Topic: correct speedometer driven gear

[MattDoscher]

Good afternoon,

My '65 w/ 289, C4, 2.80 rear, and 205/70R14 tires has a incorrect speedometer driven gear installed.  I am currently using an 18 tooth gear and my speedometer is off by 5MPH.  So when I'm driving and my speedometer reads 65 I'm actually doing 60 and 40 is actually 35. Would a 19 tooth gear fix the problem?  I'm not exactly sure how the number of teeth affect speedometer readings whether more teeth slow down the speedometer or what. 

My NPD catalog has a formula on pg 154 which they used from another catalog which states: number of drive gear teeth X axle ratio X tire revolutions per mile then divide your sum by 1000.  The result will be an approximate number of driven gear teeth to provide an accurate speedometer reading.

I'm just wondering if anyone else has run into this problem before and what driven gear they used to provide an accurate speedometer reading based on my equipment above.

Matt

[midlife]

Your speed offset should be linear with increasing speed.  Having 5 MPH difference at both 60 and 35 mph means one of these readings is wrong.

[MattDoscher]

I don't know.  Maybe it's not as noticeable a slow speeds like 35 and below.  But I know driving next to another vehicle known to be traveling at 60MPH I was looking at 65MPH on my speedometer.  Not sure what you mean by linear as speed increases?  Wouldn't having the wrong driven gear affect the speed accuracy at all speeds?

[carlite65]

you can use a gps unit to check your speed.

[midlife]

I don't know.  Maybe it's not as noticeable a slow speeds like 35 and below.  But I know driving next to another vehicle known to be traveling at 60MPH I was looking at 65MPH on my speedometer.  Not sure what you mean by linear as speed increases?  Wouldn't having the wrong driven gear affect the speed accuracy at all speeds?

The difference in speed will be linearly increasing as the speed increases.  You can't have a mechanical system with a constant speed offset.

[MattDoscher]

Carlite65, the GPS recommendation is a good idea I didn't think of that.  I'm thinking of trying a 17 tooth gear as decreasing the number of teeth on the gear would slow down my speedometer reading which is what I need it to do since it is reading 5MPH fast.  I was thinking about it last night and it would make sense that adding more teeth would increase the turning speed of the gear which would increase the speedometer needle.  Decreasing the number of teeth should slow down the gear which would in turn slow down the speedometer needle.

[Pete Bush]

Perhaps my Speed-O-Gear research may help.

http://mustangcodex.blogspot.com/2010/10/8th-box-from-left-has-code-for-speed-o.html

Based upon the formulas and original tire diameters, it looks like the original gear would have been a 16 tooth. The tires you're currently running 205/70R14 are probably the closest modern tire diameter (although they are about 1" wider).

For what it's worth...

You might want to note that the number of teeth on the speed-o-gear is a whole number and only approximates the drive solution. This means that the speedometer calibration was probably slightly off - even from the factory. This seems to be the case with reported track tests of then new cars with a 2-4% error.

[MattDoscher]

Thanks Pete.