axle swap vs speedometer

jos · Nov 18, 2007

Hello mustangfans,

I'm rebuilding this 1979 mustang.I changed the 6,75 axle for a 7,5 inch but the ratio went from 3,07 to a 2,47.
Is there a formula to figur out how many teeth the speedometer gear should be.i'm using a C4.If this ratio is not streetusable i will change the pin/gear but so far i have to work with this.Thank you Jos Teunissen (Netherlands)

D.Hearne · Nov 18, 2007

The tooth count on the cable gear will need to be reduced by about two. You can figure one tooth up or down in every .25 difference in the ratio. More teeth going up in ratio, less going down.

Rusty67 · Nov 18, 2007

1979 is the first year of the Fox chassis, why not put an 8.8 in ? I'm sure there are plenty of posi 8.8 rear ends in junkyards that will bolt right in. If I remember correctly I think the LX's came with 3.27 ratio gears.

This doesn't solve your spedo gear problem, its just a segustion.

D.Hearne · Nov 18, 2007

He's over in the Netherlands, I guess this stuff isn't as plentiful there as here.

jos · Nov 23, 2007

Mr Hearne,thank you for workeable info.I drove the car to the inspection.It passed after 10 years of sleep .Unfortunally the rearaxle with 2,47 isnt to drive.Have to change the 7,5 meanwhile i will put back the 6,75 with the 3,07 ratio.

D.Hearne · Nov 23, 2007

jos said:
Mr Hearne,thank you for workeable info.I drove the car to the inspection.It passed after 10 years of sleep .Unfortunally the rearaxle with 2,47 isnt to drive.Have to change the 7,5 meanwhile i will put back the 6,75 with the 3,07 ratio.

(for the first part) on the last part ?

axle swap vs speedometer

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