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Author
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Topic: Speedo gear change?
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stang106
Gearhead Posts: 734
From: God's Country!... Port Alberni B.C. Canada
Registered: Jul 2001
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posted 09-03-2002 11:47 AM
   
How do I know which one to use? My speedo is reading fast?
Thanks,
Dave
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Moneymaker
Administrator Posts: 19751
From: Lyons, IL, USA
Registered: May 99
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posted 09-03-2002 12:42 PM
There is a chart in the FRPP cataloge. I believe that it is on line at Fordmall.com
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Alex Denysenko
Co-Administrator and Moderator/ non 65-66 Mustang owner sensitivity co-ordinator NHRA/IHRA/SRA/MCA member
NHRA and IHRA SS/LA National Record Holder '00, '01, & '02
Fleet of FoMoCo products including 88 ASC McLaren Mustang #28
Part time secret agent license #0089
Professional Manwhore
The Barry of BarrysGrrl Quote: "I never met a magazine mechanic I liked."
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John Z
Gearhead Posts: 330
From: Morgantown, WV
Registered: Jul 99
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posted 09-03-2002 12:57 PM
If the car is operational. Drive at an indicated speed of 60 mph (Si) on an interstate with mile markers. Measure the time it takes to go one mile as indicated by the mile markers. Compute your actual speed as Sa = (1mi*3600)/(time in sec). Pull your speedo gear and count the number of teeth, Ne. Then the number of teeth you need is
Nn=Ne*Sa/Si. (To reduce the speed indicated you need more teeth on the speedo gear.) The mile markers along the interstate are close, but not exact. You could use the average time of several one mile runs to improve the accuracy of this method. If the speedo error is due to a rear gear change and you know both the original, Ro, and new rear, Rn, ratios, then
Nn=Ne*Rn/Re If the speedo error is due to different rear tire sizes, you can do a similar calc to the gear ratio method if you know the diameter of the old and new tires. [This message has been edited by John Z (edited 09-03-2002).]
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stang106
Gearhead Posts: 734
From: God's Country!... Port Alberni B.C. Canada
Registered: Jul 2001
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posted 09-03-2002 01:43 PM
Thanks Alex and John.
John: I live on an island in the Pacific off the coast of British Columbia. Although we do have a new "Freeway" it has stoplights and does not compare with your American Interstates. It does not have mile markers. Thanks for your informative answer, it helps a bunch.
Dave------------------
'70 ragtop Stang
351W - 13.70 @104.
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John Z
Gearhead Posts: 330
From: Morgantown, WV
Registered: Jul 99
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posted 09-03-2002 01:49 PM
quote:
Originally posted by stang106:
Thanks Alex and John.
John: I live on an island in the Pacific off the coast of British Columbia.
So that's what that BC stuff means in your signature - I should have looked closer.
If you have a car with a reasonably accurate odometer, you could lay out a measurement course. If you're measuring in kph, you could then drive at 60 kph on a non freeway road. You can actually drive at any indicated speed as long as you can work out the calc for the actual speed. 
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Jeff
Gearhead Posts: 280
From: Moore, OK USA
Registered: Mar 2001
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posted 09-05-2002 04:46 PM
quote:
Originally posted by stang106:
How do I know which one to use? My speedo is reading fast?
Thanks,
Dave
You'll need a speedo gear with more teeth. This will slow the cable rotational speed down and hence make the speedometer read lower. Your next question is how many teeth should the gear have? Folks have answered this question already. There's a discussion in the NPD catalog about calculating and selecting the right gear. You can also do it by ratio and proportion. If you know the number of teeth on the existing gear and know how far off the speedometer is at a fixed speed (60mph is normally used). If your speedo reads 70 when it should read 60, and you now have a 16 tooth gear, what you need is a 70/60 x 16 tooth gear. In this example 19 teeth.
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