Author Topic: Need help recalibrating my speedometer  (Read 3895 times)

Fastorange

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Need help recalibrating my speedometer
« on: November 26, 2008, 12:21:53 PM »

  welp, here goes nothing. I have not owned my 67 camaro very long. They guy I bought it off of made lots of changes threw the years.  he was a drag racer, and raced it for a wile. then bought a vega and pretty much made the camaro a street car. the camaro now has a 388 small block chevy with an m21 4speed. the speedometer is not hooked up. the car has a 66 dodge dart rear end with a 3:55 rear gear in it.  she also got bigger tires and rims.  If i hook up my speedometer im guessing its going to be way off. where should i start to try to get a better idea of how fast im really going?    Im going to take out the m21 soon and install a t350 i would like to race the car everyone in a wlie so im going to keep the rear thats in it now. but yet i still wana take it out on dates and drive to work every once and a wile.   thanks for all the input.

JohnZ

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Re: Need help recalibrating my speedometer
« Reply #1 on: November 26, 2008, 04:44:38 PM »
Remove the speedo gear "bullet" from the tailhousing and note the color and number of teeth on the gear, and write it down, then re-install it and connect the speedo cable to the bullet.

Follow someone driving a newer car at 30, 40, and 50 mph, and note your indicated speed at each of those speeds.

Post the gear color, tooth count, and your indicated speed at the true 30-40-50mph speeds and we can give you some direction; also post your rear tire size.
'69 Z/28
Fathom Green
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tom

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Re: Need help recalibrating my speedometer
« Reply #2 on: November 26, 2008, 09:44:15 PM »
Lately I have verified the calibration of my speedos with a handheld GPS. One car right on the money, One speedo about 2-3 over at 60, one speedo over by a just about exactly 10%.

The other thing I have always done with a new car - before the gps, was to drive on the highway. I would pace myself until I was actually covering a mile a minute. Note what the speedo says, no matter how far it's off, at least you know what it displays at 60.

Not a fix, or high tech, just way to know when I'm speeding.

Most older cars seem to show a couple mph over the actual speed.
69 X11 Z21 L14 glide
looking for a 69 export model (KPH) speed
o

Fastorange

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Re: Need help recalibrating my speedometer
« Reply #3 on: November 27, 2008, 09:16:47 AM »
what kinda gps are you useing? a friend of mine told me to go that route. But i would like the car to work just as it did back in 67. i know that the rear has been changed out and has a 3:55 gear in it.   does the rear gear really matter? could i put a stock size rim and tire back on the car and be fine? would the speedo be on some what.

tom

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Re: Need help recalibrating my speedometer
« Reply #4 on: November 27, 2008, 03:06:31 PM »
I have a Garmin. One of the features is a speedomoter function. It also records max speed, and eventually it even gets me where I'm going.
69 X11 Z21 L14 glide
looking for a 69 export model (KPH) speed
o

KENSTOY1969

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    • 1969 Camaro Z/28 Rally Sport
Re: Need help recalibrating my speedometer
« Reply #5 on: November 27, 2008, 04:59:56 PM »
Another way to determine with speedometer gear to use is to a little math.

Time a 10 mile drive on the highway using the mile markers as your measure doing 60 MPH consistenty.

Then use the following calculation for the "# Teeth Needed"

# Teeth                            # Teeth
on Gear Now                      Needed
-----------------------   =   ----------------------
Time in seconds                 600 seconds
for 10 miles at
60 MPH

Use the listing below to determine which color gear you need.

From the National Parts Depot Camaro Catalog
Red - 21 teeth
Blue - 20 teeth
Green - 22 teeth
Black - 23 teeth
Orange - 25 teeth
Yellow - 24 teeth
Purple - 17 teeth
Brown - 18 teeth
Neutral - 19 teeth
Silver - 22 teeth

JohnZ

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Re: Need help recalibrating my speedometer
« Reply #6 on: November 27, 2008, 06:03:19 PM »
I'm a chronograph nut, and they'll read out your actual speed right on the outer scale of the dial when you travel exactly one mile; I drive at a steady 70 indicated (doesn't matter what speed you drive at), punch the start button at a mile marker and punch the stop button at the next mile marker, and the sweep hand points directly at your actual speed. The difference between they steady-driven indicated speed and the actual speed indicated on the dial is your error; with a 4-speed or a Powerglide, a 5% error is one tooth on the driven gear.
'69 Z/28
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Fastorange

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Re: Need help recalibrating my speedometer
« Reply #7 on: November 29, 2008, 01:31:02 PM »
 Ok guys thanks for the help. The local shop here has a dyno. i think im going to take it there and run it on there to see where im at.  I guess that why the law is not going to get after me.