Got my truck back from having a new steering gear box, p/s pump and hydroboost replaced..anyways, i have 285/75r16 BFG AT's on it, so i asked them if the speedo could be recal'd, when i picked it up they told me i needed to talk to one of the tire shops to get the "clicks" or something for these tires on this truck..so i'm driving down the road and it doesnt seem right at all now... it used to be off on the low side...if i was showing 70 was actually around 76ish...well now it seems like i'm running with traffic in a 45 zone and showing close to 60!! So heres some #'s for rpm vs mph... 3.55 gears with a 5 speed... in 4th gear i was running 2000 rpm and showing 58/59...then in 5th gear i was running 1500 rpm and showing 60....is it just me? I have been driving the 06 Mega cab/6speed for the last month...anyone with this size tire and knows the speedo is right at these rpm's right? Thanks for any info....

Capn

 

You could check with [ oh no ] the dealer , or try this .

http://www.ricksontruck.com/speedometer.html
Rickson Truck Wheels and Accessories

 

i put on 285s also and my speedo was off by 8 milles an hour,tested it with my gps,used the smarty to change it,the 285s are 32.75 inches tall.

 

the other night my girlfriend was on the phone with me on our way home form an auction and she tried to pass me in her rental car and she was doing 103 right next to me and my speedo was at about 85

 

You will be 10 percent on the slow side.

Example, speedo says 50, you are actually going 55.

 

Your speedo sounds likes it is still off... with your tires/gears/trans/RPM combo you should've been running around 55 mph

 

i took my truck to the dealer and he reprogramed mine for 315s thay dident charge me any thang i gess i got lucky

 

I know the dealer can recalibrate it for like 70 bucks. My spedo is only of 5 mph with 315s