Speedo is not accurate

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reefkeeper

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74 Duster
speedo is off... says I am going faster than I am. I think ithas to do with the 15" wheels the previous owner put on the back. Is there a gear that I should change out where the speedo cable plugs into the trans?

Front has 14" wheels.
 
Below is a chart I found online, you will need to double check to make sure.
The last two digits are the number of teeth on the speedometer pinion.


axle
-------24"------26"------28"-----30" tire size
2.76 2538930 2538928 2538926 2538924
2.94 2538932 2538930 2538928 2538926
3.21 2538935 2538933 2838931 2838929
3.55 2538939 2538936 2538933 2538931
3.91 2538943 2538940 2538937 2538935
4.10 2538945 2538942 2538939 2538936
4.30 2538944 2538941 2538938
4.56 2538945 2538943 2538940
 
You can order a new speedo gear for your combo from a few places. I know year one sells them.
 
reefkeeper, formulas340 is on the mark. If the 15" tire is larger it would be
reading slower, not faster than stock.
 
I got my gear (number of teeth) based on my time size (215/70/R14) and had it checked at a local sherifs station-it was accurate. As I remember, a mile per hour slower.

Dave
 
dave273dart said:
I got my gear (number of teeth) based on my time size (215/70/R14) and had it checked at a local sherifs station-it was accurate. As I remember, a mile per hour slower.

Dave
Click to expand...

What I forgot to mention was-take the old one out, take it to a local tansmission shop and they will probably match up what you need-just tell them the number of teeth. That what I did-and it didn't cost me a thing. I got a used one in perfect condition. Look up the in information on the internet. Speedodometer Gear Ratios/calculations, things of this sort.

Dave
 
so my son advised me that with his new GPS that the speedo indicates he is going 70... he is actually going 55.
 
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