79 300SD speedometer adjustment possible?

[79W116]

As the title says I have a '79 300SD that the speedometer reads 60mph when I'm actually gps driving 55mph. I just installed new 195/70r14 winter tires as well. I thought those were the correct size.

Is there a way to adjust the stock speedometer to be accurate? I read that the dakota digital SGI-100BT will work on this forum, but then looking into the tech manuals it looks completely mechanical, with just a cable spinning in a tube.

Any ideas?

[rumb]

Was it correct before the new tires?

Are you sure you have the correct speedometer in the car and it wasnt replaced? What is the part number on the bottom of the dial?

[79W116]

I bought the car with only a ppi, and then sunk in a ton of repairs before I got it, including new tires. So I dont know if it was reading correctly or not.

The records do indicate that all is original with only 123k miles, with the part number as 116 542 23 01

[Randys01]

AS a matter of interest, in older vehicles, how does any speedo know the difference between a summer tread and a winter tread? Assuming the tyre sizes are the same, a winter tread has a perceived larger rolling circumference than an equiv summer tread.
Having to swap tyres here in Aust is not really an issue.

[raueda1]

AS a matter of interest, in older vehicles, how does any speedo know the difference between a summer tread and a winter tread? Assuming the tyre sizes are the same, a winter tread has a perceived larger rolling circumference than an equiv summer tread.
Having to swap tyres here in Aust is not really an issue. 

I'm pretty sure that it's a visual perception thing.  The size is the size.  I've found calculators like THIS helpful, doubtless others have too.  As for speedo error, when I got my car I went from 215/70-14s ==> 235/60-15s.  Visually the bigger tires look a LOT bigger in the wheel well.  Yet, per the calculator, the difference in circumference is only 21mm (~1% bigger).  And on top of that, my speedometer consistently measured about 8 kph slower than actual, regardless of speed (over about 50 kph and faster).  A couple years ago while fixing the odometer the shop also recalibrated the speedo.  It was a supposedly reliable vintage car speedo shop used by people here, the Porsche boards etc.  The result?  Still reads 8 kph slow.  Make of this what you will.  So I just kind of ignore it.   Cheers,

ps - My speedo has always been steady, not fluttery as with some old MBs.  Recently I've driven the car in cold temps, say 0C, and there's some speedo flutter.  I never noticed this before but have only driven the car in hot (or very very hot) summer conditions.  I'm thinking that it's a temperature thing with the cable or something like that.  Anybody experience this?  Thanks.......

[UTn_boy]

The 300SD models left the factory with 185 series tires.  I don't think using a 195 series tire would cause much of a discrepancy with the speedometer.  Then again, when these cars were new, the margin of error on the speedometer was always 3-5 miles per hour off one way or the other.  They were never 100% accurate. 

[raueda1]

The 300SD models left the factory with 185 series tires.  I don't think using a 195 series tire would cause much of a discrepancy with the speedometer.  Then again, when these cars were new, the margin of error on the speedometer was always 3-5 miles per hour off one way or the other.  They were never 100% accurate. 

Thanks, explains a lot.

[79W116]

Thank you all for the replies.

All my searching has brought me to 195, but you're right, at 55mph it would only raise the reading to 1mph if it was supposed to be 185.

I've also had my speedo down to about 35F with no flutter.

So, does anyone know what fancy speed shops do to the speedos then, the ones that actually are readjusted correctly?

[ptashek]

If you don't mind taking a risk of breaking the needle shaft or scoring the dial face, there's a DIY way to adjust the speedo.

It's not 100% but will get you there most of the way.

You need to measure the angle the needle is at at the indicated speed, versus the mark of the actual speed. Then you carefully pull the needle off of the shaft, rotate it by the measured angle and refit.

Personally, I'd only bother if indicated speed was significantly lower than actual.

[79W116]

If you don't mind taking a risk of breaking the needle shaft or scoring the dial face, there's a DIY way to adjust the speedo.

It's not 100% but will get you there most of the way.

You need to measure the angle the needle is at at the indicated speed, versus the mark of the actual speed. Then you carefully pull the needle off of the shaft, rotate it by the measured angle and refit.

Personally, I'd only bother if indicated speed was significantly lower than actual.

I was thinking something very similar. Moving the needle to 75 with my very thin needle nose vise grips under the needle. Then repositioning the needle to 70.

Hopefully someone on here knows the real way to calibrate and can post for all of us.