I just finished my first long trip from Annandale, Va. to Tybee Island, Ga. — 1177.6 miles (who’s counting) — and noticed a speedo anomaly on my ‘03 VDP now at just under 37k lifetime miles. If my compass App using GPS to note speed and altitude is accurate: when I put cruise control on and tried to nail 75mph on my speedo, the speed shown on the app varied between about 77.5 and 78.5mph. In the main, I think I’d rather have my speedo off in the other direction against unwanted conversations with local police. That puts indicated speed from 3.3-4.6% low against actual if the compass app is to be believed.

At the same time, the Google Maps distance shown for my route is 584 miles. I drove that exact route, did not deviate beyond getting about 0.1 miles off the road for fuel once. My distance traveled as I pulled into my destination showed 569 miles. That variance was 2.6% lower than what the mapping program projected. It was pretty much right on the money against the Maps app distance shown at the start on my Apple Iphone.

These are just observations after the first real long-haul drive in my ‘03 VDP. I’m curious if others have the same “indicating low” speedometer issue, if I actually am running faster than indicated. The world will keep turning if no one comments.

This trip alone put more miles on the car in 6 days than it got some years under its two prior owners. There were no mechanical hiccups along the way. The trip down showed 25mpg as the day was cool and I ran almost all the time without the AC. Coming back, mostly no AC, but a headwind and crosswind for about 350 miles from a coastal n’oreaster that cut my aggregate shown MPG down to roughly 23.5. That included about a half-hour of creeping along US1 at Fredericksburg detouring with AC on to get around trouble on I-95. The math for miles traveled and gallons used (51.996) put the mileage per the car’s displayed total mileage and actual fuel pumped at 22.6.

In all, it was a week of much-needed R&R for me as a caregiver, a good wringing out for the beast, and … folks, if you’re in your 70s and need to stop a lot for bio-breaks, it was a good lesson in why $15.20 in fees plus some United Miles on the direct IAD-SAV run might be a darn sight more restful than beating down the road for 9-plus to 10-plus hours, even if it is a Jag.

OK, I checked that box. Local outings and runs to the Shenandoah Valley will probably occupy this Cat’s remaining years in my hands.

Cheers.

 

Your speedometer and odometer could be off if the wheels/tires are bigger than stock. I'm not sure if a calibration can be done or not, but someone on the forum will know.

 

thanks. I know the wheels are stock. I'll double-check the tire size against the factory to see if there's an issue there.

 

I can understand the concern for the over speed scenario for you. The constables in VA aren't exactly forgiving with just a few miles over (especially south of Richmond). Have you checked your speed at lower speeds for verification? Higher speeds tend to be off a tad.

I lived out in Sterling for awhile. Fredericksburg the past several years is nothing but a detour. I drop off on exit 104, onto the 301, ..and come up under DC or Baltimore, depending on my destination. Nice scenic ride and the constables are no more forgiving.
Google maps is not exact by any means (Waze is better), I would not be concerned about that mileage difference. What you want to do is use the mileage markers (some states even have 1/10's) on the side of a highway and crank off 10 miles or so and compare.

Doing some rides on the other side of Chantilly or keep going down past Roanoke, down into W VA along the Tech Corridor will do nicely for the eyes as well.

 

To wrap things up ….

Yes, wheels and tires are stock sizes.

Yep: Fredericksburg has been a hell hole on I95 as far back as the ‘80s and, were it not such a detour, I’d probably go out 66 to 29 then take 17 or 3 back over, but it takes as long to do that as to suffer through FB. I don’t do 301 because if I cross the Potomac bridge at Dahlgren I’m committed to having to backtrack to get to where I live in Annandale right on the Beltway.

I will take a few runs at 40-50mph and check to see what “error” I’m getting just to see how the error is reflected at lower speeds. I did use Waze running parallel with the Iphone maps app when I was driving north from Savannah and Waze seemed a bit more accurate as far as road issues were concerned, but a good bit more busy with extraneous stuff.

And, yes … my favorite runs from here are out 66/29 to 211 at Warrenton, then over to the Blue Ridge, wandering around the Shenandoah Valley and eventually making my way home. 150-200 miles of real bliss … until I get back to Gainesville on 66. At least there’s Effie’s soft serve on 29 near Warrenton, or the Moo Thru ice cream dairy farm co-op on 29 at Remington to add some extra sweetener to the drive!

Thanks to all, and cheers.

 

Hey Elliott, you know what the real problem is? It's the difference between your IAS, TAS, and GS !!! That's Indicated (air) Speed, True (air) Speed, and Ground Speed !!!

 

So Mark, ..are you saying he needs a wind speed indicator as well?

This is going to be a tough decision figuring which one looks best on the A or B pillar...

 

Highhorse/Highspeed42: yeah, I was thinking maybe of pulling the Garmin out of the PA-28-180 I sometimes rent out at KHEF, or taking the attached and replacing the primary display: https://www.garmin.com/en-US/p/612862 ... or getting a unit double DIN size to replace the stock radio, then getting bluetooth/AM-FM/CD/Sirius added in.

That reminds me: I need to book a little simulator time. I'm a steam gauge guy and even SELs are getting awful complicated, not to mention I have to fly in the DC SFAR airspace! Every rental has Garmin in it and some sim time is a cheaper way to learn how to use them. So much for knowing how to read a chart and use a VOR.

Cheers.

 

Yes!!! That would help in calculating the GS. He would also need the winds aloft forecast for the higher altitudes along the way. The actual GS is what's in question here.

Getting the Garmin out of the airmobile would be the easiest way to get all the information, those things are a lot easier than getting out the E-6B. But the mileage would still have to be calculated by hand when the trip is over.

 

Mark: I've still got my E-6B in my bag. Maybe I should just mount it on the A-pillar!

 

A couple strips of velcro and you're done !!! I still have one, too, along with a plotter and probably some sectionals from 1979. But that's all in a box somewhere in a storage unit.