speedometer adjustment for a 31 roadster

O

old31

Well-known member
When checked against a GPS my speedometer reads 7-10 MPH faster than actual.

Other than my foot on the gas, is there any adjustment that I can do to fix it?
 
Speedometer drive gear must match the Ring&Pinion ratio and the tire size.
3:54 R&P, 21" tires =17 tooth gear.
3:54 R&P, 19" tires =18 tooth gear.
3:78 R&P, 19" or 21" tires = 19 tooth gear.
4:11 R&P, 19" or 21" tires = 21 tooth gear. (pretty sure but not postive).
 
How is your odometer working? If it is off the same amount you probably need a different gear that bolts to the front of the torque tube. Has it always been this way? Did you change anything recently?

Charlie Stephens
 
Check the odometer against the mile posts on the highway, if the gearing is off in the speedometer system then the odometer will be off. If the odometer is off in the same direction and the same percentage as the speedometer, then the cable needs to geared differently. If the odometer is off by a different amount or direction that the speedometer, then the speed portion of the speedometer head needs calibration. There is no calibration in the dash unit for the odometer, but the speedometer can be adjusted to make it right. I have a Stewart Warner bench "Speed Analyzer", basically a big speedometer that is accurate with an extra out put to plug your unit into to test it. Then the speedometer is tweeked to get it to run right. Contact me if you have other questions.
 
In addition to the prior comments and threads, another possible option is to purchase a USA made ratio adapter that attaches to the speedo drive gear at the torque tube. They are on ebay for about $90 and include small parts to fit the square or round tips on the speedo cable. Also they are available with many speed- up or speed -down ratios (20%, etc.). Some folks with overdrives and non stock trannies have used these.
 
MAG said:
Speedometer drive gear must match the Ring&Pinion ratio and the tire size.
3:54 R&P, 21" tires =17 tooth gear.
3:54 R&P, 19" tires =18 tooth gear.
3:78 R&P, 19" or 21" tires = 19 tooth gear.
4:11 R&P, 19" or 21" tires = 21 tooth gear. (pretty sure but not postive).
Click to expand...

I just bought the car so I am a newbie with a bunch of questions, and my manual's are on there way.

How would I check the R&P and the tooth gear?
 
Charlie Stephens said:
How is your odometer working? If it is off the same amount you probably need a different gear that bolts to the front of the torque tube. Has it always been this way? Did you change anything recently?

Charlie Stephens
Click to expand...

I am new to the car and have not changed anything. The odometer works fine but as Mark mentioned I suspect it is off as well by the same amount as the speedometer.
 
old31 said:
I am new to the car and have not changed anything. The odometer works fine but as Mark mentioned I suspect it is off as well by the same amount as the speedometer.
Click to expand...

As has been stated;

Odometer correct, speed off; an adjustment may work. Speedo info
Odometer incorrect, speed incorrect; gearing is off.
 
Should not be needed. If you know which gear ratio you have make sure the speedometer drive gear matches the gears you have. If that is correct then it is likely time to get a your speedometer rebuilt. Rod
 
How To Check Ring & Pinion

Remove all four spark plugs.
Jack up one rear wheel and mark the 12 o'clock position on the tire.
Hand crank engine 2 complete turns.
See where chalk mark on tire ends up:

Between 11 and 12 o'clock, ratio is 4.11
Between 12 and 1 o'clock, ratio is 3.78
Between 1 and 2 o'clock, ratio is 4.54

In some cases the ring and pinion tooth count can be found stamped on the banjo housing web or flange.
 
Dollar, per your instructions I checked an unfortunately I have a 4.11. I would prefer to have a 3.54 or 78. No wonder I have to shift so quick.

I removed the speedometer gear and I have a 9-34-19. I counted the teeth and I have 19.

According to an article by Ed Eaton, and if I am reading his chart correctly I should have a 37-9 with 21 teeth.

Does anyone agree or disagree with the numbers?

I am guessing this is why my speedometer is 7-10MPH off.
 
If you are running an overdrive or plan on installing one, I've been told that the 4.11 gear ratio is recommended for use with the overdrives.

If you're not going to run with an overdrive, you can have the gear ring set changed in the rear and make sure the speedometer gear drive matches with whichever gear set you choose to install.
 
Dollar Bill said:
How To Check Ring & Pinion

Remove all four spark plugs.
Jack up one rear wheel and mark the 12 o'clock position on the tire.
Hand crank engine 2 complete turns.
See where chalk mark on tire ends up:

Between 11 and 12 o'clock, ratio is 4.11
Between 12 and 1 o'clock, ratio is 3.78
Between 1 and 2 o'clock, ratio is 4.54

In some cases the ring and pinion tooth count can be found stamped on the banjo housing web or flange.
Click to expand...

Good info, can I assume you meant 3.54
 
Dunno if this the right way to adjust a speedo, but I bought 2 "restored and calibrated " kilometre ones from USA that are about 25 KPH out at 100 KPH, yes, that far out.

I moved some sort of sliding ring under the speedo head drum either way to the extremities of the slots and it changed the shown speed about 10 KPH either way.

No good for my wasted money on these 2 speedos still being out by heaps , but maybe possible to get your 6 MPH...
 
Maybe some oil from the South Pole ...

PS. I have one of those kilo speedos too. Got me worried now. Haven't tried it.
 
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