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  #1  
Old 12-28-1999, 02:26 AM
Tim_K Tim_K is offline
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Biggrin

All right, time to talk about speedometer calibration. This is really a misleading term, since you're not actually setting up the speedometer at first, you're getting the odometer right. The speedometer comes later.
I'll also say here at the start that the August '99 issue of Mopar Action has a pretty good article on the subject. I'm just going to add a few notes that I've learned of over the years.
Let's start with the example presented by Dartman in the Duster speed (street) posting.
27.7 inch tires with 4.30 gears. Since he didn't specify, I'll assume a manual transmission (yes, it matters) at first, then move to automatics.
27.7 inch diameter multiplied by pi (3.1416) to get circumference in inches. The answer is 87.022 inches.
Take 87.022 inches and divide by 12 to get circumference in feet. Answer is 7.251 feet. When the tire makes one full turn, it will move forward 7.251 feet.
Take 5280 (feet in a mile) and divide by 7.251. This gives # of tire revs per mile. Answer is 728.175.
Multiply 728.175 by axle ratio. In this case, 4.30. Answer is 3131.
3131 will be the driveshaft (and engine) rpm at 60 miles per hour. One mile per minute, 60 per hour. This will be accurate if you have a manual clutch, since there's no slippage while cruising on the highway. So, Dartman's tach should be showing about 3131 rpm in high gear at an indicated 60 mph. If this is true, his setup should be accurate.
Note that a torque converter will always have some slippage, figure 3 to 5 percent for a good street converter. In this case, multiply driveshaft rpm by 1.03 to 1.05 to get an idea of your number, then go try it out. Race converters being driven on the street might have 7 to 10 percent slippage.

Anyone confused yet? O.K.

As the Mopar Action article states, use highway mileposts to get an idea of your odometer accuracy. OK, this is true. BUT... who says the mileposts themselves are accurate? They're not always. After leaving work on thursday for the 150 mile run home on the Ohio and Pennsylvania turnpikes, I was thinking about the research for this article, so I did the 10 mile test 4 different times. I had 3 different answers. Twice I was reading .2 miles high at the end of 10 miles, once I was reading only .1 mile high at the end of 10 miles, and once I was reading .4 miles high after only 3 miles of that run, so I quit and restarted it.
The point here is that you can NOT just go out and drive 10 miles and expect that to be the end-all be-all answer of answers. You must do the test several different times in different locations. I would suggest 4 or 5 at least. Throw out the high and low readings, and average the rest if they aren't all the same. Learn from this experience!
Here's another point I learned while doing these research runs: Tire wear can have more effect on odometer reading than you think.
When I last set up the odometer in my daily driver '71 Valiant, I was reading 0.75% low. That was with nearly new 195-75-14 rear tires. Now, those same tires are nearly bald (hey, I can SEE and FEEL SOME tread, and they still hold air, so they're good tires, right?) and the odometer reading has gone from 0.75% low to 2% high. Anyone know why?

THAT'S RIGHT!!!
Due to the tread being worn almost completely off, the tire circumference has gotten smaller, resulting in more tire revs per mile, with the odometer not being adjusted to allow for this in the meantime.
So, if you're setting up your odometer with new tires, try to get it about 1% low, and over the life of the tires, the true reading will move through dead accurate to 1 or 2% high, thereby keeping you as accurate as possible for as long as possible.
When measuring tires, even those listed with the metric system sizes, the best way to get off to an accurate start is to measure around the center of a tire with string to get your circumference right.
Many years ago, I had an '86 K-car, my one and only front wheel drive. In the Plymouth shop manuals I got for it, it showed speedometer pinions having only one selection criteria: tire size. Since the drive pinion was reading off of one of the half shafts, axle ratio does not matter. Just a note there for all the front wheel drive people.

Now let's talk about getting the speedometer right in relation to the odometer. Since the odometer is direct drive from the speedo. cable, and the speedo. is magnetically operated off the odometer input, if its tension spring is not right, the speedo. won't be, either.
So here's the test of the speedometer, only AFTER you have the odometer tuned.
Get on a nice straight level road, and put the speedo. on 60. No higher, no lower. You've got to average 60 for a mile (or 2 or 3, etc.). Time with a stopwatch or something how long it takes the odometer to go however many miles you picked. A mile per minute.
Most speedometers I've checked like this were pretty close, but a few were far off (more than 5 seconds error in a mile). You can adjust the speedo. mainspring by bending it, but I never had occasion to try this myself. Seems like a touchy operation to me. Hard to reach if nothing else.
It might be a good idea (at least in my case) to have someone ride with you and work the steering wheel while you're driving and watching the speedo. to keep it on 60.
Think speedometer calibration isn't important? The next time you're going what you think is 60, and the cops' radar shows 75, try explaining to the judge how you didn't think putting on those L60-15 tires would matter because you went from a 4.56 to a 4.10 gear and so forth and so on. GUILTY!!! THREE HUNDRED DOLLARS, PLEASE!!!
Well, if anyone has any questions resulting from this incredibly long babbling session of mine, I'll be happy to answer them. Time to get down off my soap box now (for a while, anyway).
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  #2  
Old 12-30-1999, 07:44 PM
Tim_K Tim_K is offline
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Angry

Unbelievable. 3 days and not a single response to the article I put so much time and effort into. I'm very disappointed in all of you. Maybe I'll start keeping my hard earned Mopar knowledge to myself and let you figure it out on your own like I had to.
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  #3  
Old 12-30-1999, 09:07 PM
sanders440 sanders440 is offline
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Tim, that was a very imformative piece that you wrote.Being Canadian and having everything metric makes that conversion even more time consuming ,(lol), but I'm sure that I'm not alone in thanking you for the time spent in doing your topic! Every bit of information that I can pick up in this forum is invaluable to me, since I seem to be surrounded by Chevy, Ford and assorted rice burning cookie cutter money pits, that they call cars! Keep up the good work!
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  #4  
Old 12-30-1999, 11:31 PM
bruce bruce is offline
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TIM; YOU DID A LOT OF WORK CALIBRATING. TO COMMENT A LITTLE I HAVE A CHART WITH WHAT GEAR IN THE TRANNY YOU NEED FOR A SPECIFIC SIZE TIRE VS. THE REAR END RATIO. I HAVE BOUGHT AND CHANGED SEVERAL OF THESE GEARS OVER THE YEARS CAUSE I SWAPPED REAR PIGS. IF I AM UNSURE OF MY SPEED I HAVE MY WIFES CAR AND MY SONS CAR DRIVE ALONG SIDE AND GET A GOOD IDEA IF MY GEAR SWAP IS IN THE BALL PARK. THANKS FOR THE ARTICLE AGAIN. BRUCE
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  #5  
Old 12-30-1999, 11:49 PM
MoparNet MoparNet is offline
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Tim -

WOW! We were even overwhelmed!!

Keep up the great posts!!!!!
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  #6  
Old 12-31-1999, 03:06 AM
DodgeMan DodgeMan is offline
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Thanks Tim
I have personally used the Ohio State Highway Patrol Timing Association for speedometer calibration. They are very accurate and even give you a official paper with your calibration number on it, and they do it for a small fee. I believe it was $85 dollars the last time I used them. It was all for nothing though. My speedometer was accurate. It read 70 also.
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  #7  
Old 12-31-1999, 07:33 AM
speedy speedy is offline
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TIM-K once again good job, up hear in mich. over the last 2 years the cop's have ben using a big raidar box with wheel's on it.They put it near construction zones ,side streets ,high ways well anyway as you approch it it has large red numbers so you can see how fast your going.SO after i changed the gears i called the cop's and told them people were going to fast up and down my street every day so they put one of these things at the end of my street.I saw alot of hot rods that day checking their speedo's,
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  #8  
Old 12-31-1999, 09:04 AM
Larry S. Larry S. is offline
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Hey Tim, your calibration post is great! Did it occur to you that one of the reasons you didn't get any answers to it was because it was so dead on, no one could find any errors in it to nit pick about? Great work!
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  #9  
Old 12-31-1999, 04:27 PM
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The Dartman The Dartman is offline
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Tim - your above calibration was using my numbers (4.30 gears 27.7 inch tires) so I feel like I have to explain where I've been since you posted. I am a computer programmer and hardware technician, and as everybody knows the some of the problems associated with Y2K. I've been working long hours, and doing a ton of driving to ensure all of my companies sites are Y2K compliant. I read your post just a day or two ago and I have not been able to post responses until today.

Remember, this post and information is here almost indefinatly! If anyone EVER asks this question again, we can point them to your article!! You'll live in infamy!!

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  #10  
Old 12-31-1999, 06:31 PM
dbdartman dbdartman is offline
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A lot of local PD's here in NJ are using the trailer mounted speed display also. It's great for seeing how far off your speedo is! My 77 Volare (the HEAP) is dead on @ 50 MPH, reads too high below that & too low above it! Don't matter to me, *EVERYBODY* drives too fast here, except the "blue hair" crowd! "Old men in hats" are the *WORST*!!!

Oh, & TimK, that was a *GREAT* article, thanx!

Was that worth 2¢?
<font color = 990033>DB</font>

------------------
You're only young once, but you can be immature forever!

[This message has been edited by dbdartman (edited December 31, 1999).
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  #11  
Old 12-31-1999, 09:34 PM
340king 340king is offline
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Great job on the article Tim! I can tell you that the reason for the mile posts being off is because of the method most states use to set them, not to mention false posts set at most structures, bridges, box culverts, etc. These additional mile posts can be confusing.
Most states use some form of measuring device that uses a calibrated driveshaft counter to determine distance travelled. These measurements require some diligence, but are usually carried out by some summer intern or temporary help.

Hey Sanders, I thought metric conversion was easy. Just double it and add 30, right! At least thats what Doug and Bob McKenzy said.

About the speedo calibration and explaining to the local law enforcement about speeding, heres my story. I recently went to the Twin Cities to see a good friend of mine and take some time off. As I was blasting down a back road state highway, I chanced accross a Highway Patrol Officer. My speedo, the dash kind, said 75, radar 82, speed limit 65. Now, I know what your thinking. 82-65=$$$$. In this case a little local knowledge helped.

When the officer asked where I was going I told him. Then he asked if this road was a shortcut. I told him no, not really, its just the route I use when I go to guard drill. He lectured me on how speed causes fatailities and sent me on my way. I think I may need to change the speedo drive gear now. I just installed new tires on my Grand Cherokee.
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  #12  
Old 01-04-2000, 02:11 AM
Tim_K Tim_K is offline
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Biggrin

Thank you to all for the many positive comments on my calibration article. It makes me happy to know that I've contributed positively to the quality of the Mopar information posted on this site.

To add to what speedy was saying about the mobile radar displays, we get them around here once in a while, but not often, and then only on the major 4 lane highways. I saw one last Labor Day weekend. I went home, got out my '79 Aspen R/T 360-4, and proceeded to do some speed runs, since the factory speedometer only reads to 85. Only hard part was parking on the side of the road about a mile before the radar site and waiting for enough cars to get out of my way and not mess up my radar numbers.
Thanks again!
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  #13  
Old 01-06-2000, 05:26 AM
Questions Questions is offline
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Hey Tim -

Wasn't hard to find you on this board, just had to look for the gigabytes of calcualations and technical data!

It sounds like you didn't hook up w/ that guy with the cop speedo. I thought you had because he quit e mailing me.

Good article, though, a bit more detailed than Mopar Action.

I'm going to read your other post.

Ken
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  #14  
Old 02-18-2012, 10:00 AM
uwss uwss is offline
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anyone know if the 150 speedometer can be repair DIY or send it off. $150 bucks plus. Mine started jumping around and sits on zero after installing a year one cable.
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  #15  
Old 02-24-2012, 06:34 AM
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cageman cageman is offline
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Awesome, you dug up a 13 year old post to ask a question!
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Old 02-24-2012, 07:55 AM
340_GTS 340_GTS is offline
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All I can say is: why didn't this guy Tim just get a Mopar shop manual, or a Year One catalog, look up the tire size and gear ratio, and go to Mopar and get the right speedo gear? It will be close enough. That's how they were built when new!

uwss: Check to make sure the cable is installed correctly in the tranny. The housing that mounts the speedo gear and cable can be installed in several different orientations depending on the size of the gear. The gear may not be making full contact with the gear in the tranny. If that all checks out O.K., I think I'd send my speedo off to a reliabe professional for repair.
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Old 02-25-2012, 12:51 AM
cudabob496 cudabob496 is offline
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glad mine is broken. That allows me to say the officer, I had no idea how fast I was going!
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Old 02-25-2012, 11:41 AM
old woolie old woolie is offline
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Quote:
Originally Posted by cudabob496 View Post
glad mine is broken. That allows me to say the officer, I had no idea how fast I was going!
Nah, that don't work,,,,,,,you get written up for faulty equipment as well then.
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Old 02-25-2012, 08:52 PM
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cageman cageman is offline
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Quote:
Originally Posted by cudabob496 View Post
glad mine is broken. That allows me to say the officer, I had no idea how fast I was going!
This coming from the guy who has 57 guages in his car, lol
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  #20  
Old 02-26-2012, 06:27 AM
cudabob496 cudabob496 is offline
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"Honestly officer, my speedometer has been broken for years, and I really don't know how fast I was going"

"Well, you're lucky this time, because I did not catch you on radar! How about you stay away from this area from now on"

"No problem sir! You won't see me out here again!"

[Thats the last time I give my son a ride! "Come on Dad, show me what it'll do!"]
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Old 02-26-2012, 09:14 AM
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Quote:
Originally Posted by cageman View Post
Awesome, you dug up a 13 year old post to ask a question!
He is new and perhaps didn't know how to start a new thread. I normally catch those, start a new thread and lock the old one, but this one slipped by.

BTW, the OP has not posted here since July of 2005.
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  #22  
Old 02-27-2012, 08:54 AM
cudabob496 cudabob496 is offline
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Quote:
Originally Posted by cageman View Post
This coming from the guy who has 57 guages in his car, lol
too lazy to tear into the dash! There are some pretty cool after-market speedometers at Summit Racing. And you can mount them pretty much anywhere you want. However, I'll be looking at the tach needle a lot more than the speedo!
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  #23  
Old 02-29-2012, 09:20 PM
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On my trucks that the stock speedo is inop, I just use my gps speedo on my phone, it will put my speed up onto the windshield with its heads up display.
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  #24  
Old 03-12-2012, 07:27 PM
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ehostler ehostler is offline
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There is a simple formula in the performance talk archives that will tell you what pinion gear you need based on tire size and rear gear ratio. I have a spreadsheet on my other computer. I just plug in the numbers and it tells me the PN of the required speedo pinion.

That solves the odometer. For the speedo check, I just use a GPS speedometer app on my phone.
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