speedo reads different then gps

fonfixer · Post by **fonfixer** » Mon Jan 28, 2013 10:22 pm

I compared the speedo to my gps and they were pretty much the same until i get to 45 mph. Over 45 mph and it starts reading slow. I have 15 inch tires all the way around but i wouldnt think tire size would make it read slow above 45. any thoughts?

9t3svt · Post by **9t3svt** » Mon Jan 28, 2013 10:36 pm

Not sure as to the extent yours is off, but if your speedo is off, the faster you go, the farther off it will be.

71fordkid · Post by **71fordkid** » Mon Jan 28, 2013 10:55 pm

i wouldnt think too much of it, just learn what speed your going when your speedo says your off by using your gps. Thats what i did with me jeeps

mrollings53 · Post by **mrollings53** » Tue Jan 29, 2013 2:57 am

Just so you know the speedo isn't physically attached to the speedo cable. I don't know if it's a magnet in there or what, but the speedo cable drives this thing with a bar on the inside and two leaf like things around the "bell" that turns the needle. There is no actuall contact between the two. It does directly drive the odometer though. You may have some gunk built up on the "bell/needle" assembly that is slowing things down once you reach a certain speed.

BRUTUS_T_HOG · Post by **BRUTUS_T_HOG** » Tue Jan 29, 2013 11:44 pm

Its likely just that it was never very accurate to begin with. You might be able to take the speedo head out of the cluster and move the needle a little bit.

NM5K · Post by **NM5K** » Mon Feb 11, 2013 7:01 pm

Mine has always read low. I tried a different ratio plastic gear, and it didn't help.
It seems the gear that is in my tranny itself is wore out and slipping .
Or something like that.. It's been so long since I messed with it, I forgot the
exact problem. So I always use the GPS if I really want to know how fast I'm
going. You do get used to about where certain speeds are, but mine is often
flapping around a bit, so not too useful. I think if I'm doing 60, it usually reads
about 45 or so..

9t3svt · Post by **9t3svt** » Mon Feb 11, 2013 7:31 pm

If the needle is jumping, usually the cable needs lubed.

NM5K · Post by **NM5K** » Mon Feb 11, 2013 7:47 pm

I tried that with mine, but it didn't help much. It seems my gear in the tranny is
pretty wore out, and I think it slips, and sometimes makes it jump a bit due to
catching, slipping, and back and forth.. It's been a few years, but I came to the
conclusion I would have to change the gear in the tranny for it to work right..
Which for a lazy @#$% like me, too much work..

..

fonfixer · Post by **fonfixer** » Mon Feb 11, 2013 8:26 pm

mine doesnt jump continually but just as it is approaching 40mph on accelerating and 50 mph on decelerating. ill try and grease the cable see what that does.

Builtin73 · Post by **Builtin73** » Mon Nov 18, 2013 9:49 pm

[quote
Re: speedo reads different then gps

Postby NM5K on Mon Feb 11, 2013 7:01 pm
Mine has always read low. I tried a different ratio plastic gear, and it didn't help.
It seems the gear that is in my tranny itself is wore out and slipping .
Or something like that.. It's been so long since I messed with it, I forgot the
exact problem. So I always use the GPS if I really want to know how fast I'm
going. You do get used to about where certain speeds are, but mine is often
flapping around a bit, so not too useful. I think if I'm doing 60, it usually reads
about 45 or so..][/quote]

This is Exactly what mines doing thanks so much for this post... i know it's nearing a year old but new to the board and this truck ( a 73 f -100 ) so wanted to give credit from a google search to the posters here

NM5K · Post by **NM5K** » Mon Nov 25, 2013 10:13 pm

I had tried replacing and changing the "outer gear" so to speak, but it didn't really do any good.
I came to the conclusion that I need to replace the gear inside the tranny that the plastic gear I
replaced meshes with. Also, the outer plastic gears come in different ratios for adjusting for
tire size, etc as I recall.
I finally just gave up on it and use the GPS, which I hang on the sun visor.

PetesPonies · Post by **PetesPonies** » Thu Nov 28, 2013 11:37 am

wow, it's really not that difficult to get your speedo very close to correct. The drive gear rarely has any problems, the driven gear is the one easily changed. When there is an error, its a linear problem, so making a percentage change in the driven teeth to the same percentage of error is easy to do.

NM5K · Post by **NM5K** » Sat Nov 30, 2013 2:23 pm

It's the drive gear that is the problem on mine. And it's not the gear ratio that is off. It's
because of the constant slipping that is making it read low and jump around.
I tried new driven gears. Made no difference.