Speedo Gears

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70 DDDart

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Langley, BC, Canada sometimes Ca USA
I am running 255 x 60-15" BFG's on rear with 391 SG gears with A 833 4 speed. My FSM chart does not list P- metric sizing because it is for 1971 cars. There is not enough reliable info on the internet to convert the tire size. It looks like a 39 tooth gear should be close to correct my speedometer so does anyone have real experience so I can dial this in. Classic lists all gears but at $60 a pop I like to get this right the first time?

Also the FSM lists two A-833 trannie's a "Standard" and a "Heavy Duty", slight difference in 1 st and reverse ratio's and also lube capacity. How do you tell the difference without a dismantle? My trans has a 1" x 23 spline input shaft, if that helps.
 
70 DDDart said:
I am running 255 x 60-15" BFG's on rear with 391 SG gears with A 833 4 speed. My FSM chart does not list P- metric sizing because it is for 1971 cars. There is not enough reliable info on the internet to convert the tire size. It looks like a 39 tooth gear should be close to correct my speedometer so does anyone have real experience so I can dial this in. Classic lists all gears but at $60 a pop I like to get this right the first time?

Also the FSM lists two A-833 trannie's a "Standard" and a "Heavy Duty", slight difference in 1 st and reverse ratio's and also lube capacity. How do you tell the difference without a dismantle? My trans has a 1" x 23 spline input shaft, if that helps.
Click to expand...
BFG lists your tire as having an overall diameter of 27.1", so using this chart: Mopar Speedo Gear Chart : Drivin' It Home you need a 38 tooth gear, which translates to 2538938(Blue).
The FSM covers Darts, Coronets and Chargers, so the "standard" 833 is the 23 spline, and the "Heavy Duty" is the 18 spline Hemi/6bbl box. The capacity difference is due to the A body vs. B body boxes, which have different length tailshaft housings.
 
255 60-15's are 27" tall. I think the standard transmissions came in pretty much everything except the 13 spline tht was used in the Hemi's and 440-6 cars.
70 DDDart said:
I am running 255 x 60-15" BFG's on rear with 391 SG gears with A 833 4 speed. My FSM chart does not list P- metric sizing because it is for 1971 cars. There is not enough reliable info on the internet to convert the tire size. It looks like a 39 tooth gear should be close to correct my speedometer so does anyone have real experience so I can dial this in. Classic lists all gears but at $60 a pop I like to get this right the first time?

Also the FSM lists two A-833 trannie's a "Standard" and a "Heavy Duty", slight difference in 1 st and reverse ratio's and also lube capacity. How do you tell the difference without a dismantle? My trans has a 1" x 23 spline input shaft, if that helps.
Click to expand...
 
Looks like you need a 38 tooth.

speedo gear.jpg
 
Just for use in the future, with your tire inflated to the proper PSI you can measure the diameter yourself. ;-)
 
70 DDDart said:
I am running 255 x 60-15" BFG's on rear with 391 SG gears with A 833 4 speed. My FSM chart does not list P- metric sizing because it is for 1971 cars. There is not enough reliable info on the internet to convert the tire size. It looks like a 39 tooth gear should be close to correct my speedometer so does anyone have real experience so I can dial this in. Classic lists all gears but at $60 a pop I like to get this right the first time?

Also the FSM lists two A-833 trannie's a "Standard" and a "Heavy Duty", slight difference in 1 st and reverse ratio's and also lube capacity. How do you tell the difference without a dismantle? My trans has a 1" x 23 spline input shaft, if that helps.
Click to expand...

Thx all 3 of you, I will be looking now to try and get a better price for a 38 tooth gear than from Classic !!!!
 
You might want to add a GPS speedometer to your smart phone and get some good numbers driving down on the highway (at different speeds) to determine how your GPS speed reads now (compared to the dash speedometer) to get an accurate baseline on how far off you are and go from there.
 
dibbons said:
You might want to add a GPS speedometer to your smart phone and get some good numbers driving down on the highway (at different speeds) to determine how your GPS speed reads now (compared to the dash speedometer) to get an accurate baseline on how far off you are and go from there.
Click to expand...
Great idea. GPS is usually accurate to within 1 mph or less. Try Waze.
 
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