Speedometer v GPS | FerrariChat

Speedometer v GPS

Discussion in 'Mondial' started by Rapalyea, Oct 17, 2014.

This site may earn a commission from merchant affiliate links, including eBay, Amazon, Skimlinks, and others.

  1. Rapalyea

    Rapalyea Formula 3

    Jun 18, 2013
    1,511
    Georgia Mountains US
    Full Name:
    David Rapalyea alias
    50 mph indicated = 44 gps
    60 mph indicated = 53 gps
    70 mph indicated = 62 gps

    I don't live in a place where I can run 80 long enough to get a gps reading but extrapolation is easy enough. Pretty much 71. My Malibu MAXX 3.5 indicates 70 and is 66 gps. This is the car that runs my simulated drag strip at 73 indicated so make it 67. When I took delivery of the Mondial 18 months ago it turned a bit over 80 indicated which would be perhaps 72 gps. That is a pathetic performance against a similar weight ohv 3.5 engine with vvt. [ Five mph difference]

    These days the Mondial is runnning my symulated run at close to 90 indicated so that would extrapolate to 79. [Twelve mph differfence] Now THAT is a significant increase. I knew it has been running better in recent months but had no idea THAT much better. Its unprecidented in my experience. My supposition is 25 years of carbon are finally clearing out, mostly for one reason. I warm the car up entirely between 2000 and 3000 rpm. Then immediately run through the gears at light throttle to 5000 rpms. Its like a different engine in even how it sounds. And starts. And is smooth and responsive at all rpms in all gears.

    Normally a cold start takes one turn of the key but the light up is pretty much cylindar by cylindar over a period of several seconds. This morning (50F) it just started up and idled. I kept waiting for something to hickup but nothing did. One single test is not statistically significant but I was impressed since in 7,000 miles that had never happened before.

    I was thinking of putting the car up for the Winter but this is interesting. Either something is going wrong and the second shoe will drop or its just a better car. Another significant difference is I am running very heavy oil. Ten/Thirty with two litters of Lucas Stop Leak. Of course I do not rev the engine till it is runing temperature but runs like a kitten and the oil stains are fewer and smaller.

    Just more chatter from the mountains. So far so good.
     
  2. dfranzen

    dfranzen Formula 3
    Owner

    Aug 31, 2013
    1,577
    Ponte Vedra Beach , FL
    Full Name:
    Don Franzen
    30W oil is heavy ? then the 50W Im using must be sludge!

    but I will have to try that lucas treatment

    imo gps is an average speed not real actaul time speed so it could explain a little difference from odo
     
  3. soucorp

    soucorp F1 Rookie

    Sep 20, 2011
    4,814
    Old Dominion
    Full Name:
    Mike
    "The GPS is accurate for the determination of speed over ground (about 10 times more accurate than a car odometer) when moving at relatively constant speed in straight lines and is competent at determining speed on curved paths, although some overshoot does occur during transitions. Absolute error increases slightly at higher speeds but in percentage terms is less. In addition, when the system is tested under conditions of sudden changes in speed some inadequacies become evident. The system smooths the peaks and troughs of rapid accelerations and decelerations, which is attributed to inherent smoothing within the mathematical algorithm and the one sample per second output of the system."

    Tiny discrepancies between the GPS receiver's onboard clock and GPS time, which synchronizes the whole global positioning system, mean distances calculated can drift. There are two solutions to this problem. The first would be to use an atomic clock in each receiver costing $100,000. The second is to use some clever mathematical trickery to account for the time-keeping error based on how the signals from three or more satellite signals are detected by the receiver, which essentially allows the receiver to reset its clock. The latter is the less expensive solution used by Navigation device manufacturers.

    There is also an intrinsic error source in GPS associated with the way the system works. GPS receivers analyze three signals from satellites in the system and work out how long it has taken each signal to reach them. This allows them to carry out a trilateration calculation to pinpoint the exact location of the receiver. The signals are transmitted by the satellites at a specific rate.

    Unfortunately, the electronic detector in standard GPS devices is accurate to just 1 percent of a bit time. This is approximately 10 billionths of a second (10 nanoseconds). Given that the GPS microwave signals travel at the speed of light, this equates to an error of about 3 meters. So standard GPS cannot determine position to greater than 3-metre accuracy. More sophisticated GPS receivers used by the military are ten times more accurate to 300 millimeters.

    Other errors arise because of atmospheric disturbances that distort the signals before they reach a receiver. Reflections from buildings and other large, solid objects can lead to GPS accuracy problems too. There may also be problems with the time-keeping accuracy and the data onboard a particular satellite. These accuracy problems are circumvented by GPS receivers which endeavor to lock on to more than three satellites to get consistent data.
     
  4. afterburner

    afterburner F1 Rookie
    Silver Subscribed

    Jun 20, 2008
    3,197
    Hong Kong
    You can adjust the readout of both the speedo and the rpm with a tiny screwdriver after removing the instrument. Search for my thread with pics and turn direction vs. readout change.
    If you would like to correct the error.
     
  5. Rapalyea

    Rapalyea Formula 3

    Jun 18, 2013
    1,511
    Georgia Mountains US
    Full Name:
    David Rapalyea alias
    Not sure I want to correct the error since it does as I suppose is intended and keeps me less felonious then might otherwise be the case. I have had many cars with optimistic speedometers and prefer them to the other way around. Still good to know the cure.
     
  6. Rapalyea

    Rapalyea Formula 3

    Jun 18, 2013
    1,511
    Georgia Mountains US
    Full Name:
    David Rapalyea alias
    Hi Soucorp

    Fun introduction to GPS. I have one addition and it has to do with e=mc2. The total relativity adjustment requires two corrections. First, the satelites have been accelerated to at least 17,500 mph so their clocks go slower. However, they are also further away from the center of gravity of the earth which makes them run faster.

    I think acceleration is the predominent variable, but they both are substantial.
     

Share This Page