So I was coming back from central Kansas the other day with my fiancee driving my truck. She never speeds, so the speedo was right between 65 and 67 mph in the 65 mph speed limit area. We got pulled over for going 74 (so says the state trooper). What gives? Was he lying or is there really that much of a difference? My truck has the 3.55 gears and 285/75/16 tires. I realize that there might be a slight difference, but that much? She got a warning instead of a ticket, but I just need to know for the future to avoid things like this. Anybody else have any trouble?

 

just recalibrate your comp or rear end so it reads the right speed for your upgraded tirer size and neeer worry about it again

 

I doubt he was lying but the way I figure it is

If you had 245/75/16 and went to 285/75/16 and the speedo said 66 you'd actually be goin 71mph

and if you had 265/75/16 and went to 285/75/16 and the speedo said 66 youd actually be doing 68mph

Thats if my math is right

 

I run 285s and just figure 10% cause the math is easy
You could buy/borrow a GPS to verify your speedo reading or justify the $ for a smarty to correct for tire size

 

I went from 246's to 265's and mine is off a bit. i think about 3 mph. Try this site:

http://www.rocky-road.com/calculator.html

 

went to Chicago last week... i have 285s & a GPS (cause IL troopers are a pain!!!)... GPS indicated about a 3-4 MPH difference (faster) than speedometer

if she had cruise set, going up or down even a small hill will very the speed 2-3 MPH

 

"if she had cruise set, going up or down even a small hill will very the speed 2-3 MPH"

If she was stopped in a 65 MPH zone they were still in Ks. Colorado St. line speed limit changes. SO small hills can't be a factor!

But I know the problem. I have 305's and when I go out to Ft. Collins I try to count 10 mile markers, time it, and keep my cruise at the posted speed limit and see what the difference is. I get so enthralled with the scenery I forget how many mile markers I've counted!!

 

The speed is not changed by a constant number, it changes by a percentage of current speed. For example, for 245/75 size vs 285/75, the speed is higher by roughly 8%, which is about 5 mph at 66 mph -> actual speed = 66+5 = 71.
Try http://www.miata.net/garage/tirecalc.html for more examples.

 

I am running 285 tires and my gps shows 61 when speedo shows 55 so I just figure 10% off and go.


Tom

 

I fixed the whole thing by getting a Superchips programmer. It is fairly cheap, gives a great performance boost, betters fuel economy by a mile or two per gallon, reads the computer codes, and lets me set my own tire height. I set it and then go out and calibrate it on the highway. Pretty accurate and painless. You just need to remember to return it to stock when visiting the dealer so they don't reflash your computer. Then your programmer won't recognize your rig....

 

I am running about 3.5% faster than indicated with mine.

I figured 315s would be close to 9%.

Dave

 

And if you figure your fuel mileage by hand using the actual odometer reading (the odometer is running slower then actual) so mileage will be better then it appears.

 

That is not what I found out with my truck. It seems Dodge is a bit weird about speedometer accuracy.

I went from running 265/16's to 285/16's my speedo (IIRC) error went to about minus 3 mph. This was done using a GPS. I mean it is 3 mph off at 30 mph and 3 mph off at 60 mph. It is just about 3 mph off across the board, basically for all speeds. Still bakes my noodle a bit as to the reason why, but it is what it is.

I use 1.06 as a correction factor calculating mpg consistantcy. Which is just sort of sideways a weighted averaged that I came up with to correct for the error and probably not all that accurate.

Jim

 

If you mean mpg, it will actually be worse. IE: Less miles indicated for same fuel usage.

But your warrantee wears out slower!

Jim

 

I went from 265's to 285's and then reset my pinion factor with the scanner at work. Checked it with my GPS and it's right on.