# My Speedometer Wonkiness: What the Heck? **URL:** \ **Category:** Factual Questions\ **Created:** [December 2, 2016, 6:31pm UTC](https://boards.straightdope.com/t/my-speedometer-wonkiness-what-the-heck/773653 "2016-12-02T18:31:05Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![MonkeyMensch](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)\ **Post date:** [December 2, 2016, 6:31pm UTC](https://boards.straightdope.com/t/my-speedometer-wonkiness-what-the-heck/773653/1 "2016-12-02T18:31:05Z")
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? ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [December 2, 2016, 6:47pm UTC](https://boards.straightdope.com/t/my-speedometer-wonkiness-what-the-heck/773653/2 "2016-12-02T18:47:27Z")
I believe by law they are required to read above actual speed. Mine is consistently 3mph over. ---
**Author:** ![Pasta](https://avatars.discourse-cdn.com/v4/letter/p/ecccb3/32.png) [@Pasta](https://boards.straightdope.com/u/Pasta)\ **Post date:** [December 2, 2016, 7:42pm UTC](https://boards.straightdope.com/t/my-speedometer-wonkiness-what-the-heck/773653/3 "2016-12-02T19:42:34Z")
> [@Saint\_Cad](#): > > I believe by law they are required to read above actual speed. Mine is consistently 3mph over. This doesn’t pass the common-sense test. I cannot imagine there is any law that requires a gauge to report false information. ---
**Author:** ![yearofglad](https://avatars.discourse-cdn.com/v4/letter/y/b782af/32.png) [@yearofglad](https://boards.straightdope.com/u/yearofglad)\ **Post date:** [December 2, 2016, 7:47pm UTC](https://boards.straightdope.com/t/my-speedometer-wonkiness-what-the-heck/773653/4 "2016-12-02T19:47:43Z")
> [@Saint\_Cad](#): > > I believe by law they are required to read above actual speed. Mine is consistently 3mph over. 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](https://www.thrillist.com/cars/your-speedometer-is-wrong-speed-calibration-inaccuracy-in-german-american-and-japanese-cars). (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.) ---
**Author:** ![MonkeyMensch](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)\ **Post date:** [December 2, 2016, 7:53pm UTC](https://boards.straightdope.com/t/my-speedometer-wonkiness-what-the-heck/773653/5 "2016-12-02T19:53:13Z")
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. ---
**Author:** ![Whiskey\_Dickens](https://avatars.discourse-cdn.com/v4/letter/w/9fc348/32.png) [@Whiskey\_Dickens](https://boards.straightdope.com/u/Whiskey_Dickens)\ **Post date:** [December 2, 2016, 8:01pm UTC](https://boards.straightdope.com/t/my-speedometer-wonkiness-what-the-heck/773653/6 "2016-12-02T20:01:03Z")
> [@MonkeyMensch](#): > > 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. Huh? I thought you said you had larger aftermarket wheels and tires? This would cause the assemblies to rotate slower at a given vehicle speed. ---
**Author:** ![MonkeyMensch](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)\ **Post date:** [December 2, 2016, 9:20pm UTC](https://boards.straightdope.com/t/my-speedometer-wonkiness-what-the-heck/773653/7 "2016-12-02T21:20:44Z")
> [@Whiskey\_Dickens](#): > > Huh? I thought you said you had larger aftermarket wheels and tires? This would cause the assemblies to rotate slower at a given vehicle speed. 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. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [December 2, 2016, 10:23pm UTC](https://boards.straightdope.com/t/my-speedometer-wonkiness-what-the-heck/773653/8 "2016-12-02T22:23:03Z")
High makes no sense if it was reading low. Reading lower may indicate a slipping pulse generator. ---
**Author:** ![Daylate](https://avatars.discourse-cdn.com/v4/letter/d/9dc877/32.png) [@Daylate](https://boards.straightdope.com/u/Daylate)\ **Post date:** [December 3, 2016, 5:56pm UTC](https://boards.straightdope.com/t/my-speedometer-wonkiness-what-the-heck/773653/9 "2016-12-03T17:56:40Z")
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. ---
**Author:** ![MonkeyMensch](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)\ **Post date:** [January 30, 2017, 8:37pm UTC](https://boards.straightdope.com/t/my-speedometer-wonkiness-what-the-heck/773653/10 "2017-01-30T20:37:47Z")
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. ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [January 30, 2017, 8:50pm UTC](https://boards.straightdope.com/t/my-speedometer-wonkiness-what-the-heck/773653/11 "2017-01-30T20:50:52Z")
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 …