Speedo Calibration (HELP!)

[ratedZ]

Hey guys, I got a ticket for speeding the other day...if I remember correctly, my speedo showed around 95-100mph before I got pulled over. The officer told me that he got me at 80+. As you can see it’s off by a lot. Here’s some history of things I’ve done to the car that has affected the speedo. Swapped out the electronic speedo to the 79 280zx 130mph speedo. I originally used the speedo cable from the 79 280zx but I had to replace it with one I had laying around from the original set up because the speedo would jump up and down at speed. So now I’m using the cable that came with the electronic. It seems to work but it’s still off...I don’t think that using the speedo cable from the electronic set up is the problem as I’m having the same exact problem even with the 79 speed cable. Other than this mod, everything including the tires are the same 225/60ZR15 on 15x7’s...didn’t touch anything else. I’ve been to the Datsun Garage and read about the different cogs....I don’t think replacing the cog will help my speedo read accurately as I never touched the differential. Sooo, I need some help. Should I go out and grab another speedo cable for the 79 cluster? or do you think the speedo gauge it self could be faulty (it came from the junk yard). I don’t need it to be dead on accurate but I would like to have an idea of how fast I’m going...can’t afford another ticket Thanks all!!!

[Guest Anonymous]

Problem solved yet?

 

A jumping needle means either a faulty head os a dry or failing cable. If you changed cables and that went away then it was the cable alright.

 

Now that we know the head is good I will tell you that cable driven heads are activated by a spinning magnet principle and 'Auto Electric' shops used to be able to change the magnetisim to compensate things, they just needed to know by how much of course.

 

There is something I don't understand about what you said concerning electronics. If the head is electronic then it operates on square wave impulses triggered by the distriburor and no cable is involved? Following this I must say that it is not likely that the head will fall into error unless circuitry involved is failing somehow.

 

Lastly, I am not going to look in my manual to see if the speedo cable goes all the way back to the differential. I beleive it attaches to it's own little gear assy. at the rear of the trans. housing? Oh well....

[getoffmyinternet]

Wow, that's a crappy way to find out your speedo is off. It's funny how you didn't error on the side of caution though, ie. you thought you were going even faster! But I don't know if going off of what the cop says he approximated is a very good meter of your actual speed. Before I started my swap on some days it would seem that my stock speedo is way off and others it would seem right on. When I would pass one of those signs that tell you how fast you're going it would always give a very poor reading as well. I first thought it was off on the day I bought the car and was driving it home from San Diego but didn't notice the needle ever go over 55 and when I hit 60 it seemed like I was flying by everyone. Solution, go to a speedo shop and have them test and adjust it. I guess if you can figure out for sure what your gears are and factor in your tire size etc. to figure out about what your speedo should be reading at a certain speed through a few calculations, but, eh.

[Tony D]

The question to ask is if the electronic speedo is calibrated to the same input rpm as the 79 130 mph unit you have. They usually have the califration speed for 60mph written on the gauge face (some obscure number like 1048rpm). If you take a non-contact tachometer, a Variable Speed Drill, and spin up your head to the given speed, you can see if the speedometer head is reading what it should. I have all this stuff, so it's easy for me to do. That at least tells you if the speedo head is off, and by what percentage. If it's dead on, then a cog is the problem.

 

Also, if you can do a 'timed mileage' segment in your car. Measure out 5280 feet and mark it with paint, and then drive at an indicated 60 mph through that section of roadway with a stopwatch...you can also calculate the speedometer system error. Many states have highway sections set up to do just this. For every second over and under makes it easy to figure out real speed. 30 seconds to cover the mile? 120mph, and 100% speedo error.

 

When you do the 'timed mile' method, it becomes easy to just put in a different cog that 'gets you close' without going to a speedo shop and having is calibrated.

 

In my state (California) the AAA will run spedometer calibration clinics, and let you run your car on a machine to you know your error. You also get a certificate to show your car was checked. It becomes interesting when you have a certificate showing your speed error, and you can then use it to confirm your statement of your speed.

 

Short of those alternatives, you're off to the speedo shop to get recalibrated. I haven't checked the speed difference for your tires, but that is a big thing that affects the indicated speed of your vehicle.

 

You can always have someone 'pace you' and get an idea of the speed error. And be forewarned: IT MAY NOT BE LINEAR. I have had MANY cars that were spot-on at 30mph, but off by 5mph at 65, and off by well over 15mph at 120! It's a percentage error usually, but sometimes with the mechanical speedos the higher speeds just go whacky.

 

Good Luck!

[getoffmyinternet]

Yeah, a common mistake is that people check their speedo with another car at only one speed. If your speedos are out of sync with each other, they may have way off readings but still intersect at one point. On the same token, yours may read under at a certain speed and then over at a different speed. I suppose you could time yourself between known distances on the highway, like if the call boxes are almost exactly a quarter mile apart, or if there are 500 stripes on the road in a mile, etc. but there will always be some sort of error based on your reaction time. Either way, I think "close enough" is all we can expect here. If at highways speeds it's only off by one or two mph, who cares?