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speedometer gear
#1
Need help figuring out new speedo gear, I went to a 3:50 rear gear and p235/60r15 rear tires. I have an FMX trans, with cable on driver side. Thanks!!!
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#2
Hi,
Probably need to buy a set of these and try one after one? Specialist can answer better than me.
But for the story, my speedometer give me the wrong speed, I think because the tires are not the same size. Originally 205 78/14, and now 205 70/14.
Arrivederci.
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#3
Assuming your drive gear has 8 teeth, I'd start with a 22 tooth speedo gear.
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#4
This should help:
http://www.7173mustangs.com/thread-speedometer-gears

You can calculate your tire height here:
http://www.7173mustangs.com/thread-tire-sizes--11566



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#5
The width of the tires won't affect mph readings, the height does. My car came with 14"rims & now I have 15". Speedo is about 5 mph slower now.

run_horse Run Horse Run!
John 72 Q Code
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#6
Here is what I found, you would need 21.6 teeth. So a 21 or 22 one would work. This is assuming that the tranny's shaft has 8 teeth, You would need to remove the tail shaft housing and count the teeth if you don't know.

Here is a calculator that you can use,

http://www.tciauto.com/tc/speedometer-gear-calculator

-john
(jbojo)
351C 4V cc heads, 10.5 : 1 CR, 290 Herbert cam, Flat top forged pistons, forged connecting rods, Atomic efi,
C6 with Gear Vendor overdrive, 3.89 Tru Trac, Hooker Super Comp with 2 1/2" Pypes Exhaust.        

Some Mod pictures can be seen at:

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#7
Tire circumference is the issue. Tire height is used in discussions, but in the math it is circumference. If a tire is wider but the aspect ratio is the same, it will be taller as the aspect ratio is a percentage of width.

FMX's use 8 teeth, you will need a 21 tooth gear and you'll have a minor error with that. You could go to the Jeep 23 tooth gear, but they wear out quickly and will just be wrong in the opposite direction.

[Image: 1_01_07_15_8_53_18.png]

"I love my Hookers!" and "Get some Strange" probably have a different connotation to non automotive enthusiasts!
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#8
BF Goodrich lists 797 revs per mile for their tire.
Calculating based on tire diameter yielded 792 revs per mile.

Based on the revs per mile the correct one would have 22 teeth. Since I don't think that's available 21 would be the choice as Jeff mentioned.

From another calculating tool I found

The recommended gear for your vehicle has 22 Teeth!
With this gear, when your speedometer shows 60 MPH,
you will actually be going 61.01 MPH. (off by -1.68%)

The closest gear available has 21 teeth.
With this gear, when your speedometer shows 60 MPH,
you will actually be going 61.76 MPH. (off by -2.94%)

Learned a lot from this, thx

-john
(jbojo)
351C 4V cc heads, 10.5 : 1 CR, 290 Herbert cam, Flat top forged pistons, forged connecting rods, Atomic efi,
C6 with Gear Vendor overdrive, 3.89 Tru Trac, Hooker Super Comp with 2 1/2" Pypes Exhaust.        

Some Mod pictures can be seen at:

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#9
Cool, thank you for all the replies!! It never stops amazing me how helpful everybody is!! Thanks again!!!
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#10
Well, we aim to please! Smile

Doc

[Image: 6y14ea.jpg]

Project started 8-7-10
Completed: All new suspension, rebuilt 351C H Code bored .030 over with mild cam and intake, new 3.50 TracLok, custom exhaust system
Current "mini-project": interior upgrade Undecided
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