My speedometer (1998 Nissan Frontier) has gone bonkers. It used to be utterly precise albeit a smidgen inaccurate. That is to say, it has after-market wheels and tires that are bigger than production size, but through some decent observation and measurement I could calibrate indicated speed to true speed by adding 10% to the gauge reading. It was unfailingly dead on as measured against roadside speed signs.
Lately however, (like four months or so) it has started to read above true speed. How is this even possible? Everything that could be amiss (friction, dirt, slipping drive pinion) would seem to lean towards even further under-indication. To worsen the picture, it still occsionally under-indicates. What can cause something like this?
And as an aside, how are speedometers even calibrated? I understand the basic workings but how is anything adjustable in the real world short of replacement?
In the US, this is not true; there is no law in the US that governs speedometers, but the usual standard says it must be within +/- 4% of the true speed. Cite.
(The link also says that in the UK, it is illegal for a speedometer to ever show a speed slower than the actual; that’s not the same as saying they must show a speed higher than the actual, although I suspect just to be on the safe side, they probably err on the high side of reporting rather than run the risk of being under-reporting.)
The funny thing is that my current tires (by physical measurement as well as internet reference) should be rotating about 10% faster than stock tires. So the gauge should (hah!) have been reading close to dead on straight out of the factory.
Oops, my bad. Yes, you’re absolutely right. At least I got the description of which way the speedometer was indicating right before the mystery started. What I continue to take careful note of is the way it can now indicate both high and low. I’m lost.
There can be financial ramifications to an inaccurate speedometer. I read some years ago about a car manufacturer habitually providing speedometers that read about 5% too fast; this was done as a safety measure.
However, this was found to result in mileage charges in rental cars being 5% more than they should have been, and so the manufacturer stopped this particular practice.
Don’t have a cite for this, but it does make some sense.
I’m dredging up my own thread with an additional question: I thought that speedometers without digital readouts were the old style mechanical type with gear drive and spinning magnets. Is my truck (1998) late enough that it would have a VSS or some sort of pulse sender and an electrically driven gauge?
For those few, you happy few who’ve any interest in solving the mystery I am eager to hear news.
Does it matter whether the transmission is up to running temperature when you cycle the ignition on? … I ask because I had that exact same year, make and model rig and my speedometer would read wacko unless the tranny was warmed up when I went to start … for example, on a long trip blowing through the first tank of gas the speedometer would read 85 mph no matter what … but then after I shut down to take on fuel and restarted, the speedometer read fine …
That problem just got worse until all the gauges quit working … I did replace the speedometer head in the transmission but that didn’t fix the problem … and the commonality with all the dash gauges lead me to believe the problem was in the dash … and no way was I trying to take that monster apart … sheesh …