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speedometer calibration


dale

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I have a chevy 350 trns with a Explorer 3.71 ratio differential and 670-15 coker radials on my 46 Plymouth.. Getting tired of changing speedometer gears...My latest calculations are a # 7 drive gear and a 24 tooth driven gear...Had a # 8 drive gear and a 24 tooth driven gear which made the speedometer read 35 when it should have read 30...Any math whizzes or fellow experiencers out there ?  Yes, I should have kept her stock...   LOL

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That stuff makes my head hurt. If it were mine,I'd just take it to a speedo shop,and say "fix it".

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Think the solution will be solved by trial and error...When I get the #7 drive gear next week my calcs should be a 15% increase in the drive gear and 5 mph at the speedometer.  The fun in these cars is the challenge.

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6 hours ago, knuckleharley said:

That stuff makes my head hurt. If it were mine,I'd just take it to a speedo shop,and say "fix it".

I'd start with putting a GPS app on your smart phone, getting the car up to a GPS-indicated 60 MPH, and note what the speedo is displaying. THEN go to a speedo shop, give them the info, and have them fix it.  They can easily determine whether a speedometer gear would fix it or whether a gearbox for the speedo cable is needed.

 

Marty

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   Typically, a speedometer shop doesn’t need any information as to what the speedometer in a car indicates vs its actual speed, from having the car driven, or from a GPS – that’s all just a waste of time. They can put the car on rollers and run them at a pre-determined speed, and look at the speedometer themselves. The difference, ±, determines what they need to do – either a simple change of the gear in the driven head, or a small gear interface installed at either end of the speedometer cable. They can usually get the speedometer’s indicated speed to within 1-3% of the car’s actual speed, and it doesn’t get much closer than that, unless you want to spend a lot of $$$ for a certified speedometer as used in LEO’s vehicles, and that’s just overkill. Thx . . . .

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It is true that well equipped speedometer shops have rollers and can calculate actual speed compared to indicated speed for cars and make corrections accordingly. I once had a large truck that was too big to use the rollers. So the shop had a measured mile on a nearby road. They had me drive the mile with the speedometer cable removed from the back of the speedometer and a piece of tape used as a flag to count how many times the cable turned in the measured mile. One of the shop attendants road with me and did the counting so I could pay attention to where I was going. If I don't look where I am going I tend to go where I am looking.

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I suppose a shop if you want to spend the money but they by reading these post will be doing the same thing Im doing.  Ne

xt week when the drive gear arrives Ill let the board know how I did... I take the car into town where they have a speed warning device set up showing your speed...Last time the cars speedometer showed 35 when the device showed 30..

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2 hours ago, dale said:

I suppose a shop if you want to spend the money but they by reading these post will be doing the same thing Im doing.  Ne

xt week when the drive gear arrives Ill let the board know how I did... I take the car into town where they have a speed warning device set up showing your speed...Last time the cars speedometer showed 35 when the device showed 30..

Are you saying your time is worth nothing,and that there is nothing else you could be doing more important or complex than swapping speedo drive gears until you get it right?

 

Sometimes it just makes sense to turn some things over to the pros and spend your own time doing other things.

Edited by knuckleharley
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Is there anything Id rather do than work on my Plymouth ?  Cant think of any right now.    Most "professionals" are mostly professional at screwing the customer.   Im as good or better than 99% of them.

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Here is a link to a site that has the calculator which tells you which gear you need. Just fill in the blanks.

 

http://www.tciauto.com/tc/speedometer-gear-calculator/

 

You can also do a search for a ratio adapter on Google. No one makes a gear for my combination so that was the route I went.

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On ‎10‎/‎30‎/‎2018 at 7:45 PM, martybose said:

I'd start with putting a GPS app on your smart phone, getting the car up to a GPS-indicated 60 MPH, and note what the speedo is displaying. THEN go to a speedo shop, give them the info, and have them fix it.  They can easily determine whether a speedometer gear would fix it or whether a gearbox for the speedo cable is needed.

 

Marty

I went with a borrowed GPS unit to test my 38 Coupe.       My speedo was faster than the GPS at slow MPH , and the speedo was slower at higher MPH...   I believe since I lubed the speedo cable,  numbers may be more accurate...

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2 hours ago, The Way said:

Here is a link to a site that has the calculator which tells you which gear you need. Just fill in the blanks.

 

http://www.tciauto.com/tc/speedometer-gear-calculator/

 

You can also do a search for a ratio adapter on Google. No one makes a gear for my combination so that was the route I went.

 

2 hours ago, Robert Horne said:

I went with a borrowed GPS unit to test my 38 Coupe.       My speedo was faster than the GPS at slow MPH , and the speedo was slower at higher MPH...   I believe since I lubed the speedo cable,  numbers may be more accurate...

 

2 hours ago, Robert Horne said:

I went with a borrowed GPS unit to test my 38 Coupe.       My speedo was faster than the GPS at slow MPH , and the speedo was slower at higher MPH...   I believe since I lubed the speedo cable,  numbers may be more accurate...

 

2 hours ago, Robert Horne said:

I went with a borrowed GPS unit to test my 38 Coupe.       My speedo was faster than the GPS at slow MPH , and the speedo was slower at higher MPH...   I believe since I lubed the speedo cable,  numbers may be more accurate...

 

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I went the calculator route but came up 5 mph fast at 30 mph... My next drive gear change should put it right on... Doesn't anyone here work on their own cars ?

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5 hours ago, dale said:

 Doesn't anyone here work on their own cars ?

Nope! Not a single one of us. Personally,I just have a team of mechanics fly in from a CA rod shop to change my oil,check tire pressures,and other major rapairs.

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50 minutes ago, knuckleharley said:

Nope! Not a single one of us. Personally,I just have a team of mechanics fly in from a CA rod shop to change my oil,check tire pressures,and other major rapairs.

 

Wash, wax, after the interior cleaned with the other services??  ?

 

DJ

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54 minutes ago, DJ194950 said:

 

Wash, wax, after the interior cleaned with the other services??  ?

 

DJ

Of course. What do you think I am,a uncultured barbarian?

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Knuckle..........you tryin' to muscle in on my corner?............lol............after Dons going where he is looking comment and now yours I've just pee'd myself.............lol...............ah, its good not to take ourselves too seriously......no one else does.......lol..........regards from Oz.........andyd

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I don't see a beer on the table Robert  !!!    

13 hours ago, DJ194950 said:

 

Wash, wax, after the interior cleaned with the other services??  ?

 

DJ

 

15 hours ago, 52 work truck said:

I used a GPS driven speedometer from Dolphin.... Hassle free love it Stewart-Warner and Classic have them as well

Im keeping the original look with the old gauges..

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1 hour ago, martybose said:

You can get a GPS speedometer setup that drives the OEM gauge ........

 

Marty

Where can I get one of these ?

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Well for any do it yourselfer old Plymouth rodders….Took the Plymouth out this morning and going thru the speed alert  in town the speedometer was right on at 30 mph... 7 tooth red drive gear and a yellow 24 tooth driven gear......350 chevy antomatic with a 3.71 rearend and 670-15 coker narrow radial tires. 

 

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