Speedometer
 

I had to drive for a ways on a road with a 10mph speed limit today. It made me look at the speedometer and I realized that it's one of those situations where you can look at something for many years and not really see it. I 'saw' that the distance between 0 and 20 mph is considerably smaller than the distance between 20 and 40 mph. And that the distance between 20 and 25 mph is considerably further than the distance between 60 and 65 mph. As many years as I have been driving 123's I'd been thinking that the speedometer needle progressed in a linear fashion around the numbers. Apparently not.

Kerry, Now that's most interesting. I'd never noticed before, but will surely take note tomorrow morning when I start my car up. I teach at a community/ tech college and am surrounded by all sorts of intelligent folks. This matter will now be on my endless list of questions that need answering. I'll let you know what I learn. This speedo is mechanical, not electrical. Right?

I teach at a community college too. Yes, it is mechanical and something is odd since you gear ratio must be consistent. I'm wondering if what appears to be a round ring of numbers on the speedo is really an elipse, accounting for the inconsistent distances.

I just looked at pics of my gauge clusters and the spacing is the same all the way around. Got any pics of yours?

No, I don't have a picture of mine. Weird. Maybe my speedometer is not mechanical.

Just looked at my W116 79 300sd. It is not the same as my 85TD. 0-20 is much smaller than 20-40m just like the TD. But the five mph increments from 20mph upwards all seem to be identical distances whereas on the 85TD they vary. Weird.

I just looked at my (mechanical) W123 speedometers. The increments are all the same, considering that the first line is 10 mph (not 0 mph).

Mine is linear except that the rest position is not "0" but rather... I think "5". Or maybe "10". I don't have that car with me at the moment but I know it's one or the other.

How can the rest position be 10mph and not 0? Near as I can tell, when my car is sitting still the needle is at rest.
Anyone else have varying 5mph increments?

The speedometer on my w124 starts at 10 mph I think. So I have to guess at my speed if slower that that.

Did a quick search. It looks like the consensus is that the 85 TD speedometer is mechanical and not electronic. However, both I and a person with a late 84 TD have noticed that our speedometers read fast. Could it be that we have varying distances written on the speedometer face but the speedometer needle is following the same arc as ones without the varying distance.

Isn't the speedometer portion of these "mechanical" odometer/speedometer instruments a magnetic and spring combination drive? If so it is very likely that the odd arrangement at or near the bottom of the scale is due to drag and non-linearity of the magnet-spring based drive system.

I'm not sure how they work. But the scale is definitely different on both sides (10-20 and 110-120) on my 85. Would the drag issue apply at the high end of mph? Why would the 116 and TD be different?

My speedo in my 82 240D starts at 10mph.. if the car is going slower than that, naturally, the needle doesn't move.

Yep, same on my 85 and 79 240Ds.