Odometer / Speedometer Calibration

[Rolf-Dieter ✝︎]

I need some help and wonder if any of you can help me out.

My Speedometer is out by about 15 MPH and on my trip last week to Williamsburg and back home the milage is way out. It stands to reason that if the speed is out by some 15 MPH then the milage counter is out as well. I know that I put around 1,600 miles on the car for the round trip. I also know that we made a slight Detour on two occasions, however, when I compare the start milage with the final count when returning home something is trebly wrong.

So, my question to all you is "Has anyone of you had there Speedometer re-calibrated"? If so "Where did you have it done?"

Thanks a buch!

[Mike Hughes]

The question I would ask is why you would feel the need to have the speedometer recalibrated?  Have you tried any of the usual methods of checking your speedometer, such as verifying the odometer against a measured mile or checking if you travel a measured mile in exactly 60 seconds at an indicated 60 MPH?  Have you or a previous owner changed the dashboard insrtruments transmission or rear axle? Or, are your 40+ year old instruments just in need of a little TLC?  

Recently I noticed that my tachometer was a little noisy and my speedometer needle was a little wavy at certain speeds.  I sent both instruments off to Palo Alto Speedometer in California to be cleaned and serviced.  They came back looking brand new and the only caveat was that they required for me to obtain and install new speedo and tach cables before reinstalling the instruments in the dash.  On our trip down to Williamsburg my speedometer and the MPH readout on my Garmin were in perfect sync.

[pj]

Hi Dieter,
I check against the GPS in my smartphone. If you want a second opinion, let's get together and use both our smartphones at the same time.

[Flyair]

Don't forget the ubiquitous question regarding tire sizes.

BTW Why this thread is in the Full Member category? Any information that should not be channelled to the general audience?

[Peter van Es]

When I switched to Vredestein 185x14 tires, I found that my km/h speedo is now spot on, whereas I had a ~4% deviation with  195/70 tires.

The thread is now moved to the correct board... Just a post by accident in the wrong section by the OP.

[Rolf-Dieter ✝︎]

Mike, I know I am out by a country mile (so to speak) since I just traveled 1,600 mlles and when my speedo showed 90 MPH I was actually only traveling 70 MPH. Also my end milage from my trip (according to the speedo I've traveled over 2300 miles (subtracting end from start milage).

pj, Thanks Peter, I do have it under control. I purchased a new GPS and will make a detail comparison Speedo verses GPS. Then I can calculate the percentage differential between the two and can calculate my fuel consumption something that is not possible right now.

Stan, yes my bad I should have posted it under Instruments ... what was I thinking

Peter, thanks for moving the thread. I do know all about tire size having an impact on the speedo results. My tires are like yours, P185/70R14 Michelin. The speedo was changed in 1992 it might be that the speedo came from a different Mercedes model with different tires or it may have been a refurbished one I have no idea (RM Restoration/Auction have no more file on hand as to what was done to the car in 1992). It's just a huge difference (speedo shows 90 MPH when I actually only go 70 MPH according to the GPS).

My initial hope was to find out how many others have similar problems, may they be cable or connection issues. Or if someone had the speedo re-calibrated and what other issues have surfaced. May have had his serviced etc. ,,,  and I appreciate this kind of feedback. I will add a table here once I made my detail road test comparison in 10 mile increments; speedo verses GPS readings in Miles and KM.

[aussiebattler]

Hi Fellas,
I know this question is not about speed calibration, but while you're on the subject of speedos, does anyone have knowledge about the dust covers on both the speedo and the tacho, mine have both "melted" or disintegrated.

[capcgn]

Your tires ( 185-70-14 ) are the wrong size and way to small.  You need 185 HR 14. The difference is  -6%.  To calibrate the Speedometer  with wrong tire size is not optimal.

[Rolf-Dieter ✝︎]

My tires now on the car are, MICHELIN P185/70R14 XA4 M+S ALL SEASONS White Wall (previous owner had those tires mounted).

The manual shows as you mentioned 185 HR 14 (with tube).

Where did you find information pertaining to -6% I cannot find any information and/or dimensions on the Internet?

I did contact Michelin for them to let me know what the difference might be.

PS. When I travel 60 MPH (speedo readout) that would be 96.6 KM/PH my GPS reads 82 KM/PH (that is more like -18% perhaps a combination of speedo out off calibration and tire size issue).
 

[Peter van Es]

capcgn is right... On this page in the Tech Manual you'll find:

The standard Pagoda Tire Size is a 185x14 tire. When these tires were originally made there were no variations on the Aspect Ratio. In those days all tires had the same sidewall height to tread width ratio, 80 for European tires and 78 for US sourced tires

My Vredestein Tires are rated at 185 HR 14. If you go to the Tire Calculator in the Tire Encyclopedia linked from the Technical Manual, halfway down the page, and enter: 185/80/14 (the default for our cars) on the right, and 187/70/14 (what you have) on the right, it will tell you about the 6.02% difference in circumference, and hence speedo variation.

Peter

[Rolf-Dieter ✝︎]

Thanks Peter ! The link is much appreciated. The tires on my car are fine and will have to do until such time they need to be changed. Meanwhile my GPS will guide me thru speed limit zones.

Perhaps come winter I'll remove my Speedo and ship it to California for a minor adjustment since deviation of actual speed is a bit off from the 6%


Thanks everyone ... it's all good now.

[Benz Dr.]

I know my speedo is off a lot so I never look at it. I watch my tach instead and know that 3,000 RPM is very close to 60 MPH. Every 500 RPM is roughly 10 more MPH. When we were traveling on the interstates I often ran at 3,500 RPM and I bet it was close to 70 MPH.

 '' I love America. Everything is big there including their speed limits. ''