Speedometer off by 20mph

Modern K Band RADAR is ridiculously accurate assuming there is 0 cosine angle, same goes for LIDAR. Both truncate to the lower whole value. So if you’re going 70.9mph, both units will read 70 even.

 

How can you calculate mpg if the you don’t have an accurate miles. You said the speedometer is off, guess what so is the odometer.

 

Wouldn't that mean there's always a cosine error?

ETA unless you're measuring straight on?

 

Checking your speedo's accuracy:

Find a straight stretch of lightly traveled interstate, then pick 2 mile markers as far apart as possible. Set your cruise control to a specific speed - doesn't matter what as long as your speed is held constant. Remember that speed. Once you're at that constant speed, and using a reliable stopwatch, click Start as you pass the beginning mile marker. Click Stop exactly when you pass the end mile marker. Now you know the time it takes to travel a specific distance. From there it's just number crunching to compare your actual speed to what your speedo told you.

Cell phone GPS is close sometimes (depends on where the towers are), REAL GPS is better, and the cops' radar or laser (properly calibrated) is the most accurate. Usually to get the latter, it's going to come at the cost of a ticket & fine.

For the math: If the mile markers are 10 miles apart, and the stopwatch clocks 10 minutes, you were traveling at exactly 1 mile per minute. Times 60 minutes in an hour calculates to 60 miles an hour.

If you did it in say 9 minutes, then the math is: Distance/time = 10 miles/9 minutes = 1.11 miles/minute X 60 minutes/hour = 66.7 miles per hour.
If you did it in say 8 minutes, then the math is: Distance/time = 10 miles/8 minutes = 1.25 miles/minute X 60 minutes/hour = 75.0 miles per hour.

You can also check your odometer as well if you accurately note start & end readings.
Note that mile markers are not precise down to the foot, but they're close enough to get a good idea of what your actual speed really was.

You could get law enforcement to clock you, but that's not usually the recommended method.

 

Agreed. Mile markers, a stop watch and some basic math are all you need if you want accurate numbers. Apps and devices that use GPS are usually off to some degree.

 

I set mine using the airspeed velocity of an unladen swallow.

 

This only if you're looking for accuracies of up to 1/1000 of a mph

Most people don't need that

 

African or European?

 

There always is to some extent, but it’s in favour of the motorist, when the RADAR is stationary(for the purposes of a speeding ticket).

A 10 degree angle will only affect it by 0.9 mph at 60. As in a true speed of 60 will indicate 59.1

5 degrees only 0.2mph.

 

here is something for you to look at

For you tire size change. For your gear change. http://www.csgnetwork.com/rgpspeedcalc.html

 

If using a Hypertech you only change the tire size and you need to use the actual tire sizes not the 285/75/16 or whatever size as not all tires are the same diameter.

Stock tire is 30.6” IIRC most 285/75/16’s are 32.6-32.8”. You’d have to look at your tire spec sheet on the mfg website.

 

Nothing about this makes any sense.

Isn't the speedo measured at the wheels? If so...



If im wrong and it's read at the trans like most cars, then regearing would make the speedo read higher than actual, not lower.

Regearing plus larger tires should pretty much cancel each other out. Again, unless it's read at the wheels which I think it is.

 

https://www.roughcountry.com/speedo...Ax-Krpjejl3WPmqiHCt-J1Wcj-o4D2ahoCsEYQAvD_BwE

 

In the past on trucks that did read off the trans then you would be correct. Oversized tires + regear to correct them, would be the go to way to bring the speedo back to close to accurate.

Now we have trucks like Tacomas that calculate the speed from tire rotation. So, with that a given tire size will rotate one rev and cover a given distance. If you install a taller tire and not correct the input to the speedo, then the speedo still thinks it has stock shoes and that given distance traveled. So, in reality you will may be driving 69mph per GPS, but the speedo reads 55mph as it is only getting the input from the tire rotation not the ground covered.

Now, add a regear to compensate for the new tire size. With our Tacomas, this will only correct the engine rpm for a given tire rotation. The speedo still only sees the input from what the tire is telling it, and nothing from the engine or transmission. You'll get your torque back and everything will feel right, but it would read correctly on the dash until you let the speedo know the actual tire size.

In the case of the OP and his +20 offset, is probably due to the software not updated correctly, in that they updated the axle ratio in the software (which the speedo doesn't get input from) and didn't update the actual tire size (which the speedo DOES get input from).
If this is the case, then it would certainly be reasonable to see such an offset between the speedo and the GPS readings.

 

Just as a note, make sure you're getting your GPS speed on a completely flat/level stretch of road, no inclines/declines. I've noticed that some of the major map apps don't factor in the change of altitude to the speed equation.

 

Find a cop taking radar lol

 

This is what I did with my tires and hyper tech . Turned out pretty accurate

 

What about gaia gps app

 

Yes and no. GPS is simply a factor of triangulation from satellites. The more satellites, the more accurate it is. But, that being said, you're correct in that it isn't 100% accurate.

 

GPS can determine your altitude just as accurately as your longitude and latitude. But GPS units typically flatten everything to a 2D map. The end result is if you go 60 mph straight up they will likely read 0, because you aren't moving anywhere on the map.