Have you ever noticed that when drive by one of those stationary road-side radar units that displays your speed, the radar is always slower than what your vehicle's speedo indicates?

I've noticed this trend with every vehicle I've ever owned or driven and having gone past hundreds of these radar stations. The radar is always less. In my '05 F-250 for instance 75 on the speedo nets 71 on the radar, 50 on speedo = 47 radar, and so on.

Why do you suppose this is?

Maybe I'm dreaming up some strange conspiracy theory here, but I've noticed a very distinct pattern.

Are cops being conservative with their radars so they always err on the side of caution? (Although, you'd think they'd be set the other way around so they issue more/larger tickets.)

Is it a Fed DOT thing so that you end up traveling slower than you think you are for safety?

Or are Vehicle manufacturers cheating a little so that your mileage ticks away faster than it actually is? In other words, more frequent $ervice, warranties expire sooner than they're due, and they can over-state your estimated mileage?

I think its set that way so you think, I'm going 4mph slower than my speedo says so I'll just drive 79 instead of 75 on the speedo and then they hit ya with the real radar and fine you for doing 80

Nahhh...I can't imagine municipalities are really putting that much thought into it.

I'm going with my other theory: The OEM's wanting more frequent services, have warranties expire faster and be able to over rate-mileage.

Noticed the same thing. Upgraded to larger tires on my truck and figured I'd have to recalibrate the speedo. Guess what, now its exactly right.

I am pretty sure on the Harleys the speedo can be up to 4% off and considered within factory spec. I am not sure about the auto makers but with all the GPS technology you would have thought by now we would have GPS speedos in the vehicles like on boats. On my 2010 Ram I have a programmer (H&S) and I used it along with my nav device and a GPS to get the speedo correct.

My 04' F150 has always been dead on with all radars until I put bigger tires on a month ago. Now my speedo is off by 2 mph. I've even double checked and my GPS shows the same. Same with my wifes 03' Mustang. So I doubt its the auto makers but as Airpacker said, they want you to think you are going slower so you speed up and then they can pop you with a ticket.

It's in the tires. Speedo ratios calculated on theoretical tire circumference. Actual circumference is less = higher indicated speed.

Tire mfrs have DOT-allowed "tolerances" in size specs for section width of +/- 20mm. For street tires, they save money by shaving a few mm off the width; make the tire a little skinnier, multiply by the profile %, the tire's just a little smaller. Plus the change in tread depth over time reduces actual circumference also. A couple % here, a couple % there, you're down 3-4mph of actual vs indicated.

Well, ya.....it's a given that linear velocity is a function of tire circumference and its angular velocity.

But you'd think that with a stock truck with stock tires and stock programming, I'd be a little closer than 5-6%. That's significant in my opinion. Especially when my 100,000 mile warranty expires at 94,666 miles.

Honda got busted for being 10-15% happy on their speedos a few years back. It adds up to you getting out of warranty faster. Yea tires and rims throw it off too but 100% stock cars/ trucks are always faster on speedo to get the miles up faster imo.

5% isn't a surprise, probably what's allowed by DOT.