Journey with Confidence RV GPS App RV Trip Planner RV LIFE Campground Reviews RV Maintenance Take a Speed Test Free 7 Day Trial ×
RV Trip Planning Discussions

Go Back   iRV2 Forums > MOTORHOME FORUMS > Class A Motorhome Discussions
Click Here to Login
Join iRV2 Today

Mission Statement: Supporting thoughtful exchange of knowledge, values and experience among RV enthusiasts.
Reply
  This discussion is proudly sponsored by:
Please support our sponsors and let them know you heard about their products on iRV2
 
Thread Tools Search this Thread Display Modes
 
Old 09-17-2021, 08:08 PM   #15
Senior Member
 
Rob_M's Avatar
 
Join Date: Dec 2019
Location: Canyon Lake, Texas
Posts: 3,020
What's Your Speed

For what it’s worth. Performance boaters swear by GPS mph accuracy.

I have used GPS to confirm that my Mustang speedo reads 3 mph slow and my MH speedo is dead accurate. And it’s repeatable every time I’ve checked with multiple GPS sources. So I tend to trust the GPS speed readings. Your speedo has too many mechanical variances (tire diameter and gear ratios) to be trusted without comparing to GPS.

(If a couple mph is that important to you)
__________________
2002 American Tradition 40'
Cummins 8.3, Banks 431hp, 1260 tq
Canyon Lake, TX
Rob_M is offline   Reply With Quote
Join the #1 RV Forum Today - It's Totally Free!

iRV2.com RV Community - Are you about to start a new improvement on your RV or need some help with some maintenance? Do you need advice on what products to buy? Or maybe you can give others some advice? No matter where you fit in you'll find that iRV2 is a great community to join. Best of all it's totally FREE!

You are currently viewing our boards as a guest so you have limited access to our community. Please take the time to register and you will gain a lot of great new features including; the ability to participate in discussions, network with other RV owners, see fewer ads, upload photographs, create an RV blog, send private messages and so much, much more!

Old 09-17-2021, 08:14 PM   #16
Senior Member
 
Join Date: Sep 2020
Location: Boise, ID
Posts: 712
I can appreciate the concern in the original post. I like to have things work they way they are supposed to work!

I have had two BMW cars, and both tended to read faster than you were actually going. Apparently that is a “thing” with BMW. When I drive I like to set the cruise control right on the posted limit, and with both BMW I have to set the cruise at 82. I have checked it with milepost timing and with different GPS speedometers. Minor, but irritating.

My Ford pickup and my wife’s Audi are right on the button.

Recently I was on a road trip in my BMW, and was followed by by brother. My car seemed to hold the set speed closely, but my brother said my speed varied considerably. No way to know which speedometer was right, or if both were varying.
__________________
2018 F350 Limited, 2021 Arctic Fox 27-5L
300 Ah Battleborn LiFePo4, 3kw Victron Multiplus, 600W Solar, SeeLevel
JHinman is offline   Reply With Quote
Old 09-17-2021, 09:19 PM   #17
Senior Member
 
Join Date: Apr 2018
Posts: 3,441
Over any distance the gps is going to be dead on. My coach reads 4 mph slow at 62. Gets a little worse as speed increases. The garmin is always on.
shootist is offline   Reply With Quote
Old 09-17-2021, 10:12 PM   #18
Senior Member
 
FIRE UP's Avatar


 
Join Date: Dec 2009
Location: Out there, somewhere
Posts: 9,901
Well,
Our coach, an '04 Itasca Horizon 36GD with the CAT C-7 330HP, originally came with junk Michelin 255 80R 22.5's on it. Those gave me some seriously odd tire wear. So, I junked them a couple years ago and, I'm now running 265 75 22.5 Yokohama's. According to my Garmin dash top GPS, it and my speedo in the coach, are dead on together. I don't remember what, if any, my 255's would read vs the Garmin back when I had them on. But, based on my present tires and Garmin, all is just fine.
Scott
__________________
2004 ITASCA HORIZON 36GD, 2011 GMC Sierra 1500 4x4 Toad '20 Honda NC750X DCT
Retired-29.5 yrs, SDFD, Ham - KI6OND
Me, Karla and the Heidi character, (mini Schnauzer)!
FIRE UP is online now   Reply With Quote
Old 09-17-2021, 10:22 PM   #19
Senior Member
 
Arch Hoagland's Avatar
 
Monaco Owners Club
Workhorse Chassis Owner
Join Date: Aug 2002
Location: Clovis, CA, USA
Posts: 13,103
Have you ever gone through a work zone where they have a readout of your speed displayed on a little trailer by the road?

I always use that to check my speedometer. Is it accurate....beats me but I figure it must be if the government put it there.
__________________
2004 Monaco La Palma 36DBD, W22, 8.1, 7.1 MPG
2000 LEXUS RX300 FWD 22MPG 4020 LBS
Criticism is easier than Craftsmanship
Arch Hoagland is online now   Reply With Quote
Old 09-17-2021, 11:10 PM   #20
Senior Member
 
rarebear.nm's Avatar
 
Excel Owners Club
Winnebago Owners Club
Ford Super Duty Owner
Join Date: Aug 2014
Location: Santa Fe, NM
Posts: 6,789
What ever you do , don't try using three or four tire pressure gauges!

Perhaps you need an atomic clock to get the proper time portion of the equation. In fact you do have exactly that in the GPS measurements. The atomic clocks used in GPS satellites have a precision of three nanoseconds.

There is usually some variation in the accuracy of many or most measuring tools/devices. That's part of the reason high end meters and other other devices are sent to a calibration shop on a periodic basis.

Einstein’s General Theory of Relativity causes GPS satellites' clocks to be advance by about 38 microseconds per day. Thus GPS satellites are updated twice a day to correct for that error.

So the variations you see in those devices is most likely due to design, engineering and production slop. Probably as good as it gets at those price points.

Select any one of them and get along with life.
__________________
Fred & Denise (RVM157) New Mexico
2007 Excel Classic 30RSO & Coach House 272XL E450
2007 RAM 3500, Diesel, 6Spd Auto, SWD, 4x4, CC & LB
[SIGPIC][/SIGPIC]
rarebear.nm is offline   Reply With Quote
Old 09-17-2021, 11:59 PM   #21
"Formerly Diplomat Don"
 
Dutch Star Don's Avatar
 
Newmar Owners Club
Join Date: Apr 2005
Location: Moorpark, Ca.
Posts: 23,931
Quote:
Originally Posted by c92vette View Post
I'm running across I-64 this afternoon and I looked at the speedo and the needle was covering the 65 MPH line. GPS on Google Maps says I doing 66. I got the 5 Star tune SCT programmer mounted on the dash and plugged in for kicks. (Yeah, I'm kinda geeky that way) Anyway, the data on the SCT programmers bouncing between 63.5 and 64. So GPS is 66, Speedo is 65, and SCT under 64ish.

Which one is the most accurate?
Typically, your scanner is reading the same RV computer info (ECM) as what your speedo reports. So those two are the same, with some error in the actually display.

My scanner and dash read the same, the GPS in the nav system is usually 1mph faster.
__________________
Don & Mary
2019 Newmar Dutch Star 4018 (Freightliner)
2019 Ford Raptor
Dutch Star Don is offline   Reply With Quote
Old 09-18-2021, 05:34 AM   #22
Senior Member
 
MN_Traveler's Avatar
 
Join Date: Apr 2012
Location: Louisville, KY
Posts: 1,406
Quote:
Originally Posted by Dutch Star Don View Post
Typically, your scanner is reading the same RV computer info (ECM) as what your speedo reports. So those two are the same, with some error in the actually display.



My scanner and dash read the same, the GPS in the nav system is usually 1mph faster.


My electronic readout on the dash direct from the ecm (same as your scanner) reads the same as my gps. The mechanical needle reads 1 mph lower.

Like another poster, the speedometer on my mini cooper (which is built by bmw) reads 3mph low. Confused me when I first got it, because people would keep passing me (I guess that makes the difference significant…) now it is automatic for me to adjust by that 3mph at all speeds…
__________________
- 2017 Newmar London Aire -
MN_Traveler is offline   Reply With Quote
Old 09-18-2021, 12:20 PM   #23
Senior Member
 
Join Date: Dec 1969
Posts: 2,639
Quote:
Originally Posted by Rob_M View Post
For what it’s worth. Performance boaters swear by GPS mph accuracy.

....
Now that is funny!
__________________
Kit & Rita (in memory)
37 foot ‘98 HolidayRambler Endeavor diesel pusher
followingsea is offline   Reply With Quote
Old 09-18-2021, 12:21 PM   #24
Senior Member
 
SuperGewl's Avatar
 
Winnebago Owners Club
Ford Super Duty Owner
Join Date: Aug 2004
Location: Oregon
Posts: 2,728
Well lets see. yo stated the needle was covering 65, but I have to ask were yo looking directly down on it or at a slight angle? This can make a world of difference. You also state the Google maps GPS read 66 but your Tuner read, lets say 64.
Being that the GPS is the most accurate I would tend to believe it and your dash gauge most likely actually read closer to 66 than 65. Your Tuner is the least accurate because it is only calculating speed using the inputs that you have given it. Ie Tire size, the biggest factor. Changing the actual tire size will change the output. Basically if the circumference of the tires are smaller or larger than what the tuner is using yo will get a different speed displayed. It's all about revaluations per minute.
Your speedo and tuner are affected by RPM's. GPS really does not care, it uses radio waves from satellites and calculates the time and distance to and from the satellites (3) minimum to give you actual speed across ground or air if you were flying. The system is also very accurate, as in enough to shoot rockets and missiles and navigate with.

OK to answer your question. They are all correct. Why you say?
you looked at each one at a different time which in it self would allow them all to say something different.
Basically with all of them being that close to one another it just doesn't matter.
Get your head out of the clouds and enjoy the journey, stop and smell the roses and see America, this Great United States.
__________________
Retired Navy Submariner
2014 Itasca Sunstar 35F; 5 Star tuned; 2014 Jeep Cherokee TrailHawk
SuperGewl is offline   Reply With Quote
Old 09-18-2021, 12:23 PM   #25
Senior Member
 
Join Date: Dec 1969
Posts: 2,639
Quote:
Originally Posted by Arch Hoagland View Post
...... it must be if the government put it there.
Now that is really funny!
__________________
Kit & Rita (in memory)
37 foot ‘98 HolidayRambler Endeavor diesel pusher
followingsea is offline   Reply With Quote
Old 09-18-2021, 12:27 PM   #26
Senior Member
 
davidcbaker's Avatar


 
Newmar Owners Club
Freightliner Owners Club
RV Trip Wizard
Join Date: Jun 2021
Location: Nashville, TN, USA
Posts: 118
Unless it's differential GPS (I'm a pilot--it's not), GPS is only accurate for SPEED over longer, and very straight distances.
__________________
____________
David C. Baker (Nashville): author | speaker | advisor | airplanes | helicopters | motorcycles | photography | woodworking | rving
davidcbaker is offline   Reply With Quote
Old 09-18-2021, 12:27 PM   #27
Senior Member
 
mountainkowboy's Avatar
 
Monaco Owners Club
Workhorse Chassis Owner
Join Date: Dec 2006
Location: Socal/NE Oregon
Posts: 602
Life's to short to care
__________________
Chuck, Ruth, with 4 legged Molly
2007 Tiffin Allegro 30DA
2011 Ranger, 1987 FLHTP
mountainkowboy is offline   Reply With Quote
Old 09-18-2021, 12:37 PM   #28
Senior Member
 
Join Date: Dec 1969
Posts: 2,639
Quote:
Originally Posted by rarebear.nm View Post
What ever you do , don't try using three or four tire pressure gauges!

......
Four tire pressure gauges and three ways to measure temperature. None are traceable to a standard.

I happened to check the pressure of the MH next to me as he was leaving and when he got back the following week. One tire was very hot to the touch. He had picked up a nail.
__________________
Kit & Rita (in memory)
37 foot ‘98 HolidayRambler Endeavor diesel pusher
followingsea is offline   Reply With Quote
Reply



Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Allison 2500 5 speed transmission converted to a 6 speed LJB Allison Transmission Forum 3 05-07-2012 07:58 AM
5 speed vs 6 speed Donmystic1 Class C Motorhome Discussions 0 07-11-2011 04:50 AM
4-speed or 5-speed frankdamp Ford Motorhome Chassis Forum 4 11-23-2010 11:04 PM
Anyone been to Bonneville Salt Flats for Speed Week or World of Speed? CheetahGirl RV Events & Gatherings 1 06-15-2008 04:31 PM
Convert 5 speed to 6 speed Rocky 2 Workhorse and Chevrolet Chassis Motorhome Forum 4 04-19-2008 05:02 PM

» Featured Campgrounds

Reviews provided by


All times are GMT -6. The time now is 12:15 AM.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2024, vBulletin Solutions, Inc.