[Thomas J]

It seems the speedometer on my 500sec (and my former 3 merc's) is showing around 10-13 km/h too much; this would be according to other newer vehicles riding along side of me, as well as my GPS (that probably is the most reliable source of the two) - and my tires are 215/55/16, so the diameter is the same as stock-tires...!

Have any of you had that same experience with your speedometers??

 

[jupiterpbc]

As I stated in a recent post my speedometer indicates I am traveling at 67 1/2 MPH when my GPS shows 65 MPH. I am running OEM size tires and they are Bridgestone G 009 (excellent tires, BTW).

The speedometer reading is a rather linear function, i.e., when I am traveling at 35 MPH it registers 37 1/2 MPH. Only ay lower speeds does the relationship narrow. Conversely, I checked it while traveling 85 MPH (GPS reading and my speedometer registered 89 MPH.

Unfortunately, the mechanics of speedometers is not an exact science. Digital configurations may be more accurate.

Gary

Gary

 

[Thomas J]

2½ mph is 4 km/h, right? - so your speedometer is very accurate!!

Like yours, my speedometer seems to have almost the same inaccuracy all the way up the scale - from around 40-50 km/h or so... although at very high speed (180-200 km/h), the inaccuracy is a little more (but that's quite normal, I guess...)

 

[jupiterpbc]

Yes, you are correct, 2 1/2 MPH equals 4.02336 kilometer/hour, however, I would consider that accurate when it comes to a Yugo or when changing tire size, but not with a Benz.

At any rate, the only reason I put a GPS to it was to make sure I wasn't driving at an excessive speed and hence a traffic ticket. BTW, the faster you go the more accurate a GPS becomes.

Gary

 

[cyberdrakon]

Yes, GPS is pretty accurate, but it doesn't seem to update fast enough, what programs are you using?

I have been using Delorme street atlas.

Also, if the difference is not progressive, then you can try removing the needle, and pegging it back in at the right calibration?

 

[Autootter]

Thomas J - 3/2/2005 7:02 PM - and my tires are 215/55/16, so the diameter is the same as stock-tires...!

Are you sure about those measurements? The standard tires for your car (in Europe anyway) is 205/65 R 15. They have a height of 13.3 cms and a diameter of 64.3 cms. Your 215/55 R 16s have a height of 11.8 cms and a diameter of 61.3. That is enough to cause a 4.67% difference in speedo readings - the optimistic way. So if the speedo reading is 100% correct at 100 km/h on the standard tires, with your tires the actual speed will "only" be 95,3 km/h even though the speedo reads 100 km/h. So I guess your tires can be part of the reason for the optimistic speedo anyhow? I run the standard tires and have had the speedo reading checked. At 100 km/h actual speed the reading is almost 100% correct. The small discrepancy can be explained by tire wear from new to worn tires.

 

[Thomas J]

Autootter - 3/2/2005 11:10 PM

Thomas J - 3/2/2005 7:02 PM - and my tires are 215/55/16, so the diameter is the same as stock-tires...!

Are you sure about those measurements? The standard tires for your car (in Europe anyway) is 205/65 R 15. They have a height of 13.3 cms and a diameter of 64.3 cms. Your 215/55 R 16s have a height of 11.8 cms and a diameter of 61.3. That is enough to cause a 4.67% difference in speedo readings - the optimistic way. So if the speedo reading is 100% correct at 100 km/h on the standard tires, with your tires the actual speed will "only" be 95,3 km/h even though the speedo reads 100 km/h. So I guess your tires can be part of the reason for the optimistic speedo anyhow? I run the standard tires and have had the speedo reading checked. At 100 km/h actual speed the reading is almost 100% correct. The small discrepancy can be explained by tire wear from new to worn tires.

I own an '82 euro model, and the standard tires for that is 205/70 R 14; which have the exact same meassures as the 215/55 R 16's I'm using now... it's the post-'85 models that uses 15" rims! []

check this link for calculating revs/mi...

http://www.miata.net/garage/tirecalc.html

And cyberdrakon; I use tomtom - and it's also slow in updating the speed - but I've been checking when driving at the same speed for a longer period of time... on the freeway and the highway... and I've actually never thought about moving the needle manually - haha, what a great idea!!! I'll definately consider that!

 

[MBeige]

Mine too. Reads 50mph when I'm doing 40mph in reality. Does anybody know how to correct this? New speedo? Retighten? Bad ground? It's also "unstable" sometimes and would wonder up or down a few millimeters. Then it would go back.

 

[H-TownBenzoboy]

karugs - 3/3/2005 1:03 AM

Mine too. Reads 50mph when I'm doing 40mph in reality. Does anybody know how to correct this? New speedo? Retighten? Bad ground? It's also "unstable" sometimes and would wonder up or down a few millimeters. Then it would go back.

I think what'd you need to do is take a GPS, and record your actual speed, and the speed shown at certain points on the speedometer at the same time. Then, take it to a speedo shop, and have it recalibrated with your readings.
-Joe