Imperial speedometer - driving car while removed

[Asleep at the Meal]

My 1964 Imperial speedometer is highly inaccurate (at about 65 MPH the reading is approximately 90 MPH), so I will be having it removed and repaired. Are there any problems with driving the car while the speedometer is removed with the cable exposed?


While the speedometer is out, I will use my GPS to monitor my speed. The car is equipped with Auto Pilot, and I know it cannot be used until the speedometer is repaired and reinstalled.

 

[Boydsdodge]

Have you had any transmission work done that would coincide with the speedo being off?

 

[Imperialist67]

The speedo on mine does the same, although not by such a margin........The "free standing" GARMIN GPS also shows your speed, you MIGHT be able to just use that for the interim.......

 

[Asleep at the Meal]

Have you had any transmission work done that would coincide with the speedo being off?

Not since I purchased the car in November 2021. I do not know the previous history. An earlier owner installed a tachometer on the car, which seems strange.

 

[Asleep at the Meal]

The speedo on mine does the same, although not by such a margin........The "free standing" GARMIN GPS also shows your speed, you MIGHT be able to just use that for the interim.......

I do use a GPS since I become lost nearly anywhere. However, I want the speedometer repaired so that I may use the Auto Pilot. My concern is driving the car with the cable exposed.

 

[Imperialist67]

I was thinking about being able to tell your speed vx. having it exposed - sorry to confuse..........

 

[Big_John]

My 1964 Imperial speedometer is highly inaccurate (at about 65 MPH the reading is approximately 90 MPH), so I will be having it removed and repaired. Are there any problems with driving the car while the speedometer is removed with the cable exposed?


While the speedometer is out, I will use my GPS to monitor my speed. The car is equipped with Auto Pilot, and I know it cannot be used until the speedometer is repaired and reinstalled.

Before you send the speedo off for repair, have you checked that the speedometer gear in the transmission is correct for gear ratio and tire size?

Be sure to use the chart for 1962 - 1965... The later gears are different.

 

[Big_John]

An earlier owner installed a tachometer on the car, which seems strange

I'll bet someone put a different rear axle or changed the rear axle gears.

The more I think on this, the more I'm thinking that's what the issue is. If your speedometer reads 90 at 60MPH, that leads me to that theory. Usually, if the speedometer is bad, it will tend to read low... Which makes a lot of sense when you think about it... The mechanism gets dirty and sluggish.

Since you have a tach, this may be even easier to figure this out...

MPH = (RPM x Tire Diameter) / (Gear Ratio x 336)

or

Gear Ratio = (RPM x Tire Diameter) / (MPH x 336)

In this case, we know the RPM and we can use your GPS to get MPH. Measure the tire diameter with a tape measure.

I'll bet if you do the math (using the second formula) the gear ratio isn't what the stock ratio would be for your car.

I checked the FSM and the ratio should be 2.94:1.

So, at (let's say) 28" diameter tire and the stock ratio of 2.94 and 2000 RPM

2000 x 28 = 56,000

2.94 x 336 = 987.84

56,000 / 987.94 = 56.7 MPH

So.... If you hold 2000 RPM your GPS should say 56.7MPH.

This is assuming the tach is correct and you have 28" tires.

Or you can do some simple algebra (see you should have paid attention in class LOL) and figure the ratio, but I'll let you do that.

 

[57fury440]

If you want to get an idea if the rear gears have been changed there is an easy way. Jack up the back of the car so both wheels are off the ground. Be sure to use jack stands. Put the tranny in neutral only after blocking the front wheels so the car doesn't shift. Mark a line on your driveshaft and count how many times your wheel turns to get the driveshaft to make one revolution. If the wheel turns 3 and 1/2 times you have a 3:55 to 1 ratio. If it spins slightly less than 3 turns, then you have a 2:93 to 1 ratio and so on. This will at least let you know if the gears have been changed.

 

[D Cluley]

Another test is the odometer.
If the odometer works properly, then the speedometer pinion in the transmission is correct, and the speedometer itself has a problem. If the pinion is wrong, then the odometer will be over or under the same percentage as the speedometer.

 

[Asleep at the Meal]

Another test is the odometer.
If the odometer works properly, then the speedometer pinion in the transmission is correct, and the speedometer itself has a problem. If the pinion is wrong, then the odometer will be over or under the same percentage as the speedometer.

Thanks, Dan...the speedometer it is. The odometer only has the normally expected error (about 5%), whereas the speedometer reads 35% high.