I have just purchased a set of 20 inch chrome wheels from someone who also owned a similar year and model pickup. I'm going from 17 inch wheels with 260 70 R17 tires to 270 60 R20 tires.

I believe the new tires will be almost 2 inches taller than the old so I am wondering if I need to have the speedomoter recalibrated so the computer information is as accurate as possible?

I have read conflicting information from people outside this forum that having the firmware recoded can actually make the accuracy worse according to their GPS units.

Is this something I need to worry about, do I have the dealer do it?

I appreciate any feedback from anyone with experience in this.

 

I didnt find 260 or 270 tires but with 255 and 265 your speedo is off 2.8mph and with 265 and 275s your speedo is only 2.6mph slow. Here is a tire calculator.

 

guys,its not a certain number of MPH that the speedo goes off.
its gonna be a percentage.
so it could be a 1 mile of at 20MPH and by the time you get to 60 ,it would be 3 miles off....

 

The 4G's take their speed reading from the ABS Tone Ring in the rear diff

 

Assuming you have index rims (i.e., the rim size that the manufacturer based the tire deminesions on in terms of rim width) your old tires are 31.33" tall and your new tires are 32.76" tall. That is a difference of ~5.4% or at a highway speed of 70 mph a difference of ~4 mph. Remember that the vehicles computer calculates speed, odometer milleage and many other items based on a combination of factors including gear ratios and tire height. The cost of changing the vaules in the computer (nothing is recalibtrated really) is about $100 more or less depending on the garage. You can buy a unit and do it your self for about 200 but obviously that would only be worth it if you are going to need it a few times. Since the computer utilizes the tire size data point for so many calculations, it is ALWAYS recomended that you have it changed when you modify your tire hieght or gear ratios.

 

like the other guy said, its a percentage, higher speeds it will be off by more mph. you can pick up a tuner and re-adjust it, thats what I did.

or if its not a problem to you, just figure out how much your off by and remember that while driving. I drove through a radar sign and found it wasnt much of a problem at lower speeds, but the highway can have you off by enough to maybe upset a local law enforcement officer.