speedo gears

[John Cailey]

My 74 Duster with a 904 in it - the speedo runs 4 mph slow. - actual 50mph reads 46 on the speedo. I obviously need a speedo gear change. Does the 904 and 727 take the same speedo gears? What is the approx. speed reading change for a one tooth change? Assume I have to go to a speedo gear with at least one tooth less than existing? Thanks, John

 

[Dodge73Dart]

My 74 Duster with a 904 in it - the speedo runs 4 mph slow. - actual 50mph reads 46 on the speedo. I obviously need a speedo gear change. Does the 904 and 727 take the same speedo gears? What is the approx. speed reading change for a one tooth change? Assume I have to go to a speedo gear with at least one tooth less than existing? Thanks, John

Give these guys a call. You will need your tire size height and rear ratio. They can get your gear
speedometer cable-custom built speedometer cables

 

[Alaskan_TA]

Check the chart below & 904, 727, A833 & A230 speedometer pinion gears swap over, but I am sure it may depend on the year of the transmission. (I have a 1976 727 here that takes a different type)

 

[69_340_GTS]

50 and reading 46 , so the speedometer is reading about 8% low. You just need a gear 8% smaller to speed up the speedometer. So take whatever gear you have and reduce it by 8%... 36 tooth ÷ 1.08 = 33 tooth, 34 tooth ÷ 1.08 = 31 tooth, etc...
There is nothing wrong with the charts either. But if you really know you are reading 8% low my way will work too.

 

[Alaskan_TA]

I checked my newest ride with a GPS. Spot on up to 70, then the speedometer reads about 0.5 slow.

 

[John Cailey]

My 74 Duster with a 904 in it - the speedo runs 4 mph slow. - actual 50mph reads 46 on the speedo. I obviously need a speedo gear change. Does the 904 and 727 take the same speedo gears? What is the approx. speed reading change for a one tooth change? Assume I have to go to a speedo gear with at least one tooth less than existing? Thanks, John

Thanks all for the info. Tire dia. = 25.5. Diff = 3.2:1.. Existing speedo gear = 34 tooth. Per the chart, looks like I need a 33 tooth gear.