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Discussion Starter · #1 ·
As the title says, I got to wondering- my '97 Mustang came with 245-45-16 size tires, instead of the 50 or 55 series tires it was supposed to have. The speedo is off by about 10 mph...at 70, it reads about 80! With so many tire sizes out there for Cobras,, how do I get a Stewart Warner speedo (the brand I plan to go with on the build) or for that matter, any speedo calibrated? Also, if I use a used donor speedo- does the state of Ct get picky about indicated milage, or does that matter since I could simply buy a new SW one?I know that there are drive and driven speedo gears, but how to find out which combo??????
 

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FFCobra Fanatic
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510 Posts
XL:

There are two tools that I use. One is the FMS catalog. It has a couple of tables in there that describes which speedo gear combinations work with each axle ratio assuming stock Mustang tire size.

The other is the tire size calculator at http://www.miata.net/garage/tirecalc.html

Based on this information, I can get into the ballpark. If it is still off a little (test drive), I can fine tune it by changing the driven gear to one that is one step up or down.
 

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Senior Charter Member
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616 Posts
xl,
best bet is to go with aftermarket gauges, some are very easy to calibrate and work better than the SW ones.

p.s. how are those pics coming, i cant wait to see them!!
 

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XL,

After rereading your post I'm not sure I would change anything if I was you
. Is it a bad thing for the speedo to read about 10 mph high at 70? It would probabaly keep me out of some trouble, since these cars way too much attention to begin with ;) !

Mike
 

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After reading this it got me to thinking.

I had just assumed I used the same Driven Gear for my smith speedo as the Mustang.

Is this true?

Thanks,
TAZ
 

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FFCobra Fanatic
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My Autometer reads about 5-7 mph high. I learned this by passing one of the speed warning signs that show your speed from the radar gun attached. I figure it's better to be showing 70 and doing 65mph. Safer, in that I probably won't be pulled over for speeding at speeds 25-75mph.

San Rafael Mike
 
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