Speedometer accuracy: RPM + Final drive size + tire size = speed?

[robert m fukumoto]

For my fellow GMCers who are smarter than I (which is probably all of =
you), it sure seems to me that a way to check the speedometer accuracy, =
if you have a tachometer, is to convert the RPMs, final drive size and =
tire size to the correct speed. Seems to me that there must be a way to =
convert rpms from the engine through the final drive to revolutions of =
the drive wheels. Add to that the tire circumference and you've got =
speed. My only problem is I don't have a clue how to do this. Any =
engineers out there that can help?

BTW, after my maiden voyage from hell my wife did make me put an ad in =
the paper to sell my GMC. However, she is softening and that combined =
with not a single call in two weeks might lead us to giving it another =
try next year. (I'm still working and with two teenage kids we really =
can only use the coach once a year.)

Bob Fukumoto
76 Palm Beach

 

[samuel pickens]

Regarding Speed: Seems to me that the question is speedo accuracy, not
exactly how fast are you going. Probably the easiest way: At a given mile
marker and over 10 or 20 miles using a stop watch, log time, distance,
speedo, odometer and you get an apparent accurate reading, very accurate.
Mine is 3 mph faster that speedo says.
- -----Original Message-----
From: Bartz, Paul
To: 'gmcmotorhome'
Date: Tuesday, August 18, 1998 11:03 AM
Subject: RE: GMC: Speedometer accuracy: RPM + Final drive size + tire size =
speed?

>Bob:
>
>Go to http://sura1.jlab.org/~grippo/auto.html, scroll down the top
>skinny panel and you will find such a program titled Bowling's RPM and
>MPH calculator.
>
>However, unless one has a digital tachometer readout, I doubt that you
>can get an exact, accurate number of RPM's from the average analog tach
>gauge to use in performing the calculation.
>
> Paul Bartz
>
>-----Original Message-----
>From: Robert M Fukumoto [mailto:fukf19]
>Sent: Monday, August 17, 1998 1:40 PM
>To: 'gmc motorhome mailing list'
>Subject: GMC: Speedometer accuracy: RPM + Final drive size + tire
>size = speed?
>
>For my fellow GMCers who are smarter than I (which is probably all of
>you), it sure seems to me that a way to check the speedometer accuracy,
>if you have a tachometer, is to convert the RPMs, final drive size and
>tire size to the correct speed. Seems to me that there must be a way to
>convert rpms from the engine through the final drive to revolutions of
>the drive wheels. Add to that the tire circumference and you've got
>speed. My only problem is I don't have a clue how to do this. Any
>engineers out there that can help?
>BTW, after my maiden voyage from hell my wife did make me put an ad in
>the paper to sell my GMC. However, she is softening and that combined
>with not a single call in two weeks might lead us to giving it another
>try next year. (I'm still working and with two teenage kids we really
>can only use the coach once a year.)