Skip to main content ↑ ↓
Topic: Speedometer error (Read 680 times) previous topic - next topic

Speedometer error

MY Garmin GPS says we are going 56MPH but the speedometer says we are going 62MPH.
I tend to set my speed based on the GPS. Right? Or Wrong? Tire size is apparently slightly different as they are not the 19.5 or whatever that odd size was originally installed. Woud this make the 6MPH difference?

Anyone else have this type of problem? How did you resolve it? Different speedo drive gears? Ignore the difference?
Nitehawk,  Demolition Lady, & our NEW master, Zippy the speeding BB cat.
1989 Grand Villa 36' ORED
Oshkosh chassis, 8.2 DD V8
2006 Saturn Vue AWD

Re: Speedometer error

Reply #1 –
Yes, GPS is more accurate BY FAR than any speedometer.

And some speedometers can be calibrated.  Contact your speedometer manufacturer for information.

I know I calibrated my VDO speedometer against a GPS (level, straight road).

Did the same for the sailboat knotmeter.

Brett
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Speedometer error

Reply #2 –
On our U295 with all stock sizes and equipment, GPS shows 65 mph, speedometer shows 66 mph. On Jeep with slightly oversize tires, GPS shows 60 mph, speedometer shows about 55 mph. Other vehicles show minor differences (less than 4%) with recommended tire sizes.

In our case, I generally set speeds by GPS. Jeep is the only vehicle with a significant difference. We don't drive it much, so we haven't tried to correct it.
J D Stevens
1997 U295 CAI 36' Build #5085
2002 Subaru Outback
Motorcade 16869
Bellville, TX

Re: Speedometer error

Reply #3 –
Also my VMSpc agrees exactly with the GPS on level ground, going up or down will give you a different reading between the two as expected.

Dave M

Re: Speedometer error

Reply #4 –
Quote from: Dave M –
Also my VMSpc agrees exactly with the GPS on level ground, going up or down will give you a different reading between the two as expected.

Dave M
                            My GPS and VMSpc also agree exactly. I never look at the speedo anymore.
Dean & Deirdre

2012 Arctic Fox 22GQ
Was 2003 U270

Re: Speedometer error

Reply #5 –
GPS and stock speedo agree withing a mph at all speeds.
Pierce and Gaylie Stewart
'93 U300/36 WTBI
Detroit 6V-92TA Jake
1140 watts on the roof
SBFD (ret)

Re: Speedometer error

Reply #6 –
When I bought this coach the ewner told me to be careful as the gauge shows 10% less than true road speed and this is confirmed on the GPS. So when I got the VMSpc I changed the parameters and that too now reads the same as GPS.
John H
Coachless, now use aircraft. 2003 Ford Travelair TC280 class C. Super shape. Just for 1 yr .
1994 Ford E350 ClassC,total renovation inside and out. Now sold.
2000 U295  36' Cummins 350 c/w Banks Stinger, Resonator upgrade,Solar, LED lites.Residential fridge with slide out pantry. Build 5674. Sold
ex 92 GV 022C ored Cummins. Sold
ex 95 GV240 cat 3116. Sold
2017 Mini cooper s & 2016 land Rover LR2 HSE  LUX.
jhaygarth@aol.com    SKP #130098
treat everyone as you would like to be.

Re: Speedometer error

Reply #7 –
My speedometer is the OLD Morse brand, along with all the other gauges. I kinda doubt whether I could get any parts for the speedometer drive train (gears and cable) or the gauges. I did a search on the internet a couple weeks ago with little success.
Nitehawk,  Demolition Lady, & our NEW master, Zippy the speeding BB cat.
1989 Grand Villa 36' ORED
Oshkosh chassis, 8.2 DD V8
2006 Saturn Vue AWD

Re: Speedometer error

Reply #8 –
GPS indicated speeds are not affected by tire size, wind, or hills. They *are* affected by solar flares, however; but generally these types of errors are so large that they're readily noticed (telling you that you're in Brazil, for instance).

So many of us drive with our GPS units on that speedometers are almost irrelevant nowadays. I watch the tach a lot more than I do the speedo. Some seriously useful applications for smart phones with built-in GPS systems are available now, too.

I almost always set a "route" in my Magellan/Co-Pilot GPS even when I know the route by heart because I like a reminder of where I need to slow down to make a turn. It's also handy to determine whether it's worth while passing a slower vehicle (yes, it happens in a U225 now and then) as one's exit approaches.

Craig
1993 U225 36' Unihome GV with PACBRAKE exhaust retarder, Banks Stinger and Solar Panels.
Toad: 1999 Jeep Wrangler 2-door soft-top.

"No one has ever had to evacuate a city because the solar panels broke."

Re: Speedometer error

Reply #9 –
Craig,
I did get a laugh over the goofy GPS, I have to assume that you also have experience with the Loran C system, I had it in the Piper Arrow, and had the experience of seeing 450 knot speed reading. Everyone knows the Arrow will not get anywhere near that number even in a vertical dive, I was going into Oshkosh, the crazy part, it showed my about 2 mile out when I was about 60 miles out, had to scamper around and change the stations the Loran C was using as two had lined up.  Glad when it said 140 knots and 55 miles out.  Sometimes you gotta laugh when you calm down.  Enjoyed Oshkosh show big time.
Dave M

Re: Speedometer error

Reply #10 –
Quote from: Dave M –
Craig,
I did get a laugh over the goofy GPS, I have to assume that you also have experience with the Loran C system, I had it in the Piper Arrow, and had the experience of seeing 450 knot speed reading.

Loran C had problems with the way radio waves propagate and it was not uncommon to have these errors early and late in the day as the ionosphere began to change. Loran A was even more fun. I also worked with Omega systems and "Transit" satellite navigation system. GPS is far superior but nothing is perfect.

I flew a 1946 Taylorcraft BC-12D from Manassas, VA to Martha Lake Airport north of Seattle in 1970 with no electrical system at all; no starter, no radio, no lights. It was an interesting trip but I don't think it would be possible today. Luckily for me there were lots of highways. I flew north until I saw the Pennsylvania Turnpike and then turned left. :)

Craig
1993 U225 36' Unihome GV with PACBRAKE exhaust retarder, Banks Stinger and Solar Panels.
Toad: 1999 Jeep Wrangler 2-door soft-top.

"No one has ever had to evacuate a city because the solar panels broke."

Re: Speedometer error

Reply #11 –
Craig,

I did a lot of cruising in Asia around 1980. Shipped a CRT wave matching Loran C to Taiwan while the boat was being built. Soon found out that only Japan used C. Every other country used A. Using a sextant in the South China Sea in winter was more than challenging. Inside Tokyo Bay, matching wave forms LOP was within a pencil width.

After radio failure in a wind storm, I had to drop down to read road signs to find I was outside of Albuquerque. One of those storms where it took full throttle to catch up with the airport. With the wind still blowing and no radio the next day, the Grand Canyon suddenly appeared underneath me. Always loved DEAD reckoning.

Pierce
Pierce and Gaylie Stewart
'93 U300/36 WTBI
Detroit 6V-92TA Jake
1140 watts on the roof
SBFD (ret)

Re: Speedometer error

Reply #12 –
Dave, when the EAA was in Rockford, IL I worked at a company on 21 Airport drive. I would go out front and lay on the lawn and watch the aerobatics above me on the north end of the field. Awesome!
When the EAA moved to Oshkosh the held the aerobatic competition at the Fond du Lac airport. We lived just east of the airport and would drag out our reclining lawn chairs and watch the competition there..this until 1976 when we moved to Antigo. Sure miss watching those guys.
Nitehawk,  Demolition Lady, & our NEW master, Zippy the speeding BB cat.
1989 Grand Villa 36' ORED
Oshkosh chassis, 8.2 DD V8
2006 Saturn Vue AWD