Tonight I was driving down Reseda Blvd near the 118 Freeway in Northridge. Along side me comes Mr. Chippy in his Crown Vic. We both pull up to a red, I am in the fast lane, he is next to me. When the light turns green, I gradually make my way up to the speed limit of 40. My speedo needle is pegged right on the 40 mark and I hold my speed there. I glanced over to see what officer was doing right next to me but he's looking straight ahead. For a quick moment, my eye catches a box affixed atop his dash. It read MPH's and was displaying "45." We were going the EXACT same speed and this went on for a good five seconds with no variance. I looked over a second time to be sure what I saw and it indeed read 45.
So it got me thinking. My car showed 40. His car showed 45. We were moving at indentical rates. This looked like some sort of certified speed reading box (maybe for radar?) and likely what he reads to view his speed. I started wondering if my car was off or if his was. I have a hard time believing it would be my car, which is essentially stock, save for some very worn out front tires. Could that be throwing off my own speedometer? Could his car be calculated incorrectly (on purpose dare I say)? 5 mph is quite a difference.
Ultimately, the whole thing does not sit well with me at all. I am very much ontop of my speed and avoiding tickets but now it seems I can't trust my own speedometer or possibly even the CHP's.
Thoughts?

 

This "box" was probably a radar unit. the speed you saw was most likely the speed of a vehicle coming at the CHP car.
All Police vehicle have calibrated speedos and are checked on a regular basis

 

^yup. Speedo's are rarely displayed on any sort of HUD. It was probably the built in radar locked onto a target ahead. And unless it was a handheld unit, vehicle mounted radar only fires forward and behind. Even if whipped out the handheld radar (which is never done while driving), you cannot lock onto a target moving at a 90 degree angle moving in the same direction. The speed will read 0, unless you pull away.

Also, worn out tires (if you somehow managed to shave off a few mm), would make you read slower than your actual speed. Your speedometer measures wheel rotations, and a smaller diameter results in a smaller circumference, which results in less distance traveled per rotation = slower. Upsizing your wheels (to something like 20"+), will effectively make your speedo read slower than your actual speed.

 

CHP Lidar units (laser) are very accurate.

 

Thanks for the clarifications. That explains it... it was radar that was set on something previous. There were no other cars around.

 

AFAIK, laser isn't used by moving CHP cars. I believe they do use Ka while moving though since my V1 picks up Ka from many CHP cars.

 

Yes, laser can only be used when stationary.

 

I think you are correct, CHP only uses laser when stationary.

Ka radar is more common, I often pick it up w/Escort detector.

 

most of California law enforcement uses Ka.

 

Law enforcement in general. K band radar is near non-existent, except for radar speed signs. Remember, K-band units are 15+ years old. I still laugh at people who swear they're getting tagged by X-band, using their 50 billion band $20 Cobra radar detectors.

 

I've definitely been hit with K by Santa Clara County Sheriff within the last 2 years. Came with no warning while I was the only car on a 4-lane road.

I've turned X off completely on my V1.

A lot of CHP seem to drive with Ka on continuously - I'm not sure what kind of speed monitoring system they have, but I'm imagining it's pretty sophisticated and can pinpoint the fastest object in their vicinity.

 

Agreed, I've noticed that too (use of KA).

CHP in many areas now have Lidar (laser units).

 

^ The nice thing about lidar is that they have to be stationary to operate it (to use the targeting sight and for accuracy). Radar will never disappear for that reason. And many times, lidar requires a 2 man team, one to operate the trap, and a secondary (or multiple) chase vehicle(s). I'm sure you've seen them camped out on 280 (SF-SJO). But even with the lidar traps, you'll still get tagged plenty of times with good old Ka band.

 

^ Agreed on how they use laser, seen it used on 280, 101, 580.

Also Lidar units are expensive.

 

K Band is still used by departments up here in NH, especially in the more rural area's, thus I leave it on. I don't think I've ever picked up a legitimate X band, that's why It's turned off. Ka is by far the most common, including use by the NHSP. Laser is almost unheard of in NH. Go over the border though and that's all the state police use.

What you saw was probably an old reading or another car on the road. If he was right next to you he would just pace you.