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Making a 165 mph speedo accurate

 
Old 03-13-2019, 06:34 AM
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brassplyer
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Default Making a 165 mph speedo accurate

There's a 165mph speedo in my recently acquired '80. It works but it's way off. For example gauging with a stopwatch and mile markers at 75 on the speedo I'm at just about 55. What do you think most likely has to be done to make it accurate?
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Old 03-13-2019, 07:11 AM
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bashcraft
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You'll need to replace the speedometer gear in the transmission with one that has more teeth.
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Old 03-13-2019, 07:40 AM
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Check odometer against mile markers also. There are 2 things in a speedo. A spinning disc that is calibrated to the speed of the driveshaft. And the needle is driven by a magnet up against the spinning disc. So, most likely needs gear changed. But if your odometer matches road,the speeds itself is off. But usually magnet gets weak and speedo reads low, not high.
What rpm are you turning at that speed and how tall are the tires? And did seller mention Nything like different rear gear ratio?
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Old 03-13-2019, 09:29 AM
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Originally Posted by brassplyer View Post
There's a 165mph speedo in my recently acquired '80. It works but it's way off. For example gauging with a stopwatch and mile markers at 75 on the speedo I'm at just about 55. What do you think most likely has to be done to make it accurate?
You will need to remove the speedo gear going into your transmission and count the number of teeth on the plastic gear.
Then you will have to find a gear that has about 26 percent more teeth on the gear.
Should be close
Also check your speeds with a gps… a little more accurate then a stopwatch
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Old 03-13-2019, 09:34 AM
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red topless gator
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I used the speedometer app on my cell phone to accurately gage the speed. It took a couple different try's with different teeth on the transmission gear but finally got it perfect.
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Old 03-13-2019, 09:35 AM
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Down load a speedometer app for your phone. What kind of tranny
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Old 03-13-2019, 01:29 PM
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Grumpy 427
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I had a stock 85 mph speedo from my 80 Vette that i sent to redline to make me a 150 mph speedo. I have 3.55 gears, 255/60/15 rubber and a Grey 22 tooth speedo gear, mine is bang on the money.

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Old 03-13-2019, 01:30 PM
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Will need to know the tire size plus rear gear to correct this. or at least get it closer.
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Old 03-13-2019, 01:38 PM
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wadenelson
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Ha! I seemed to always be speeding in my 280ZX until the day I figured out the speedo was showing 79 while doing 67.

Cops must have hated when I drove 49 mph whenever they got directly behind me in a 55 zone.

Likewise, I could never figure out how my K1500 truck got 5% better gas mileage than anyone else on Fuelly.com. Even with doing
the speed limit, keeping my tires fully inflated, fresh tune-up.

Undersized tires.

I've got a radar detector with GPS. It shows my actual speed.

Last edited by wadenelson; 03-13-2019 at 01:41 PM.
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Old 03-14-2019, 12:03 AM
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Gale Banks 80'
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Back when these Spedo's were sold threw most of the major catalogs they came with an in line reduction gear. The reduction gear meant one more spedo cable and two more things to go wrong. When I went to the Nevada 100 open road race I took my car to a local Spedo Shop and had them dial it in.( this is back when there were spedo shops ) They quickly told me I had the choice of having it accurate at 140 mph ( My Dial in for the Race ) or at the 55 national speed limit but not both as the GM spedo wasn't capable of doing both. I went for the 140. Keep in mind that there was no GPS that was small enough take in a car at the time. I am no longer running the reduction gear I am able to get it in the ball park using the spedo gears in the trans. Oddly enough when I take my car to a car show of all the things to look at the most common thing Kids notice is 240mph spedo !
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Old 03-15-2019, 09:10 PM
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Originally Posted by Gale Banks 80' View Post
Back when these Spedo's were sold threw most of the major catalogs they came with an in line reduction gear. The reduction gear meant one more spedo cable and two more things to go wrong. When I went to the Nevada 100 open road race I took my car to a local Spedo Shop and had them dial it in.( this is back when there were spedo shops ) They quickly told me I had the choice of having it accurate at 140 mph ( My Dial in for the Race ) or at the 55 national speed limit but not both as the GM spedo wasn't capable of doing both. I went for the 140. Keep in mind that there was no GPS that was small enough take in a car at the time. I am no longer running the reduction gear I am able to get it in the ball park using the spedo gears in the trans. Oddly enough when I take my car to a car show of all the things to look at the most common thing Kids notice is 240mph spedo !
Got THAT right for sure.....my GPS said everything was fine till 80 which was really 78 100 was really 94mph.....ran outta room test over......tach, I put a digital audio oscillator with amp on the electronic tach input.....dead on till ~2300 rpm or so.....then it over indicated by 600 rpm by 4k, and worse yet at 6k......
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Old 03-19-2019, 11:35 PM
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Guys.. I've been building speedometers for over 40 years.... In most cases you can dial in the change on the head.... or you can go to the trans to change the gear. This depends on the issue.

The issue is that I've never had any problem re-calibrating a speedo with a new face when switching from the standard speedo on the 1980-1982 cars and I suspect the PO may have just changed the face without the calibration... There is no way to be sure unless you have the speedo in hand.

The change in calibration has to be linear to the speedometer when this is done and sometimes it's tricky and you are splitting hairs when you do this because of the variations allowed by GM/AC Delco... The old speedo's were not a masterful piece of design... and you can put a charge in the head, adjust the tension spring and be way off... or you can get lucky and nail it... you just have to find a common ground within the allowed variance. .

I use the original GM Borg calibration equipment and then you must combine it with the fact that any speedometer... according to the original AC Delco calibration chart a good calibration can be off 3-5 mph +/- at any speed. The speedo's from 1953-1982 were no master piece of quality that's for sure.

So... depending on what the gauge is doing...

If it's off at "about" the same amount at any given point then I'd probably change the speedo gear in the tranny.... 1 tooth is about 5 mph.... If it's erratic and close at some speeds, then gets way off.... I'd have the actual head calibrated.

IMHO,

Willcox

Last edited by Willcox Corvette; 03-20-2019 at 09:44 AM.
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Old 03-21-2019, 10:02 AM
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Bills72n84
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ok Willcox my 72 is ten mph off ,it shows that when im really going 60mph ,it shows im going 70mph ,so do I need a tranny gear with two more teeth ,and do you sell them ,thanks Bill oh I have a automatic.

Last edited by Bills72n84; 03-21-2019 at 10:06 AM.
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Old 03-21-2019, 10:44 AM
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Grumpy 427
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Originally Posted by Bills72n84 View Post
ok Willcox my 72 is ten mph off ,it shows that when im really going 60mph ,it shows im going 70mph ,so do I need a tranny gear with two more teeth ,and do you sell them ,thanks Bill oh I have a automatic.
What gear ratio do you have, and what tire size do you have. From there you determine the color and number of teeth you need for a speedo gear.
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Old 03-21-2019, 11:29 AM
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7T1vette
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If you want a 165 mph speedometer to be accurate throughout its range, you will need to install a fully electronic speedometer head and drive it with an electronic speed signal. That system will still need to be 'adjusted' to match your rear axle ratio and rear tire diameter.

Last edited by 7T1vette; 03-21-2019 at 11:30 AM.
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Old 03-21-2019, 11:40 AM
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Grumpy 427
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Originally Posted by Bills72n84 View Post
grumpy ,im pretty sure that I have 273 rear end and 65 by 15 inch tires!
need the exact aspect. i have a 255/60/15 on my car 20 that tire is 27" tall. so look when you can to see what the exact tire size is please.
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Old 03-21-2019, 12:22 PM
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Originally Posted by Grumpy 427 View Post
need the exact aspect. i have a 255/60/15 on my car 20 that tire is 27" tall. so look when you can to see what the exact tire size is please.
grumpy427 here is my tire size (P215/65R15 )
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Old 03-21-2019, 01:32 PM
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Ok so with that tire, you are at 26" tall, with a 2.73 6ear, tach should be at 2174 rpm @ 60 mph and 2536 rpm @ 70 mph.

You will need to pull the speedo cable off the trans, remove the gear, catch the oil that will run out, and with a flashlite, look and see what colour drive gear is on the tail shaft inside the transmission.

teeth Colour

7 pink
8 black or orange
9 green
10 purple.

Once you know that dive gear you can get the correct tooth driven gear. That is assuming you have a Th 350 trans in the car.

Last edited by Grumpy 427; 03-21-2019 at 01:41 PM.
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Old 03-21-2019, 03:16 PM
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Just for you AT guys...1968-1982... I uploaded and fixed the chart from GM

You can view this at the link below.

http://repairs.willcoxcorvette.com/w...securedpdf.pdf
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Old 03-21-2019, 08:05 PM
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SWEET!!
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