Speedometer accuracy

H

hatter ed

New member
Aug 3, 1998
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After picking up the GMC in Kansas City in route to Omaha, my wife followed
me in the other car. Monitoring my speed, she said we were doing 70 mph
when the speedometer in the GMC was reading 62 mph. I assume that the tires
are oversized causing the speedometer to read slow. I did catch a post
yesterday about something like this. Can this can be corrected with a gear
reducer in the speedometer cable?

Or is the a better way to tell how fast I am going other then timing the
mile markers?

Thanks

Ed Hatter
Omaha, NE
'73 26' Sequoia
 
I have a Tripmate GPS and use Street Atlas USA 5.0. When I set up a route
there is a window that gives me my current speed. I would think that it is
fairly accurate. My speedometer agrees with the GPS within 1 MPH @ 65 to 70
MPH. I like the GPS and would recommend it to anyone.

Russ Bethel
rbethel

- -----Original Message-----
From: Hatter, Ed
To: 'gmcmotorhome'
Date: Tuesday, August 11, 1998 11:07 AM
Subject: GMC: Speedometer accuracy

>After picking up the GMC in Kansas City in route to Omaha, my wife
followed
>me in the other car. Monitoring my speed, she said we were doing 70 mph
>when the speedometer in the GMC was reading 62 mph. I assume that the
tires
>are oversized causing the speedometer to read slow. I did catch a post
>yesterday about something like this. Can this can be corrected with a gear
>reducer in the speedometer cable?
>
>Or is the a better way to tell how fast I am going other then timing the
>mile markers?
>
>Thanks
>
>Ed Hatter
>Omaha, NE
>'73 26' Sequoia
 
>Monitoring my speed, she said we were doing 70 mph
>when the speedometer in the GMC was reading 62 mph.
>
>Or is the a better way to tell how fast I am going other then timing the
>mile markers?
>

We use a GPS unit to check the speed Eagle Explorer, it also has the
speed display, reads in tenths as well, if you want accuracy.

And FWIW: our speedo error is the same as yours, 8 mph
coincidence I,m sure.. :)

John & Brenda Szalay
73 GMC
 
What a coincidence!

Although I have a McNeal instrument panel, I was convoying with Leigh
Harrison last year along I-10 coming back from the GMCMI Boerne TX
Rally. I found that his speedometer also read 8 mph high.

Paul Bartz

From: John Szalay [mailto:john.szalay]
Sent: Tuesday, August 11, 1998 3:45 PM
Subject: Re: GMC: Speedometer accuracy

Monitoring my speed, she said we were doing 70 mph when the speedometer
in the GMC was reading 62 mph. Or is the a better way to tell how fast
I am going other then timing the mile markers?

We use a GPS unit to check the speed Eagle Explorer, it also has the
speed display, reads in tenths as well, if you want accuracy.
And FWIW: our speedo error is the same as yours, 8 mph (coincidence
I,m sure.. :)
 
Ed here is a thought:

Maybe your pointer on the speedometer has slipped. I would check with a
shop that repairs them, however, before you do, make sure you know how
many miles per hour the pointer is off so they can reposition it on the
dial to correct the error.

Bob.

>After picking up the GMC in Kansas City in route to Omaha, my wife followed
>me in the other car. Monitoring my speed, she said we were doing 70 mph
>when the speedometer in the GMC was reading 62 mph. I assume that the tires
>are oversized causing the speedometer to read slow. I did catch a post
>yesterday about something like this. Can this can be corrected with a gear
>reducer in the speedometer cable?
>
>Or is the a better way to tell how fast I am going other then timing the
>mile markers?
>
>Thanks
>
>Ed Hatter
>Omaha, NE
>'73 26' Sequoia
>
>
 
Forgot to mention that mine was off 10mph, took it to the shop and the
pointer was advanced 10 mph on the shaft and bingo it is right on the money
within 1 mph.

>In a message dated 8/11/98 1:04:23 PM Central Daylight Time, robshim
>writes:
>
> Maybe your pointer on the speedometer has slipped. I would check with a
> shop that repairs them, however, before you do, make sure you know how
> many miles per hour the pointer is off so they can reposition it on the
> dial to correct the error.
> >>
>Like I said in a letter a couple days ago, when I took mine to the
Speedometer
>shop they said the speed portion of the speedometer is magnetic run and said
>that after all these years, it had weakened somewhat causing the error. It
>could be, and was, corrected by an adaptor, but that then made the odometer
>off because it is gear driven. I opted to have my speed as near correct as
>possible.
>
>
 
Does anybody know a way to calculate speedometer error if you know the =
size tires (my are 8.75 x 16.5 R) and the final drive (mine is the =
standard 3.07?). There must be a way. I know I have read that you can =
get speedometer gears for the aftermarket final drives to compensate for =
the ratio changes.

Bob Fukumoto
76 Palm Beach
 
Bob

There is a tire size comparison calculator at
www.dsm.org/tools/tiresize.htm.

Ed Hatter
Omaha, NE
'73 26' Sequoia


> ----------
> From: Robert M Fukumoto[SMTP:fukf19]
> Sent: Wednesday, August 12, 1998 9:17 AM
> To: 'gmc motorhome mailing list'
> Subject: RE: GMC: Speedometer accuracy
>
> Does anybody know a way to calculate speedometer error if you know the
> size tires (my are 8.75 x 16.5 R) and the final drive (mine is the
> standard 3.07?). There must be a way. I know I have read that you can
> get speedometer gears for the aftermarket final drives to compensate for
> the ratio changes.
>
> Bob Fukumoto
> 76 Palm Beach
>
>
 
Hi bob, as far as I know, I used the tried and true method of computing speedometer error. I took a trip, took my mileage on the speedometer and at the same time, the mile marker on the highway. when I hit 100 miles, I noted the two and calculate the diff. Then you can see accurately what's going
on. I did that for a trip of about 700 miles and when I computed the varirance, for every 100 miles, the speedometer was 4 miles more than the road.

Have a good time trying... it's worth a trip to just see

Al Chernof

> Does anybody know a way to calculate speedometer error if you know the size tires (my are 8.75 x 16.5 R) and the final drive (mine is the standard 3.07?). There must be a way. I know I have read that you can get speedometer gears for the aftermarket final drives to compensate for the ratio changes.
>
> Bob Fukumoto
> 76 Palm Beach
 
I guess I'm old fashioned, but the easy way to check the accuracy of your speedo
is to use a stop watch and time how many seconds it takes to travel some
distance at a steady speed. I usually try to time over at least 10 miles. Then
calculate the average number of seconds per mile. Divide 3,600 by that number
and the result is your average speed in MPH.

Example, 3,600/57.4 seconds per mile = 62.7 MPH

Clark
 
Regarding this speed thing. My two cents. Mile markers, time, speedometer,
AND odometere.
- -----Original Message-----
From: Clark Searle
To: gmcmotorhome
Date: Friday, August 14, 1998 11:14 PM
Subject: Re: GMC: Speedometer accuracy

>I guess I'm old fashioned, but the easy way to check the accuracy of your
speedo
>is to use a stop watch and time how many seconds it takes to travel some
>distance at a steady speed. I usually try to time over at least 10 miles.
Then
>calculate the average number of seconds per mile. Divide 3,600 by that
number
>and the result is your average speed in MPH.
>
>Example, 3,600/57.4 seconds per mile = 62.7 MPH
>
>Clark
>
>
 
It would be great to use a time measurement over a single mile if you had the
luxury of a electric eye to measure the elapsed time to at least a tenth of a
second accuracy. You can get a fair amount of variation in the time readings
over a single mile. It's like the people who calculate MPG on a single tank of
gas. But if you keep track over several tanks the error in measuring the amount
of gas to top off the tank diminishes as a percentage of the total gas
consumption.

Unfortunately, I only have mile markers and my wrist-stop watch. Measuring time
over 10 miles reduces the error effect by one tenth. If you use your cruise
control over a relatively flat section of highway, the result is a pretty good
indication of your average speed over that distance. If your speedo readings
have also been steady over that distance, it's a reliable comparison.

If you don't trust the average speed approach, speed by a radar cop sometime and
he'll give you an instantaneous reading (grin).

Clark
 
Hi guys, for those of you, like me, that have difficulty with
mathematical formulars and counting seconds and miles, ask the local cop
with radar to check your speed.

Jim Bolick
'73 Painted Desert
 
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