speedometer off

[wlf89]

my 720 speedo is reading off, I still have close to stock tires on it 205/75/14 but It reads off a good bit.

according to my gps and phone app to go,

45 speedo has to read about 53

55                                          65

65                                          80

 

is this normal for these trucks? would a speedo gear change cure this?

[DanielC]

If is is a mechanical speedometer, there are two parts.  The odometer, measures miles driven, and the speedometer.

The odometer is a set of many gears that count wheel revolutions.  Check the distance against a measured standard.  Drive down a freeway, with mile posts, and compare it to the odometer.  That will tell you if the tire diameter, rear axle ratio, and speedometer pinion in the transmission are correct.

 

The speedometer is just a magnet, spinning inside an aluminium cup.  There is no direct mechanical connection.  The spinning magnet induces an electrical current in the aluminium cup, and that creates a drag, that moves the speedometer needle against a spring.  If dirt, or crud builds up between the magnet, and aluminium cup, that will tend to drag the speedometer needle higher than it should.

[Guest Rick-rat]

:thumbup: Mine used to be off too, after rear gear change and bigger tires , it is right on now 

[JoeCool]

Aren't they all off? I thought that was normal!

[ggzilla]

Yes, its normal. The specified tolerance is +5% -10%. So at actual 60mph it can read 57 to 66 and still be acceptable.

 

But check the odometer first. The gears are to make the odometer correct, than if needed the speedometer can be adjusted.

[wlf89]

the odometer is correct according to the mile markers on the highway. so how is the speedometer adjusted?

 

the truck is a 82 720 with the z22 engine.  thanks

[DanielC]

I would look for Speedometer repair shops in your area.  On a 521, if you take the combo meter apart, you can remove the speedometer from the rest of the gauges, and there is a small note, printed on a covered up part of the speedometer face that says X RPM = 60 MPH.

Then  I think they just spin the input shaft of the speedometer at that RPM, and adjust the spring tension that wants to return the needle to zero, tighter or looser to indicate 60 MPH.

 

Does your truck have a tachometer?  If it does, use your GPS app to figure out what RPM, in what gear equals X MPH.  On my 521, 2,000 RPM, third gear, about 25 MPH.

[ggzilla]

Send it to a speedometer shop is the easy way.

 

 

620 speedometer says "60 MPH = 1024 RPM" on the bottom edge.

 

 

You can:

 

1. Change the pointer position. This is good if the speedometer is off by say 2mph at all speeds.

 

or

 

1. Change the magnetic tension. This will adjust the scale across the board. Be careful as it is easy to ruin it. For ideas, see: Recalibration of Speedometer http://homepage.ntlworld.com/johnstonrh/a30a35/speedot.htm

[bryant2482]

does anyones else's speedometer needle have a slight bounce to it as you go faster?

[ggzilla]

Sure, that happens when the cable is not turning smoothly. A new cable is only $15.

[Z-train]

720's are known for speedo issues.

[datzenmike]

First I wouldn't believe the GPS application at all. So this begs the question.... what does the speedometer read??????? 

 

Follow someone at a set speed or drive 60 on a highway. It takes one minute to go one mile.

 

Odometer isn't that important, (you don't get a ticket for going too far) so either recalibrate or change the transmission pinion gear to bring it closer.

[JoeCool]

^^ DITTO on what Mike said. Don't trust an app! ^^

 

I found out the easiest way is to check it against one of those speed limit warning signs with the digital readout of your speed underneath.Those are fairly accurate.

 

 

A worn out speedometer cable will read way off as well, until it breaks! Mine read 8mph off (high) for about a year until it broke last year. Got a new one, now it is maybe 1-2mph off.

[bryant2482]

Joe cool that's exactly the method I use!

[Yedi]

Yea something is up overall your tire size is REALLY close to stock at 65 mph you should have less than a mph difference stock vs yours is like 65mph / vs 65.4 mph aprox

[datzenmike]

The cable may bind and let go very rapidly flicking the needle higher than normal. Try a tube of lock cylinder graphite lube on the inner cable. Undo the transmission end by twisting it by hand. (you may need plier) Pull the inner cable out. It's very long slippery and floppy so be ready with a clean cloth to catch it and not get it dirty. Lube and replace it The gauge end will find it's own way back in as soon a you drive it and it starts turning. If when pulled out it is all crusty and rusted... replace as the outer sheath probably has a hole in it.

[Onderma]

My speedo is bad as well. Reads 85 with the truck on and stopped, any time I'm rolling it reads 100. Driving the speed limit is like playing an instrument.

[datzenmike]

If you can, replace the gauge. You can mess with it... might just be a stuck needle.