.45

Had the opportunity to do a lot of highway cruising over the past week. I did multiple comparisons (40+ tests) of ODO Mph against the trip computer average Mph and my ODO consistently indicated lower speeds than the trip computer. Is this common? Expected? Anyone else note this or have this problem?

Granted this was "not scientific" but I tried to control the variables as much as highway driving would allow to ensure accuracy. I tested multiple times under cruise control only, multiple speed settings (i.e. set to 70Mph, 75Mph, 80Mph), multiple road condtions (flat, subtle up and down hill), various traffic conditions from congested to completely open road, I minimized lane changes, tried multiple road surfaces, outside air temp varied from 55 degrees to 30 degrees, cold and warm tire pressures etc. Regardless of the environment the outcome was pretty consistent with a 2-3Mph difference. If I set cruise to 70Mph the trip computer would always come back with 72-73Mph (mostly 72). At 75Mph setting the computer would read 77-78Mph.

Car tested: Stock 02' GLE, auto, stock 215/55-17 EL42's on stock 17" GLE rims. I would expect more accuracy with no changes to the stock drive train, rims or tire diameter. So where is the "mis-read" problem, is the ODO reading low or is the trip computer high?

Why am I asking? I will be installing Michelin Pilot Sport A/S's on the stock rims within the next 1-4 weeks. I have been struggling between 225/50's and 225/55's and I want to factor this "ODO issue" into my decision.

Thnaks in advance for any insight.

Bluesbrekr

iTrader: (4)

My speedometer reading is very accurate. (based on the radar dispaly units they set up around here)It wouldn't surprise me that the trip computer is wrong.....the MPG reading on the computer generally reads about 3 MPG high.

.45

It's ironic you mention this Blue because I too compared my ODO to one of those "trailer speed units" about 3 or 4 weeks ago. I drove by one 3x at 40Mph on the ODO and the digital readout displayed a "flickering 40Mph" give or take. That was in-city, at less-than-highway speeds (lower tire RPM) and limited testing obviously but...seemed accurate as you indicated.

Bluesbrekr

iTrader: (4)

Mine has tested accurate at 60 mph. There was one set up in a 45mph zone and on my second pass I took it up to 60 to check my speedo.

E55AMG2

iTrader: (4)

There is error built into the speedometers of our cars. The only proper way to test it is with a photogate stopwatch. Basically to accurately measure time/distance and get speed.

Anuj

how come my trip comp avg's at like 30 when i drive 60 a lot? i just look at it and move it back to the temperature....am i supposed to leave it on the speed one?

unknownpmp

gles come stock with 225/55/17s?
my se came with 225/50/17
and my winters i'm runing are 45s - my speedo is slighty lower than what it reads, if the 55s are not what your car was calibrated to then maybe that's where the diff is coming from

Bluesbrekr

iTrader: (4)

You answer is in your question....it's an average. Even if you drive 60 a lot, you still slow down and stop, too. That is figured in the average also. Switching functions has no effect on how they work or what they are monitoring. They're all working all the time, but you can only view one at a time.

.45

typo, meant stock 215/55-17's