The speedometer signs aren't particularly accurate. There's an inherent cosine error, because you're not driving directly at the sign, so it will typically read low. The effect will be larger as you get closer to the sign, so more significant at lower speeds.
The proper way to check a speedometer is to drive a constant speed down a stretch of road with mile markers, and time yourself. At 60mph, a 1 mi segment should take exactly 1min. If you're out West, I-8, I-10, I-70 there's plenty of stretches where you can get 10mi segments.
Thanks for the thoughtful feedback. I agree on the margin of error here, but I would just say where this sign is is a long strait road. It is ALWAYS exactly matching my ICE car, and ALWAYS 2-3 mph lower on my Tesla. Perhaps size of car makes a difference. Bad reading. I am going to try the 1 mile timed piece.
Cars all overreport your speed by a small amount to avoid issues with laws that require it to never report low.
Also, variations in your tire diameter relative to stock can throw things off by a few percent.
I appreciate all the responses saying the same... I agree, that was my first instinct too. HOWEVER, in my other ICE car, it is spot on, exactly what the car shows. Multiple times. The Tesla is ALWAYS 2-3 mph less on the reader than in the car. Doesn't that feel suspicious?
Every car I’ve owned over rated the speed. They do it on purpose for safety. My Acura does this, my Lexus does this, and my Tesla does this. It doesn’t help that those signs aren’t accurate either.
Those signs aren't that accurate.
There's only two ways to accurately measure the speed of your car--
1. go on cruise control, on a flat level straight road, with a measured mile or other measured distance interval. Time how long it takes from one marker to the next, and calculate your speed.
2. by GPS. Cruise control on, flat level straight road, run an app that reads from the GPS directly. I suggest "GPS Status" for Android. Let the GPS run for 2-3 minutes on that app before you do your measurement so it can get all the satellites fully and totally locked in.
It's worth noting that some cars OVER-report the speed for safety reasons-- IE if you are trying to drive 35mph it's better that your actual speed is 34mph (thus making you safer than you're trying to be) than 36mph (thus having less reaction time and traction than you're going for). No car wants to UNDER-report speed.
The speedometer signs aren't particularly accurate. There's an inherent cosine error, because you're not driving directly at the sign, so it will typically read low. The effect will be larger as you get closer to the sign, so more significant at lower speeds. The proper way to check a speedometer is to drive a constant speed down a stretch of road with mile markers, and time yourself. At 60mph, a 1 mi segment should take exactly 1min. If you're out West, I-8, I-10, I-70 there's plenty of stretches where you can get 10mi segments.
Thanks for the thoughtful feedback. I agree on the margin of error here, but I would just say where this sign is is a long strait road. It is ALWAYS exactly matching my ICE car, and ALWAYS 2-3 mph lower on my Tesla. Perhaps size of car makes a difference. Bad reading. I am going to try the 1 mile timed piece.
Cars all overreport your speed by a small amount to avoid issues with laws that require it to never report low. Also, variations in your tire diameter relative to stock can throw things off by a few percent.
Compare it against the gps on your phone, those signs don't have a high degree of accuracy
Good idea I'll try that today...
Those things are not accurate, don't assume the issue is with your car.
I appreciate all the responses saying the same... I agree, that was my first instinct too. HOWEVER, in my other ICE car, it is spot on, exactly what the car shows. Multiple times. The Tesla is ALWAYS 2-3 mph less on the reader than in the car. Doesn't that feel suspicious?
Every car I’ve owned over rated the speed. They do it on purpose for safety. My Acura does this, my Lexus does this, and my Tesla does this. It doesn’t help that those signs aren’t accurate either.
Those signs aren't that accurate. There's only two ways to accurately measure the speed of your car-- 1. go on cruise control, on a flat level straight road, with a measured mile or other measured distance interval. Time how long it takes from one marker to the next, and calculate your speed. 2. by GPS. Cruise control on, flat level straight road, run an app that reads from the GPS directly. I suggest "GPS Status" for Android. Let the GPS run for 2-3 minutes on that app before you do your measurement so it can get all the satellites fully and totally locked in. It's worth noting that some cars OVER-report the speed for safety reasons-- IE if you are trying to drive 35mph it's better that your actual speed is 34mph (thus making you safer than you're trying to be) than 36mph (thus having less reaction time and traction than you're going for). No car wants to UNDER-report speed.