speedo slow

[sean sherry]

An interesting old post from Styria on inaccurate speedos. My 280 SL is 11% slow on the odometer. I have fitted a higher ratio Diff but the speedo was also too slow with the original diff. Now I know that diff Ratios and speedo heads are matched. The speed reading was easy to alter , just move the Needle to start from about 15 MPH. A quick twist removes most needles. Finding a second hand speedo head for a 280 107 is well neigh impossible but one to match the higher ratio diff a lost cause. I have found a device that interrupts the pulse from the gearbox and claims that the signal can be increased or reduced.
How it increases the pulse speed I don't understand.
Has anyone experience with such a device sourced from "J Car " .I don't want to proceed with fingers crossed.

 

[Styria]

Sean, would it be incorrect for me to assume that a W116 Speedo from a 280 Sedan would be suitable for your car ? I am just thinking aloud - your car is a 1981 model - correct ? Hmm...there may be some difficulty...well, just not sure where I am coming from, even less so where I am heading in this case. Regards. Styria

 

[sean sherry]

Again Styria "Great minds think alike" The same thought had just occurred to me. The speedo head from a Sedan should just bolt right in . The clusters may look different but should be the same inside. The 280 Sedans will have a higher diff ratio than a 280SL, so maybe that's what is now in my SL ? I don't remember what it was changed to.
Nothing ventured nothing gained ,and probably an easier option than experimenting with electronic signals.

 

[SEL_69L]

Sean:
Non spec. tyres with a different outside will make a difference up to 5% or 6%, either way.
Non spec. diff ratios will also make a difference, you need to know the standard spec. ratio for yor car, whatever it is, and what the current diff ratio is in your car.
Non spec. speedo and odometer will also make a difference. Could it be possible that you have a speedo instrument set that is from a 350 or 450?

Together two or three of these factors may explain the difference as large as 11%, that you are observing.