In past years a vehicle had different gear ratrios for the speedo cable in the transmisssion for different size tires. My '09 Caravan and my GPS agree on the speed I am driving.
I don’t see a comment thread for the guys Plymouth Voyager that keeps clicking the door locks, interior lights and the “Door Akar” dinger.
Tom and Ray got that one wrong, I think, and I know the answer to that problem. My Chrysler Town and Country of the same year did that. Trying to adjust the door lock didn’t work and the little button switch in the door seemed to not be getting pushed in far enough but it had no adjustment.
I bought some small stick-on rubber feet like the kind on the bottom of boxes and electronic devices. I stuck one on the door jam in the exact spot that contacts the button so that it pushed the button switch farther.
That fixed it.
If you have an HP48 calculator handy you can write a simple program that will give you a super accurate speed readout for just two key presses - one at the beginning of the measured mile and the other at the end. I used to drive a city bus from Eugene to Cottage Grove, Orygun on I-5. I noticed that one of the miles about halfway there always gave a different reading than the others. So I measured that mile and the next together and discovered that the milepost in the middle was misplaced because one of the miles was fast and the other was slow by the same amount, i.e. together the measurement was 30 MPH when I was doing 60.
Here is the code: - HP geeks only, sorry.
‘MPH’
<< TIME ltmph OVER
’ltmph’ STO HMS-
DUP ABS
IF .1 <
THEN HMS-> INV ‘1_
mph’ *
ELSE DROP 1 FIX
" INIT MPH" CLLCD 5
DISP 2 WAIT
END
>>
‘ltmph’ is the only reference that isn’t a standard 48 function. It retrieves the time at the moment of the previous key press for calculating the speed of the current mile, also saves the current time for the next iteration. It is self initializing.
The accuracy is very good because the time lapse between pressing the key and the time measurement is essentially the same at both ends of the measurement.
Enjoy!
Correction that should be
IF .1 < THEN HMS-> INV '1
etc
Also note that there is a built in intentional error. No manufacturer wants to be responsible for someone getting a speeding ticket being involved in an accident, so they intentionally calibrate the speedometer, to indicated a higher speed than the car is really going.
For years, car manufacturers offered sized speedometer gear sizes to compensate for different size tires shipped on their vehicles. I’m not sure how that flexibility is being provided in recent years.
Most manufacturers only warrant speedometer accuracy to within some tolerance - like +/- 6%.
Joel949, I do know that on the Toyota Scion tC they offered 18 inch wheels with 225x40x18s as an option to the 215x45x17s. Both are almost the same diameter. You have to know more than the wheel size, you have to know the tire size too.
Different tire and rim sizes only affect the speedo and odo if the combination results in a different overall rolling diameter.
If mile markers line up on both sides of the roadway, then they must not consider the effect of curves or changes in elevation (including bumps, potholes, etc) which would be different for each direction - these effects could be in the range of a few %, would not all be corrected by GPS, and have a tendency to underestimate distance therefore speed.Tire size effects are plausible, but then speedometer would not be a consistently high result. I hypothesize that the mile markers are biased, but the uncertainty is too high to prove it.
European regulations state that a speedometer may never indicate less than actual speed but may indicate as much as 10% faster than one is actually moving.
The long form is here: EU Regulation 39 (pdf)
@ Davidkelly most cars I’ve had have over read vis a vis the “mind your speed” signs. 55kph on the speedo equals about 50. So there is your 10%. Don’t think it is linear though, I remember 120 on the speedo reading 112kph.
Of course, the accuracy of a roadside advisory only speed check is another matter. I know the ones that give you tickets have to be calibrated.
My own speedo reads about 2mph faster than I’m actually going according to the radar things that the cops put on the sides of the road, regardless of my speed. I have correcting that on my list of things to do.
I’ve learned my truck’s speedometer is consistently off by about 4mph. When it reads 50, I’m going about 46. This is consistent with those “Your Speed Is” signs and with my TomTom GPS in the truck. And it’s consistently about the same if the tires are brand new or nearly dead.
The good news is that modern instrument clusters are far easier to remove than they used to be to reset the pointer. Mine just requires popping a garnish off, removing one screw, and unplgging it from a wiring harness. Then the plastic lens pops off and the needle can be removed and reset.
The easy way to check the speedo is to use Garmin GPS.
I created an account here just to comment on this story. It was a delightful surprise to listen to Mike talk about his partner as a “him” and Tom and Ray jump into the conversation without a moment’s hesitation and engage the caller’s opinions on the issue as well as his partner’s. Thanks, Car Talk Team, for treating this gay couple the same way you treat every couple who calls in. It made my day.
JP, I didn’t hear the call and was unaware of this, but in all honesty I couldn’t care less. I hang out here to help people solve car problems. It matters not who they are. Well, I suppose if John Stephens (Supreme Court Justice) posted in I’d have a lengthier response than usual. But that’s another story.
When I met my wife-to-be in 1974- she had a '69 Javelin. We hung on to it until 1983. I’m still hanging on to her (but not for her car.).
Garmin GPS units can download trip info to your computer using their free MAPSOURCE program. Connect the unit [I have a Nuvi 260W, but all Garmins do this] and download the tracks using the Tracks-download from device menu option. The tracks are listed by date, Each one shows data collected as you drove… lat/lon, instantaneous speed, etc-- even your altitude.
I’ve attached a screenshot of my trip home from work tonight, showing the data that is displayed for each trip. Click on any waypoint and you can see more detail. They offer a “filter” option that allows you to choose to place waypoints at 5,280 foot intervals among other things. This feature will give you the data to check your car’s speed any time after the trip
They even provide a plotting feature… it will show the trip with altitude on the Y-axis [who knew your car’s GPS was recording altitude!
Besides, it’s easier and more precise than driving to see the family in Utah by way of Idaho.
All the caller need do is spend ~$100+ on a GPS device that includes a readout of the vehicle’s speed, which they mostly all do now, and then compare it to the speedometer readout. I think he’ll find that his speedometer (and that of most other cars) reads slightly higher than the actual speed.
The summer tires on my mini van are too small and the speed and odometer read higher than the actual speed and distance traveled. Normally I estimate that I can drive three milles faster than what I see on my speedometer, and even more so at higher speeds. In winter I need to be careful as my snow tires are about two inches taller than my dinky summer tires and I need to stay a little under speed.