Final Drive Ratio

[fgordon]

I hate those kind of threads where the answer can be calculated but. In this case I calculated the Ratio and I'm not too comfortable with my answer considering the cost involved. I have a 3:42 original rear in the car but I've added 275/65 15 Drag Radials (28" tall). I have a Ford 9" housing with disc brakes that I am removing from my Malibu. It has a Detroit Locker in it so I will be replacing the entire chunk.
I calculate that I need a 3:70 gear to make the speedometer read correct (actually 3:68.8). Does anyone have a better number?

 

[bison]

So what's your question?


BPE2013@hotmail.com

 

[fgordon]

So what's your question?


BPE2013@hotmail.com

The question is does anyone have a better number for the final drive with the changes I am making to the tire size. Just want the speedo to read correctly without changing the speedo gear.

 

[Nascar83Fan]

What size tires did you replace the drag radials with and how tall were those tires the drag radials replaced?


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[pacecarta]

changing the speedometer gear isnt too difficult , you only need to change the driven gear on the end of the speedometer bullet and thats just a 10mm bolt and pull it out
if you can change a rear or contemplating changing the gear in the rear but cant change a speedo gear i cant help you
what trans are you talking about
closest gear choice on the 9" to the 3.42 is 3.45 but with the 28 tall tire (which are 275/60-15) would require lower gear around 3.25 to keep speedo closer
like i said just change the speedo driven gear

 

[pacecarta]

and the speedo would be off just over 7.5 percent with a 28 tire (88in per rev ) vs a 26 in tire (82" per rev) , thats a whopping 3 mph slower at 50 provided the speedo was accurate to begin with

change the driven gear in trans to match your rear gear /tire combo not the other way around

 

[bison]

The question is does anyone have a better number for the final drive with the changes I am making to the tire size. Just want the speedo to read correctly without changing the speedo gear.

Whatever % change you make in tire height is the same as changing the axle ratio that same %. 10% taller tire then 10% steeper gears are needed to not have a change. You'd always need a comparison point. Like you went from 27" diameter to 28" diameter. Or 3.50 to 3.70. I've almost always had to change the driven and or drive gears to achieve speedometer accuracy. You could also break it down to tire revs per mile and aim for the same number of revs per mile less or more the % change to the axle ratio. Pi times diameter to to get circumference.


BPE2013@hotmail.com

 

[pacecarta]

there are speedo gear calcs at tci
http://www.tciauto.com/tc/speedometer-gear-calculator

 

[fgordon]

Whatever % change you make in tire height is the same as changing the axle ratio that same %. 10% taller tire then 10% steeper gears are needed to not have a change. You'd always need a comparison point. Like you went from 27" diameter to 28" diameter. Or 3.50 to 3.70. I've almost always had to change the driven and or drive gears to achieve speedometer accuracy. You could also break it down to tire revs per mile and aim for the same number of revs per mile less or more the % change to the axle ratio. Pi times diameter to to get circumference.


BPE2013@hotmail.com

Thanks, using the percentage method going from a 26" to 28" gives me a 3:68.3 ratio. Looks like the 3:70 will get me close enough. I've changed speedo gears in 3 other applications and only got a good speedo reading on the first try once. I'll stick with changing the final drive since I'm replacing the chunk anyway.

 

[bison]

The tire height is usually an issue. Unless it's measured it's probably not accurate. Also many speedometers are wrong even with the correct gears in the trans.


BPE2013@hotmail.com

 

[regan j]

Just me being stupid, If i are trying to avoid a ticket, keep up with traffic. Like Bison said there are many variables and it may not be exactly correct on paper as it is in the real world.

 

[fgordon]

Avoiding a ticket is always good. I just want to keep the indicated close the actual.

 

[bison]

Avoiding a ticket is always good. I just want to keep the indicated close the actual.

Id use a gps to see what actual is


BPE2013@hotmail.com