speedo question

[64shortbed]

Ok i have a 64 GMC shortbed. Now i have put a 350/350 combo in it. It has the original dana 44 rear with 3:54 gears. I put 255/60/15 on the rear. Now i have searched and searched and figured out what my speedo gears should be. I have a 7 tooth drive gear and a 19 tooth driven gear. Which gives me a 2.71 ratio. What i should have to read right is a 2.75 ratio.

Now my question is my speedo is like 10 mph to high ruffly. Why is this? There is not enough differance between 2.71 and 2.75 to matter that much! So the speedo is original and i have a spare i put in and it sticks (as it sits in the garage right now in says it doing 20 mph) do these speedos not run the 1000 rpm? Is that what my problem is? I cant remember were i read that but it said that they started the 1000 rpm speedos in 1965?
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[Wrenchbender Ret]

1000 revs per mile is the standard. Take out the 19 tooth & put in a 21 tooth. Then compare the odometer reading to the mile markers on the freeway. If it looses tenths put in put in a gear with less teeth. If it gains put in a gear with more teeth. If you are not near a freeway mark out 2 miles using a late model vehicle you know is right. Like from the oak tree in front of the smiths to the light pole by the church.

[64shortbed]

Thats what i was thinkin too. Just wanted someone else opinion on it too. Thanks for helping out. Ill let you know what i end up doing. Thanks.
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[fleetsidelarry]

shortbed, the only problem I see with wrenchbender's suggestion is that it is trial and error, requiring you to have different gears to swap BEFORE you know what to get.

I don't know how you determined that you only need to change to a 2.75 ratio from your current 2.71 but, for instance, if you are off 10 mph at 50 (just for example) you are off 20%. changing from 2.71 to 2.75 is only about a 1.4% change if my math is right.

I like to offer another suggestion.
1.as wrenchbender suggested, time your truck through a measured mile or two at a steady speed.
2. note your speedo reading.
3. divide 3600 by the number of seconds it took to go a mile (if you go 2 miles, divide your total time be 2 to get an average time per mile)

this gives your actual speed versus your indicated speed. now you're not guessing

example, say you put your truck on 60 by the speedo and time the mile at 55 seconds: 3600÷55=65mph. if it takes 70 seconds then 3600÷70=51.4mph.

since you know what gears you have you can determine a driven gear to achieve the change you need. if you are reading too high as you said, you're going to need to slow the speedo down, which means MORE teeth on the driven gear.

[T Smith]

You can also use GPS or a speedometer app on your smartphone and tune accordingly. I've seen several instances where the speedometer and odometer were not in sync so doing the mile marker thing is sometimes not so accurate. This is still trial and error though.

[fleetsidelarry]

yes, for those of you who are more into the 21st century, GPS should work fine for measuring the miles and speed.