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Cobra Nutcase
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1,633 Posts
Discussion Starter · #1 ·
I am running the stock T5, 3:55s, and 275/40/17s, what speedo gear do I need? I was told I needed the white 23 tooth speedo gear, so that's what I bought. At 70mph my speedo says I'm doing 80...obviously this is wrong. Now someone is telling me I need the red 21 tooth or the black 20 tooth gear. Anyone know for sure???

Thanks,
Rob
 

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Premium Member
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9,698 Posts
need to know what drive gear you have in the trans. Post the year of your transmission, and based on the year, a determination can be made as to what gear you have. It will be either a 7 or 8 tooth drive. The 23 tooth gear you bought is not a Ford Speedo gear, it's originally a Jeep part, and probably won't last long anyway.
here is the T5 calibration utility from Hanlon Motorsports, Plug in your data, and it will tell you what gears you need. The tire diamenter method is the most accurate, and you can get that info at the Tire Rack web site. John
http://www.hanlonmotorsports.com/t5speedo.asp
 

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section 8
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5,136 Posts
Your ford racing catalog has the calculations needed to get correct readings . You have all the info except the tire revolutions per mile , which can be had at the tire rack website. Page 95 in the 2000 ford racing catalog drive gear teeth
(7)most T5 x axle ratio 3.55 x tire revs per mile divided by 1000.should look like this
7x3.55x815(guess)= 202.5275 divided by 1000 =20.25 teeth the closest is 21. Now substitute the revolutions per mile for your tire size and no more guessing. I just did this I needed an 18.5 tooth gear needless to say they dont make fractional gears .I installed and 18 tooth gear and checked it with the local "Radar cart" that the police have it was off by 2mph at 45. good luck Bob
 

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Cobra Nutcase
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1,633 Posts
Discussion Starter · #6 ·
The trans came out of a 1990 Mustang GT...is that a 7 or an 8? Thanks for all the info, Rob
 

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section 8
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5,136 Posts
just looked 90-95 8 tooth drive gear . . Your welcome don't need any tickets due to innaccurate speedo if you going 180 you want to know !!! bob
 

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Junior Charter Member
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I have the same setup, WC T-5, 3:55, and 275x40R17s. I initially used the white 23 tooth gear, which indicated 10% slow. I was indicating 55 when I was doing 50. When I had to drop my trannie last winter, I took out the 23 tooth gear, which was totally stripped, and replaced it with the black 20 tooth one. I havented checked it out with a radar check yet, but it feels OK, if maybe a little on the fast side. Pete
 
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