So I got a speeding ticket, 14mph over, 49 in a 35 zone.
I saw the Officer on his bike waiting for speeders with his speed gun.
Thing is I was watching my speedo at the time and it was only
showing 2-3 mph over.
I double checked it later, side by side with my brithers truck which is dead on and my truck is way off.
My truck came with 245/75-16
I now have 265/75-16

So how do I correct this ?

 

Well I don't think going from 245's to 265's will give you a 14 mph difference(I think you were speeding). But, you can go to the dealer and have them change it, or by a Hypertech tuner or a SuperChips tuner.

 

I guess I was speeding, but had no idea I was.
Do you know if someone makes something like a code reader that would be able to input the info. ?
I don't really need a tuner and they are costly

 

the speedo effect on a tire size change from 245 to 265 would be minor, probably less than 5 mph.

however - most of our trucks came stock with 225 (see door jamb), so from 225 to 265 you might see a little more.

you can use a tire size chart to determine circumference, and then calculate the percentage increase of each, and then apply that percentage to your speed to see what it comes out to.

the 99 speedo is electronic, so its not a simple gear change, but is instead a parameter in the pcm called pinion factor. a superchips will adjust it to any tire size you can dream up, just by entering the tire diameter. hypertech is iffy. i've got one that's supposed to adjust speedo, but it doesn't work. superchips does. any dealer can program your pcm for 3 factory tire sizes 225, 245, 265 with their standard menus. an INTERESTED dealer can program any size by entering OTHER, then revolutions per mile.

there is also an electronic box that you can splice into your rear diff speed sensor, but it costs a couple of hundred, so a used superchips for the same price is a much better deal. i got a superchips to do this and have been happy with it.

 

On mine 245 to 265 resulted in the speedo reading 3mph slower than actual sleed.

 

When my '72 says 25mph I'm really doing almost 40 Depending on which set of tires I have on at the time of course.

 

Just my . I run 285/75 16's on my 01 1500 Sport. My speedo is only off by like 3mph. I know this cause of a radar speed limit sign that I drive by every morning on my way to work. Going from 245 to 265 you shouldn't really see a difference. My 285's are equivalent to 33x12.5 15. I think I would have to agree with Zman on this one. Just sayin'. lol

 

Here is a great Java based calculator for determining how far your speedo may be off due to tire change.
http://www.miata.net/garage/tirecalc.html

 

So changed to 235/85/16 tire from 245/75/16 and trying to fix the speedo. In the pinion settings I'm not seeing revolutions per mile or tire size. Just see the 3.55 gear setting and "tire count of 400" cannot for the life of me figure out what this is. Any thoughts? It was with a Snap On tool not a Dodge DRB or DART etc.

 

What year is your truck?