Editor’s Note: TFLtruck reader Bernie Kressner recently sent us this interesting story about how he calculated his trucks real speed vs his speedometer. I thought it was interesting so I’m sharing it with you. Thanks Bernie for an interesting take on calculating your truck’s real speed.
Did you ever listen to someone recite a sad tale that goes like this: “Gee, I received a speeding
ticket last night. But the dealer told me the speedometer had a 15% over-reading error, so I
compensated for that AND still got nailed!” Or, you’re traveling on the highway, and everyone else
is passing you at what seems to be ridiculous velocities. Or worse, YOU are passing everyone
else, and then sheepishly ask what they know that you don’t! Well, after 54 years of driving
experience, I can say that most speedometers, even in luxury vehicles, are simply not giving you
a reading remotely resembling your vehicle’s linear motion. They are not even diverging the
same way at all speeds.
What’s more, if you change wheel sizes, tires sizes, or sidewall heights,
then all bets are off, — and you have to plan through your estimates all over again. None of
this even considers tread wear: new versus old tires of the same nominal size! So, your
perceived speed often becomes little more than wishful thinking.
Well, after I recently put new tires on my 1996 Dodge Ram truck, using the recommended size, I
discovered what I thought were two or three of the situations described above. It seemed that
some sort of calibration was in order. But how to do it? As it turned out, the interstate (I-41)
wrapping around the north side of Appleton, WI, has blue mileage markers in the medium every
0.2 miles; and my cellphone has a handy stopwatch built in (as most do). So, gathering a fat mug
of coffee for a late-night adventure (which became wee morning hours), I mapped out the plan of
going 25, 35, 45, 55, and 65 MPH on the interstate whose speed limit was 70! As they say in
televised football, “upon further review”, I decided to drop the lowest speed as perhaps a bit
risky, especially at 1:00 AM, when most traffic was 18-wheelers desperate to make good time. I did get really great timing data on 35, 45, 55, and 65 MPH, which unfortunately devolved
into 35, 45, and 55 MPH, since I sleepily and accidentally deleted all the 65 MPH runs! 🙁
Oh, well…moving right along.
I guess I could work with three data points, if they were good ones.
(They say that a mathematician needs only 2 points to determine a straight line; a theoretician needs 3; an
experimentalist needs 5; and an engineer needs 49,237…(^_^)…). So, the Excel® data and plots
show the results on my truck, in a somewhat self-explanatory fashion. Observations are:
- The displayed speed (Disp Speed) on the speedometer was about 4 MPH greater than the
actual speed (Act. Speed) for all the nominal speeds used, constituting an offset; - The percent divergence was not constant at all, and varied from 22% at a calculated 25
MPH, to 7.1% for a measured 55 MPH. So, no single accurate “percent over-reading”
quoted by anyone is possible: it would depend on vehicle speed (!); - For this vehicle and these particular tires, the equation relating the actual speed (y) to the
displayed speed (x) is: Y= 1.0264 X – 5.1610, with r^2 = 0.99994. - The equation for calculating actual MPH speed (AS) from timing data (T) in a one-mile run
is: AS = (5,280 ft / T sec) * (60 mph / 88 ft/sec). Fine. Great. Now I’ll have to do this all over again if ANYTHING changes with these tires, and
that includes routine wear from a couple of years of driving. Might be safer using a back road
with a GPS locater on a decent cellphone!
