Speedometer accuracy... can it really be this bad?

[MopaR&D]

I got this 1972 D200 with a 360 and 727 last November as a non-running project which I've gotten back on the road in the last few weeks. I knew when I put 33" tires on it whatever speedo gear was already in it would be incorrect and I was right; speedo read about 50 when I would be going a tad over 60 (checked with GPS phone app). So going off the chart here Mopar Speedometer Pinion Numbers I got a 28-tooth gear since the truck has a 3.55 rear axle ratio, but now the speedo reads quite a bit over, like 60 when I'm actually going 50. Just to make sure I once again crawled under the truck and counted the turns of the driveshaft vs. tire and it is most definitely 3.5x:1. Tire diameter is 32.7" from the manufacturer they are 285/75-17 size.

What gives? Only thing I can think of is the speedometer itself is just way off, is that possible? This is a pain, speedo drive gears aren't exactly cheap and easy to come by to just buy 15 and try them all out until I find the right one.

EDIT: As a comparison my Duster has an AutoMeter 160-mph speedometer and with 2.94 gears and 28" tires the speedo reading was pretty close to actual speed with a 28-tooth drive gear as shown in the chart (29-tooth actually turned out to be perfect)

 

[moparmat2000]

Figure out what it reads compared to a GPS speedo on your phone at various road speeds and just go with it. Get a different speedometer, or have yours rebuilt.

 

[Dana67Dart]

Assuming nothing wrong with speedo. There is a ratio between speedo reading and GPS .

In your second example speedo showed 60, GPS showed 50 ( close enough for example) and you have a 28 tooth gear.

If you put in a 34 tooth gear your speedo should read 49.5 at a GPS reading of 50. A 33 tooth should read 50.9 at a GPS of 50.

What was your old speedo grear count?

By using this method the tire size and rear gear ratio are irrelivent

Speedo reading / GPS reading x current number of teeth = new number of teeth

Rounding up or down gives you slightly over or slightly under GPS speed

Current speedo gear count / new speedo gear count x speedo reading = speedo reading after replacement.

Speedometer Reading 50 MPH
GPS Reading 60 MPH
Current Number of Teeth 28 Teeth
New Number of Teeth Rounded Up 24 Teeth
New Number of Teeth Rounded Down 23 Teeth
New Speed with 24 Teeth 58.33 MPH
New Speed with 23 Teeth 60.87 MPH

 

[Mattax]

I've always used the milemarkers on the road to get the odo as close as possible.
That might be the best thing for you to do here.
The odometer is directly driven by a worm gear off of the speedmeter cable.
If that's right, and the speedometer is off then something is wrong with the speedo.

On the tire, sometimes the manufacturer will also provide rolling radius. That will be a little more accurate than going from the diameter.

 

[MopaR&D]

Assuming nothing wrong with speedo. There is a ratio between speedo reading and GPS .

In your second example speedo showed 60, GPS showed 50 ( close enough for example) and you have a 28 tooth gear.

If you put in a 34 tooth gear your speedo should read 49.5 at a GPS reading of 50. A 33 tooth should read 50.9 at a GPS of 50.

What was your old speedo grear count?

By using this method the tire size and rear gear ratio are irrelivent

Speedo reading / GPS reading x current number of teeth = new number of teeth

Rounding up or down gives you slightly over or slightly under GPS speed

Current speedo gear count / new speedo gear count x speedo reading = speedo reading after replacement.

Speedometer Reading 50 MPH
GPS Reading 60 MPH
Current Number of Teeth 28 Teeth
New Number of Teeth Rounded Up 24 Teeth
New Number of Teeth Rounded Down 23 Teeth
New Speed with 24 Teeth 58.33 MPH
New Speed with 23 Teeth 60.87 MPH

I figured I'd have to do something like that I was just surprised the table is so far off and annoyed I need to buy another gear. Previous speedo gear was a 39-tooth (!), splitting the difference I would need around a 33-tooth.

 

[Projectile Dart]

That sounds way more accurate than mine! No matter what speed I go my speedometer just reads "0"

 

[Dana67Dart]

my speedometer just reads "0"

Put the car in gear and push on the right pedal

 

[Dana67Dart]

Remember the New Speed is based on the GPS speed and the accuracy of your readings, but 33 looks like it will get you in the ballpark