Speedometer jumping, and inaccurate
Note: this topic came up in Member's Restorations, but I decided to address it here instead.
The car: a 65 Dart wagon with an unknown 69 4-speed transmission.
I have put 800 miles on it since then acording to the incredibly inaccurate odometer. The speedo rattles and the needle jumps around. I went by a sign that tells you how fast you are going and it said 24 while the guage said 40.
The speedometer and odometer do not necessarily share the same accuracy. If you had a factory transmission and rear gears and tire size, the odometer would likely be just about perfect (mine is off by less than 0.5% as measured against 100 miles worth of highway markers.) With your unknown 4-speed, you may need to calculate how far off the calibration is, then find out how many gear teeth are on the speedometer pinion, then figure out how many teeth you
ought to have, and get a new speedo pinion. The odometer rollover is simply a function of gear reduction (rear end ratio, transmission tailshaft, and pinion gear). When the cable rotates 1000 revolutions per mile, the odometer is accurate, and it stays accurate despite the age of the vehicle.
(The pinion gear is just inside the transmission, at the end of the speedo cable-- watch out you'll probably need a bucket to catch tranny fluid when you pull it out. The 1965-older style gear is made of unobtanium, but the 1966-newer style is still fairly common.)
Once the odometer is accurate, THEN worry about the speedometer needle. If the magnets have lost some of their power (very likely) then you may need to have someone recalibrate (and clean and lubricate) the speedo head unit.
It might be tempting to do it in the other order (compensating for an inaccurate needle by changing the pinion gear), but you'll find it unsatisfactory because the speedometer is typically off by different percentages at different speeds. Mine showed more than 10-15mph error at city speeds, down to 5mph error at legal highway speeds, then back to 10mph error when even faster.
A jumping needle is often the sign of a poorly lubricated speedo cable, or a cable which is asked to make a sharp bend. The cable insert is actually speeding up and slowing down as it whips around inside the sheath, and the needle is giving an accurate indication of what it's being fed.
Please do fix the speedo head unit before it self-destructs. There aren't many replacements in the junkyard anymore. Don't ask me how I know...
- Erik
64 Valiant, 170 3-spd
82 Volvo wagon, 5.0 5-spd :)
