Posting for friend: He has 71 Cutlass where the speedometer is about 50mph off from the real speed. I would replace the parts one at a time starting with the plastic gear.

Any elimination process is appreciated?

 

50mph? So when he's going 10mph, it says 60? Or when he's going 60 it says 10?

Sounds implausible.

- Eric

 

Assuming it's the right tranny, I would figure out the rear end ratio first. Then I would figure out which gear should be in the adapter (teeth/color). Then pull the adaptor and see what color the drive gear is. I've never heard of being that far off. Maybe the speedo needle fell off before.

 

But he says it's off by 50mph.

The best case is that that would be a 2:1 error at highway speeds; a worse case would be as I suggested above, that it was 50mph off at lower speeds, resulting in an error of 3:1, 4:1, or even 5:1.

This is not possible from the gearing that normally exists, unless you actually have the best case scenario - if you had a 2.41 rear and used speedo gearing for a 4:56, you would be off by 2:1, and you would notice that the speedo was reading double your actual speed. If that were the case, you would be likely to say "My speedo's reading double," and not "My speedo's off by 50mph."

So what are the odds that his '71 Cutlass has either a 2:41 or a 4:56 rear end?

- Eric

 

An inaccurate speedo is usually off by a specific MPH at one speed. It is however, off by the same % at all times. Eric touched on this in his second reply.

Or like mentioned already, the needle fell off and was installed 50mph advanced?

Maybe he meant it's off by 50%? Still sounds far fetched.

Does the speedo start at 0 MPH and begin moving as the car first starts to roll?

 

I'm with Eric. The OP statement doesn't tell enough. And it really is difficult for a speedo to be off 50 mph....unless your driving about 200 mph, or the speedo is stuck at 50 while the car is at rest.
Btw, the Muncie speedo driven gear only go fro 17 to about 24 teeth, so that probably wont fix a 50mph differential at any speed.
How about telling what trans, rear, tire size, if speedo is original first. And what speedd he's actually going, what speed the speedo is reading.

 

Thats the only thing I see possible

 

Or... the OP fat fingered his numerical input of 50 and that threw everyone off.

 

Take it to a speedometer shop and have it remagnetized and calibrated.

 

This car does not have a great deal of original equipment. Has been modified with many performance engine parts, different rear end. I have rode in the car and noticed needle bounces a little in between gears. It is as he states, 50mph out. Going to take a look at the color of the gear and see what stamped numbers appear on the rear end.

 

Then he needs to state more clearly, or we can't help him. That is little more than gibberish.

- Eric

 

I second the speedo shop. I'd bet on a bad cable or a bad speedo head, not gears.

 

at 60 MPH speedo =10 (progressively climbs as speed increases)
I originally suspect they never replaced the cable or thought about rear end ratio's with the transmission and the gear installed. I'm going to contact a local shop to see what their diagnostic fee is to roll that car in. I've asked him to purchase new cable.

 

This is like pulling teeth.

So, does it read 5mph at 30mph? Or does it read zero?

Does it read 12½ at 75mph?

What is his rear end ratio? Is it extremely high or extremely low?

For a speedometer to read low because of gearing, the rear end has to be numerically lower than the original rear end was. Most people install rears that are numerically higher when they build a "hot rod."

It could just be a bad speedo. A cable is possible, but usually they either work or they don't (or they get jittery).

- Eric

 

It does not read at all until it gets to 60MPH

at 60 MPH speedo =10
at 70 MPH speedo =20
(progressively climbs as speed increases)

 

Ah. Thank you.

So the needle only begins to move once you get to around 60, and then it reads linearly.

Could possibly be some sort of an odd cable problem, where the cable only starts spinning the speedo once it gets to a certain speed. Speedo cable cores are dirt cheap, and available at your (or his) local parts store, so this is an easy one to rule out.

Unlikely to be a problem of incorrect gearing.

Could be a bad speedo problem, where poor magnetism or dirty pivots is not allowing the needle to respond properly.

Does the odometer work?

- Eric

 

WOW! THAT was difficult...

FYI, if you want help, please provide as much info as possible. The mind reading thing STILL isn't working.

Your friend's problem is most likely a stripped plastic drive gear in the trans. This happens when the speedo cable starts to go bad, causing friction, which in turn causes the teeth on the drive gear to wear down. As you go faster, the trans oil causes a sort of hydraulic coupling between the stripped drive gear and the driven gear, so it starts to register. I had exactly this problem on my 70 W-30. The trans tailhousing needs to come off to replace the drive gear.

 

Damn, Joe, that was a good one.

- Eric