but in cruise control it should be pretty accurate specially if you reset it just after/before u set the cruise.

But as far which of the speed information is more accurate (atleast the speed data from OBD vs Speedometer), compared to GPS speed data, the digital speed data is more closer to the actual speed.

MadocOwain · Mon Mar 31, 2014 8:30 pm

An informal road test I performed:

GPS unit that shows MPH - shows 72mph
Kiz dash "avg speed" reset during level interstate travel at 75mph by the speedometer - shows 72mph

Tests at lower speeds showed the discrepancies didn't start appearing until speeds over 45mph.

smsmart · Tue Apr 01, 2014 4:59 pm

MadocOwain wrote:GPS unit that shows MPH

Are you referring to the navigation unit? If so, I wouldn't rely on the speed data it shows as it can be +/- several mph off from what you're actually travelling at. For example, at times I've had it show that I was going 5mph when I was actually at a standstill in traffic

MadocOwain · Tue Apr 01, 2014 5:06 pm

No, I don't have the built-in navigation unit. I used a Garmin mounted on my dash.

smsmart · Tue Apr 01, 2014 5:28 pm

MadocOwain wrote:No, I don't have the built-in navigation unit. I used a Garmin mounted on my dash.

The built-in navigation is also a Garmin unit. Either way though, I wouldn't rely on a GPS unit's speed data for accuracy.

MadocOwain · Tue Apr 01, 2014 5:40 pm

Eliminating the possibility that the average speed indicator in my Kiz is influenced by a GPS signal was the point of my stating I don't have the built-in navigation unit.

If the GPS I used were slaved to the odometer in some way it would certainly be suspect. I found the output from the console matched perfectly with the independent GPS unit at every speed, while the speedometer was increasingly out of sync with the GPS and average speed indicator as speeds increased beyond 45 MPH.

Sluggo · Wed Apr 02, 2014 5:10 pm

My Garin GPS has agreed with my last two cars speedometers.
In my Kizashi is shows that an indicted 70 mph is actually 68 mph.

If I reset my average speed indicator when I have the cruise set to an indicated 70 mph on the speedo after a minute my avg speed and GPS will show 68mph.

No doubt the speedo is optimistic. This was also noted in Car and Drivers long term test of the Kizashi.

LPSISRL · Wed Apr 02, 2014 5:30 pm

Keep in mind, that if the speedometer is off, the faster you go, the further it will be off. Five percent off at 40 is 2 MPH. Five percent at 70 is 3.5 MPH. The best way to check is to on level interstate to put the cruise at 60 MPH and then start a stopwatch when you pass a mile marker. Stop it at the next mile maker and see if you have more or less than 60 seconds. For more accuracy, do a 5 or 10 mile test.