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Guest Anonymous

speedo accuracy question,,,,,

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Guest Anonymous

My '76 has 175-70-13 tires and ever since I bought it the speedo reads about 5 mph higher than the actual speed. Usually the reverse would be true with tires that are larger than stock diameter. Any ideas why this might be the case? I'm probably lucky the error is in my favor when being surprised by radar, but wondered if perhaps my car has some part that's not the correct replacement item. Is there a speedo drive gear in the transmission that the cable connects to that may be incorrect? Some older Fords can be corrected for error when going to bigger tires by changing plastic drive gears. Perhaps the diff was replaced with another ratio or the speedo is from another year. This is not serious, but somewhat annoying and on my list of issues I'd like to address sometime. I'm about to go to 205-60-13 tires which are just slightly larger than the current ones, so I may get a little closer to accurate then. Anyway, any help or suggestions would be appreciated.

Don

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Guest Anonymous

I have 195 50 r 15 tire and according to my calcuations my speedo should be just a little slow, but it is at least 5 mph slow at 60

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Guest Anonymous

a 185/70x13 or a 205/60x13 tire. A 175/70 is actually a little smaller in diameter than the three proper tires. Also, 02 speedos are not the most accurate in the world. With a 3.64 final drive and the stock 4 speed, if your tach is accurate, 4000 rpm works out to 74 mph in 4th gear. You can do the math for other revs.

The speedo itself has different drive gears, and are marked on the back of the housing with a stamp that says W=x.xx. The 4:11, 3.64 and 3.90 final drives all use different speedo heads. Don't have my figures in front of me, but someone in FAQland can tell you which W number goes with which final drive.

Just don't expect your speedo to be dead-on accurate--watch your tach (and use a radar detector!)

Cheers

Mike

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Guest Anonymous

(nt)

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Guest Anonymous

gary_allen.jpg

I had a GPS unit that told me my speed. As I traveled, I found that my speedometer was off about 6 miles per hour at all speeds (except stop). If the gear ratio for the speedo was incorrect for my drivetrain/tire combination, I should have found an exponential variance (ie. correct at 0 mph, 3 mph off at 20 mph, 9 mph off at 40 mph, etc.). Because the speedo was off 6 mph equally throughout the entire range, I assumed that the error was "built-in" to the speedometer.

Okay engineers...proove me wrong!!

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Guest Anonymous

the same amount regardless of speed..because the tire would stay the same diameter (except for miniscule growth due to centrifugal force as speed increases) regardless of how fast you're going. Thus, a consistent error. Right?

Cheers

Mike

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Guest Anonymous

Its a consistent error, but its not a consistent amount of mph (like 6mph at all speeds). Its a consistent PERCENT of difference. So If your speedo is 5% more optimistic at 10mph, it will also be 10% more optimistic at 100mph. Make sense?

An exampe of the tire size change would yield differences like this. If its 1 mph high at 10mph speed, it will be 10mph off at 100mph speed.

Dan Curtis

'73 2002 Atlantik near the Atlantic

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