Several weeks (months?) ago while on my weekly drive to Bethlehem I noticed that the radar detectors Penn-DOT had set up along the road reported my speed at 2 miles/hour slower than my speedometer. I did not think much of this since that sounds well within the range of error of a stationary radar device, but later checked the speed reported by my GPS-based navigation aid, which also agreed with the external data. I would not expect the GPS data to be highly accurate in determining the exact location of the vehicle at any instant in time, but it should be nearly perfect in measuring displacement over time with a constant speed. I began thinking about how a speedometer would operate and why it’s data might be inaccurate.
It made sense to me that the vehicle would calculate speed simply by multiplying the number of rotations per minute of the drive axle by the expected circumference of the tire (distance traveled per rotation), and then scaling to miles per hour. If this were so, reporting my speed at 103.2% of its actual value (65 m/h rather than 63 m/h) would be a result of it believing the circumference of my tire was 3.2% larger than actuality. I made a note to check the treads for wear, but promptly forgot about it.
A few days ago I noticed that my front driver’s side tire looked significantly flat, so I tested it and found a pressure of only 8 PSI. There is supposed to be a sticker on the inside of my driver’s door frame with recommended pressures, but I could not find one so I filled it to 30 PSI. The rear driver’s side tire was only inflated to 18 PSI, so I raised it as well. The passenger side tires were filled to 32 PSI and 29 PSI, confirming that 30 was around the correct value. I have no idea what caused the driver’s side tires to slowly deflate, but will be keeping a closer eye on them for a while.
On Friday I drove up to a hunting camp in Shunk, PA that my father is a member of to visit him and my uncle, enjoying my newly pressurized tires. I noticed no specific improvements in handling or performance, but the car ran exactly as expected for the entire hour-long drive. When I arrived at the cabin and began turning to park, the engine suddenly cut out, while all of the electrical systems remained operational. I attempted to restart it, but could not get the starter to turn over or even make a sound. I had not been intending to stay the night, but was at least stranded in a place with food, a bed, and friendly faces rather than by myself along one of the country roads with no cell reception on my way there.
In the light of the morning I tried starting the car again with the same results, and verified that the lights, radio, and windshield wipers were working at full power. Still, we cleaned the battery contacts a bit and jumped the battery. After this the starter would turn over but the engine never caught and sounded decidedly unwell while I continued to crank the starter. We visited the one local pseudo-garage but could find no one working. Fortunately, we were able to find a high ridge with cell reception and my Geico insurance covered the cost of a tow to the nearest Ford dealership, in Dushore. They expect to be able to look at it on Tuesday, at which point I will post an update about what is wrong.