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Topic: Speedometer MPH vs GPS MPH (Read 1038 times) previous topic - next topic

Speedometer MPH vs GPS MPH

 On our '89 GV the speedometer reads 62 MPH while the GPS says we are doing 56 MPH.
The front tires are 280's and the rears are 235's.(Michelin)
Is the speedo error due to the sender unit being on a front tire or just a normal error due to being an old mechanical Morse speedometer?
We bought the coach like this, but Wayne at FOT says when it comes time to replace to put 235's all around. The coach sits level, handles great, and rides great for an ORED. Don't know why previous owner(s) made this change. Any one else have two different sizes on their coach?
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 MPH vs GPS MPH

Reply #1 –
Can't answer your question directly, but many of the VDO speedometers can be calibrated.  I calibrated mine and it is within 1/2 MPH of GPS speed.

I suspect some of the very high MPG claims are the result of a really optimistic speedometer.

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 MPH vs GPS MPH

Reply #2 –
Have checked our speedo and odo against GPS and mile markers at different speeds and we are well within 1%. 295/75s on a '93 U300.
Pierce and Gaylie Stewart
'93 U300/36 WTBI
Detroit 6V-92TA Jake
1140 watts on the roof
SBFD (ret)

Re: Speedometer MPH vs GPS MPH

Reply #3 –
Larger diameter tires will cause the speedo/odo to read slower than smaller tires at a given speed. Advantage is lower piston travel per mile so better mileage that more than offsets the bigger footprint on the road. Larger size tires may also have a higher load capacity, but read specs. on side of tire. Can go overboard in larger sizes as tires may contact airbag mounts in certain driving conditions. U300 operators should avoid the temptation to fit truck 11.00/22.5 as they are TOO tall and will contact parts of the chassis occasionally.
Pierce and Gaylie Stewart
'93 U300/36 WTBI
Detroit 6V-92TA Jake
1140 watts on the roof
SBFD (ret)

Re: Speedometer MPH vs GPS MPH

Reply #4 –
One of the happiest good surprises was when installed the VMSpc, found the mph was identical on the VMSpc as the GPS, the total engine hours and total coach miles were real close in agreement. Also liked the total fuel burnt history.  It is amazing all the info collected and stored in the Cummins ECM.
Today, I would feel lost / bad trying to drive without the VMSpc.  The speedo does not have a great display and not as accurate as I would like, however the odometer is very close.
It is still a possibility of having the Glass Dash installed by Silverleaf, we have been yaking about it.
After seeing brothers Glass Dash, makes it harder to forget the whole idea.  Just $ $

Re: Speedometer MPH vs GPS MPH

Reply #5 –
I agree with Pierce and Gaylie.  I recently replaced the tires on our '97 U295 with Michelin, XZE2, 295/80 R 22.5's.  These tires have the re-enforced sidewalls and stand a bit taller than original equipment.  This caused them to rub against the air bag mounts when turning.  Gary Omel ground off about 1/4" of the mounts and now all is fine.  I expect the diameter is greater and may well have affected speedometer accuracy.

Fred
Fred, Dawn and Bandit
1998 U295, 36', build # 5233, Motorcade # 16961
2007 Black Saturn Vue

Re: Speedometer MPH vs GPS MPH

Reply #6 –
Fred, Are you still running around nekid ?  :) Or have you put new graphics on your coach ?
Gary B

Re: Speedometer MPH vs GPS MPH

Reply #7 –
Casper is no longer.  Need to get some pic's to post, painted similar to original.

Fred
Fred, Dawn and Bandit
1998 U295, 36', build # 5233, Motorcade # 16961
2007 Black Saturn Vue

Re: Speedometer MPH vs GPS MPH

Reply #8 –
I am wondering why anyone would put larger tires on their coach. 
With the tire rubbing issue on the older coaches, it just amazes me Why someone would intentionally put on larger tires and cause rubbing/grinding problems.
I am sure there is a brilliant reason for this.
I am always trying to learn or is it the bigger Johnson issue? ??? 

Re: Speedometer MPH vs GPS MPH

Reply #9 –
Dave, I wish I knew also. Maybe they thought bigger up front was better than bigger behind.
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 MPH vs GPS MPH

Reply #10 –
For anyone wanting to view the Michelin Truck/RV manual for all your questions, here tis.

Re: Speedometer MPH vs GPS MPH

Reply #11 –
Quote from: Dave M –
For anyone wanting to view the Michelin Truck/RV manual for all your questions, here tis.

Thanks, Dave.  I put a copy in the forum files library as well.
Learn every day, but especially from the experiences of others. It's cheaper!  - John C. Bogle

2000 U320 36' non-slide / WildEBeest Rescue
2003 U320

Re: Speedometer MPH vs GPS MPH

Reply #12 –
There's just no way to keep up with those Metzger boys. One has souped up the engine so much that he is in danger of overloading the rear axle because of weight transfer on acceleration, and the other has so many electronic gadgets that he may have to hire a driver/valet to operate everything.  :P  :P
J D Stevens
1997 U295 CAI 36' Build #5085
2002 Subaru Outback
Motorcade 16869
Bellville, TX

Re: Speedometer MPH vs GPS MPH

Reply #13 –
Dave, At the time of the new tire purchase, I was a motorhome owner for lessthan a month.  The tire purchase was made with the original size specifications but with what was proposed to me as a safer more reliable tire having the reenforced sidewalls.  I was not made aware of the tire profile being taller and only learned of the issue a couple days later when I heard the rubbing noise while going around a corner.

I thought I would simply add what happened to me as a way of preventing another "Newbie" from being misled as I was.

I am hoping to continue learning myself while helping others along the way.

Fred

 
Fred, Dawn and Bandit
1998 U295, 36', build # 5233, Motorcade # 16961
2007 Black Saturn Vue
 

Re: Speedometer MPH vs GPS MPH

Reply #14 –
Fred,
Dont worry, You are not alone here, this is a continual learning forum, we all learn a lot here, and we all make mistakes, do things we wish we could take back and do it different. 
I am aware some people like getting larger tires, I am not sure of the reasoning, but none the less it happens.  This is not the first time this has come up and I am sure it is not the last.
I have been a Foretravel owner since Jan 2008, I have been RVing for 20 years before with an MCI Bus conversion, I can assure you I have made plenty mistakes, spent way too much money on some real dumb things, but I laugh and cry at the same time.
Enjoy
Cheers.