Speedometer vs GPS

[stationjj]

A few months ago I had to replace my speedometer and Speedo cable. Speedometer from LMC and I found a speedo cable on Ebay that stated it fits a CUCV. The speedo cable fit perfectly and works. GM Part Number #25032661. I did notice the reading from the Speedometer vs GPS aren't exact.

What I noticed is that as I increase speed the difference between what the speedo and GPS reads grows.
At speeds of around 40 mph on speedometer. The GPS will read 38-39 MPH about 1-2 mph off. Not bad 1-2 mph off. Now as I increase my speed the difference between what the speedometer and GPS read will grow as much as 10 MPH. For example the speedometer will read 80 MPH, but the GPS reads 70 MPH.

The speedometer and speedo cable are new, and my GPS is a newer Garmin Nuvi. I don't expect the speedometer and GPS to read the same all the time. A few MPH difference between the speedometer and GPS isn't a big deal. What caught my eye was the difference between the devices grew as the speed increased.

What is showing the correct speed? I think the GPS is. I wish I had a tach installed.

Has anyone else noticed this? With the age of the CUCVs and other factors is this a common issue? Is there anything I can do to fix this?

Thanks for your help and input.

stationjj

 

[Keith_J]

Common. At indicated 65 MPH, my M1031 is doing 62 MPH per GPS.

 

[jw4x4]

Tire size determines speedo accuracy. Even tiny differences effect the mph reading- tread depth, tire pressure, wheel width, etc. The difference is barely noticeable at low speed. As speed increases, the gap between actual speed and that which is displayed on the speedo also increases. Think of it as a percentage, or ratio. The higher the speed, the bigger the difference. I am certain the GPS is dead on.

 

[txmytx_catahoula]

Ditto what jw4x4 said, speedometer error is a ratio or percent so it shows more at higher speeds. If you're interested in correcting it or calculating the error you might try this site http://chevelleengineer.home.comcast.net/~chevelleengineer/speedo.htm

 

[stationjj]

Thanks,

I am new to this, learning as I go. I do understand what your saying.

I bought my CUCV from a previous owner about 9 months ago. The PO had put new tires on it. I honestly don't know what size tires the PO put on them. The tires are in great shape and serve my purpose for now. I never had any reason to question my tires. That may change later? I will check tomorrow, but I suspect it's a tire size issue. I can deal with the speed difference issue.

Thanks for clearing this up for me. I just learned something!

stationjj

 

[cpf240]

Just keep in mind that this stuff is not perfect, and was never intended to be. Tire size and air pressure in the tires, affecting the diameter of the tires, are the two biggest factors in speedo error. Even tires from different lines or manufactures will vary, even if they say they are the same size. Yes, a tach would help, but then there could be slip in the torque converter or in the trans. Even the GPS has an accuracy factor in it, so while it is probably much more accurate then the speedo, it still won't be as accurate as a measured mile and a good stopwatch or perhaps a properly calibrated radar gun.

If this were an aircraft, I'd want it to be as accurate as possible. For the typical car, "close" is good enough for me.

 

[Gripy]

I'd be more worried about your engine if you are trying to push it to 80mph...

 

[stationjj]

I don't think I am not pushing the engine 80 MPH. Only the speedometer shows 80 MPH. The GPS shows 70 MPH. I trust the GPS as more accurate.
On the highway I keep the M1009 between 60 and 70 MPH.

stationjj

 

[wayne pick]

I've owend GM trucks all my life, and never had a speedometer or fuel guage work properly for any extended period of time in any of em. I bought my truck with a 700R4 conversion and 285/16 tires. the speedo was off by 20mph with the usual jumping around, ect. The cable finally broke so I just use my Tom Tom GPS as a speedo and trust it to be accurate. My M1031 bounces too much to go over 60-65 MPH anyway, but for M1009 guys that can actually break the speed limit, an accurate speedo is good to have.

 

[Scarecrow1]

It sounds like you need to trust the GPS and start replacing parts one at a time starting with the cable . Most times it is the culprit with winter the grease may be hardening or just worn out. considering you replaced the cable already then you may want to look closer to the back of the odometer to see if it has wallowed out the connection there. Good luck and be sure to let us know when you find the problem...

 

[MarcusOReallyus]

Under normal operating conditions, your GPS will be easily better than +- .5 MPH.

In dense urban environments (Manhattan, Chicago downtown) it might not be quite that good.

 

[shaker]

I am having speedometer trouble I replaced the cable and am running stock 31x10.50.15 tires BF Goodrich AT KOs. I have narrowed it down to the speedometer gear in the tranny my questions is how many teeth does the gear have so I can get the right part at this point in time I can not take out the part until I have the new one due to it is my only way of going. So if anyone who knows the correct color and tooth count on the gear I would be very appreciated. In addition, I am running the stock gears, tires, trannys.

 

[Kloakndagr]

My speedo cable just went out tonight on my way home from recovery. Where does one fine a replacement cable? Do you have a part number and/or source? Thanks. I did a search and was brought to this, but nothing about where to get a replacement.

 

[cpf240]

My speedo cable just went out tonight on my way home from recovery. Where does one fine a replacement cable? Do you have a part number and/or source? Thanks. I did a search and was brought to this, but nothing about where to get a replacement.

Have you tried the -20p or -34p TMs that are available on this site? They list all the part numbers.

 

[Kloakndagr]

Have you tried the -20p or -34p TMs that are available on this site? They list all the part numbers.

Ok..what is a -20p or -34p?

ive downloaded some TMs but they have long numbers, so please explain what a -20p and a -34p. Thanks

 

[212sparky]

-10 operators manual -20 repair manual -34 overhaul manual

 

[GreenBull]

2-3% positive bias is pretty much standard for automotive speedometers. If you are lucky the odometer is slightly more accurate.

 

[Kloakndagr]

-10 operators manual -20 repair manual -34 overhaul manual

Alright, I was in the Air Force, so bear with me....can someone provide a link to a -20 or -34 TM, theses numbers are code to me. Thanks.

now if ya wanna know how to get a confession out of someone, or figure out whether your ex gf is lying to ya, then I'm on track. He he

 

[Kasper31]

GPS will be way more accurate than the speedo. I use the GPS most all the time for getting me where I'm going and speed readout.

 

[MarcusOReallyus]

Ok..what is a -20p or -34p?

ive downloaded some TMs but they have long numbers, so please explain what a -20p and a -34p. Thanks

Look at the last part of the TM number.

http://www.steelsoldiers.com/showthread.php?77609-TM-s-for-the-CUCV