Speedometer help

Garrett

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Hey everyone, don't know if this is the right thread to post in for this question but, What would make my speedometer read 20mph off in my 1968 chrysler 300?
 
Tires and/or gear in the transmission. I have the incorrect size tires on my Polara, and the speedo is 5-10 off.
 
If the gear in the transmission doesn't match the rear tire size the odometer will be off a similar percentage as the speedometer. If your odometer is correct, then it is a problem with the speedometer itself.
 
Speedo gears are the easy part and part which have to be matched to the basic tire size/rear axle ratio information. THAT gets the odometer to read correctly ONLY. For example, the odometer on my '67 Newport is dead accurate according to the Interstate Mile Markers on a 10 mile run at 60mph (60 secs/mile = 60mph). BUT on that car, the speed readings are 10% higher than they should be. How might that happen? The magnet in the speed cup on the speedometer head needs adjustment.

The speedometer cable turns a bar magnet in a "speed cup" near where the speedometer cable attaches. That magnetism turns a "mating" soeed cup which is attached to the speedometer needle shaft. As one spins, it takes the other one with it. With the second speed cup turning against a clockspring attached to the speedometer needle shaft (which returns the needle toward "0" as speed decreases). The speed reading is a function of the magnet's strength or lack of. Too much and the speed is high. Too little and the speed is lower. ALL for a given speedometer cable turning speed.

At speedometer shops, they had either a bench-located calibration mechanism. Place the speedometer head on it, engage the drive mechanism, turn it on and increase the speed as you watch how the calibration mechanism and the vehicle's speedometer head's needles increase or decrease together. THEN, to get the speed calibration more correct, a "zapper" tool was used to increase or decrease the bar magnet's strength to get a more correct speed reading. Usually, the spec range of speeds would err on the slower side of things. I have seen this done, in the past, and it is a purely "cut and try" method! One short zap and speed decreases. Another short zap and the speed increases. Hopefully a third short zap gets it within a mph or so, plus or minus.

Knowing these things, it is a mystery to me how my '67 Newport has an accurate odometer and an incorrect speed reading . . . considering how difficult it is to remove the speedometer on those cars. So, I'm more concerned about an accurate odometer to check mpg and can compensate for a very optimistic-reading speed reading by mentally compensating for this.

As for conversion of the '67 or so tire sizes to more modern metric sizes, take the old "inches" section width number on the tires listed in the FSM and multiply them by 25.4 to get the approximate metric tire size in "75 section width". As for the alpha-numeric sizes on C-bodies . . . G78 would be P215/75, H78 would be P225/75 (which is actually a bit closer to the old J78/78, but works well), and L78 being P235/75 (which can also work for the short-lived L84). The P235/75-15 is also available in "XL" extra load (6-ply rating) variations, if needed.

Now, back when the cars were newer, many complaints of "inaccurate speedometer" (due to traffic tickets or such) were usually addressed with speedometer driven gear (where the cable attaches to the transmission tailshaft housing), going up or down one tooth, depending upon how far off the speed reading allegedly was. But it seemed that most of these things were more related to GM products than Chrysler products, back then. Nobody came back with "My gas mileage dropped after y'all did my speedometer work" complaints, though, not sure how the speedometer work affected how well their engine ran. LOL.

Sorry for the length. Hopes this might explain how that system works, plus my own experiences.

CBODY67
 
I just want to add that we have 4 of those cars and the speedometers read high on all of them, between 10 and 20 mph. Sure tire size will affect indicated speed, but 20 mph seems excessive. If the odometer is accurate for the most part and the speedometer is off, then I would assume the speedometer is out of adjustment. Probably something just wear and tear in the mechanism.
 
If the gear in the transmission doesn't match the rear tire size the odometer will be off a similar percentage as the speedometer. If your odometer is correct, then it is a problem with the speedometer itself.
Ive got 225/75/r14's on the original wheels. I know the original bias plys that would have come on the car are a 885-14. How would those compare? And the gear in the transmission would be the factory one in my case.
 
I just want to add that we have 4 of those cars and the speedometers read high on all of them, between 10 and 20 mph. Sure tire size will affect indicated speed, but 20 mph seems excessive. If the odometer is accurate for the most part and the speedometer is off, then I would assume the speedometer is out of adjustment. Probably something just wear and tear in the mechanism.
Yeah between 0 and 30 its fine. 30 to 40 its 10 off then after 40 its 20 off
 
Yeah between 0 and 30 its fine. 30 to 40 its 10 off then after 40 its 20 off
That sounds like a speedometer issue, but let's try something first.

You need to check the odometer... Pick a mile marker on the highway and note the odometer reading. Drive 10 miles (or more) and check it against another mile marker.

If the odometer is correct, it's the speedometer.. If the odometer is wrong about the same percentage, then you need to match the speedometer gear to tire size and rear axle ratio.

Pay no attention to the red circle... This is just a chart I found on the intrawebs.

7390568-speedogeartoothrequirements-jpg.jpg
 
In 14" sizing, the 8.85x14 would replace the 9.00-14, I suspect, with the 8.55x14 being replaced by P225/75R-14. Based on the metric conversion AND rhe revs/mile specs. Revs/mile can relate to the tire's rolling diameter, not necessarily its static diameter (sitting still). Usually, only wagons came with the 9.00x14 tires on 14x6.5" wheels.

Before the tire manufacturer's spec sheets at TireRack.com, the "revs/mile" number was only mentioned in the AMA Specs document which each OEM did for their vehicles. AND it varied by speed for the bias-ply tires.

For my general use, the P255/75R-14 tire is very close to the P225/70R-15 tire in revs/mile and such, with most being in the 754revs/mile range. As is the older P245/70R-14 BFG Advantage T/
As I have on my '67 Newport. I later found a real H70-14 tire casing which measured exactly to the OD of those BFGs, fwiw. Those older BFGS have aged WAY OUT from being street usable, at this time.

FWIW,
CBODY67
 
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