OK, I found how much my speedometer is off in a search. But what I am wondering is.... my speedometer reads 4-5 miles/hour faster at 25 actual. my odometer is putting more miles on than it actualy has. How many more miles is my question??

 

What tires are you running and what was on your truck stock...?

Your speed difference is not a set amount, but a percentage difference. This same percentage affects the odometer. If you are running smaller tires than stock, your odometer will accumulate miles faster than it should along with your dash speedometer over stating actual ground speed. Larger tires will cause your odometer to accumulate miles slower than actual and your dash speedometer to understate actual ground speed.

From the information you have given, it sounds like your speedo is reading fast by ~18% which would cause your odometer to accumulate miles 18% faster than with the stock tire size.
But an 18% smaller tire in terms of revolutions per mile is quite small... Are you sure that it is that far off at 25MPH?

 

I am at work now and do not have the tire size. but yes I have a speed sign in a school zone just down the street. It reads your speed... it will read 25 and the speedometer reads 29-30. When I run 60(in the dash) I seem to be going a little faster then the flow of traffic, so I am guessing It is not as far off at a higher speed which does not seem right.. I was thinking someone had a link to figure this out. I was guessing the % thing would work..

 

I would use a GPS to indicate your speed. That's more reliable than other traffic, and a radar detector sign.

 

Yeah, those speedo signs are kind of iffy.

Someone could have swapped gears in your truck sometime before you purchased it (going to a numerically lower gear ratio)... But that would require a drop from 3.73's to 3.08's to get to a ~18% increase in actual vs indicated speed (4.10's to 3.42's is ~17% change, but 4.10's are kind of rare in the s-series trucks). But if you have a 4wd, then they would have to change out both the front and rear gears.. That would be going backwards for most 4wd purposes.

Tire size calculator from miata.net will give you the difference from tire sizes. You have to know what you had originally and what you have now for it to work though.