First, I’m not looking to start any lengthy discussion, merely passing on an observation that seems to run counter to my understanding about speedometer accuracy.

During long runs over the past two days (long holiday here in US), I was using one of those map services with GPS-based speed monitoring in it (Waze). I have long thought our speedometers show a fraction higher than our actual speed to make us feel like we’re buzzing along and beating the system. Over the course of these two long drives I saw that my speedometer was reading under the actual speed per GPS by about 5 percent or so. An indicated 70 was actually 74. An indicated 69 was actually about 73 up or down a decimal or two, The same held at lower speeds down to about 60mph where I quit doing comparisons and shockingly kept my eyes on the road in the heavier traffic, As a doublecheck on what the map service was telling me, I switched over to an Apple compass/altitude/groundspeed system and held the phone on the dash to be sure of the signal integrity. I had the same results.

Is it a big deal? No. At 100 mph indicated the cops would probably write me up for 105. This just runs counter to all I had believed from reading various posts here over the years. It being a holiday weekend and the roads being saturated with the local and state constabulary, I didn’t speed, and at my age I don’t do much of that any more anyway.

The speedo aside, my Good Kitty performed like a dream. 24mpg and a rock-steady cruiser. A guy came up to me in the hotel parking lot and other folks at an outside party I attended gushing about how beautiful she looked. It even drew a crowd of its own one space away from the birthday girl’s ‘50s Rolls. The attention in both cases was gratifying. Thanks, Sir William…

Cheers.

 

My particular car is bang on accurate at all speeds. My German cars are 2 to 3mph faster on the dash than gps

 

To under-read suggests someone's done something - different size wheels to factory or some such.

 

My 01 XJR is spot on according to both google maps GPS and those speed reader things on the side of the road here and there. I have factory sized tires on it, but when I bought it it had significantly shorter radius tires from the prior owner and I noticed my speedo was definitely reading high.

I would check your tire size compared to the sticker in the driver's door and see if you might be running a different size.

 

I think if the tires had shorter radius, the speedo would have read higher than actual speed, not lower.

 

You are correct, I had it backwards. Edited the original comment.

 

The Duchess seems to indicate the exact speed she is traveling according
to everything I have tried in my efforts to gage it.

 

I can’t find the plate with the tire size/inflation data on it on any of the doors/jambs/sills (I think I had one once but the adhesive probably gave out) … but as far as I know nothing’s been done to this plain vanilla VDP. Wheels are stock 16”, tires are the prescribed 225/60R16 with perhaps 20k miles on them. Tire pressure at the rear wheels is close enough to spot-on as to be a negligible matter.

Oh well, I have a couple of more opportunities for long or longish drives in the next two weeks or so to run a few more tests. If they have the same results as before, I’ll just factor that into my driving awareness. I do recall checking against GPS in the 25-30 range and the speedo and GPS matched.

Cheers to all.