Speedometers

Cacti_Ken · Post by **Cacti_Ken** » Sat Dec 27, 2008 3:48 pm

The used speedometer I got for my M38 is 5 mph slow. I think I have that right. If I'm going 45 as shown by the speedometer, I am actually going 50 mph.

I do not know if it is the correct speedometer or not. The speedometer gear I belive is original to this jeep.

Question I have, are the speedometers which were made for 50's, 60's and 70's model Mv's the same as far as calibration. What I mean, are they innerchangeable from one MV to the other?
Would the number on the face of the speedometer tell what MV it was design to function in?
The photo is the one I am using presently.
The numbers are MS-39021-2 and S.W. 503-C
Ken

Post by **Ryan_Miller** » Sat Dec 27, 2008 9:58 pm

Ken,

The M38 used the AC speedometer.

I do not know of any difference in accuracy between the AC and SW speedometers.

As far as speedometer gears go, I do not know about the different vehicles interchanging.

What about your tires - correct size?

Cacti_Ken · Post by **Cacti_Ken** » Sun Dec 28, 2008 12:32 am

I am using the 700 16LT NDCC,

DaveO · Post by **DaveO** » Sun Dec 28, 2008 9:53 am

Ken,

I have the same spec number (39021-2) on my M37, and it's accurate. Going out on a limb, I would have to say that they didn't standardize the speedometers between the different sized vehicles. Pure speculation on my part, but matching the cable speeds would be difficult and expensive.

Nice looking cluster, by the way...

Dave

Post by **Ryan_Miller** » Sun Dec 28, 2008 10:04 am

Ken,

How did you measure your speed to check the speedometer?

Maybe you were off in your measurement?

Cacti_Ken · Post by **Cacti_Ken** » Sun Dec 28, 2008 2:51 pm

Ok here is how I checked it. I had my wife helping me So just between me and you maybe that is where the error is, LOL.
I had my wife tow me down the hiway with a tow strap connected to her vehicle. We communicated by cell phone, I told here to pull me at a constant 50 mph. And let me know when she was doing 50. So that's how I did it.

Post by **Ryan_Miller** » Sun Dec 28, 2008 3:21 pm

Ken,

Well, as long as you can trust your wife

that should be fairly accurate as long as the car is accurate - tires, etc.

The only other thing I would say that you could take out the speedometer and have it checked and/or take out your drive gear in the transfer case and check it.

Cacti_Ken · Post by **Cacti_Ken** » Sun Dec 28, 2008 3:28 pm

Well I figured the tow vehicle a 2005 Yukon with factory tires would be accurate.

And about the drive gear I cant' see why anyone would change that out to a different one. But that is a guess.

Post by **Ryan_Miller** » Sun Dec 28, 2008 6:36 pm

I would do the easiest thing first: take out the speedometer and go to a speedo shop to have them check it out.

DJ · Post by DJ » Sun Dec 28, 2008 10:51 pm

Wow,50 mph,in a jeep, with your wife driving the tow vehicle ,she must really love you and your hobby.

DJ

Cacti_Ken · Post by **Cacti_Ken** » Sun Dec 28, 2008 11:00 pm

I wasn't worried I had all new brake stuff. Yeah I do love it too.

Post by **wesk** » Mon Dec 29, 2008 11:58 am

All the following numbers work on the M38/M38A1/M38A1C/M38A1D/M170 group.

SW 503A
SW 503C
AC 1580260
AC 1582373
G742-7760396
G742-8376375
WO 118101
WO 808913
WO 809819
AL EO-12205
FSN 6680-282-6165
NSN 669-00-933-3599
MS39021-2

Yes there was a significant amount of standardization between the M series vehicles. Shaft speed was easily matched to the standard speedo using an appropriate driven gear at the transfer.

DaveO · Post by **DaveO** » Mon Dec 29, 2008 12:41 pm

Wes,

I stand corrected. Thanks for the education...again.

Dave

Cacti_Ken · Post by **Cacti_Ken** » Mon Dec 29, 2008 8:46 pm

Maybe I'll try another test sometime when the weather gets better.

Post by **wesk** » Tue Dec 30, 2008 2:03 am

Two really good accurate test methods are:

Drive too fast in front of a cop.

Drive by one of those radar traffic monitors and see what she reads. You can call your local PD's and ask when and where you can find one set up.