If 275/65 to 275/70 is 3.4% increase then why is speedo only off 1.6%

Got new tires and checking speedo vs GPS I get about a 1.6% difference at 60-70mph. But the tacoma world tire comparison (god bless whoever created that---it is truly one of the great tools of the internet age) shows a 3.4% change.

Is the factory speedo (and more importantly odometer) understating speed and miles driven?

One other question- I'm trying to figure out real MPG on these new tires. Do I add 1.6% to the odometer reading to check this?

 

Because from the factory your off, it's never (or hardly is) dead nuts. And they usually run so that the end user is going slower than what is displayed.

And yes, say you drive 300miles.
300 x 1.016 will give you true miles driven.

 

Yes I went from 275/65 to 275/70 and now I am spot on with GPS and radar.

Yes I made that jump and now my truck is spot on.

 

I have to same setup. I believe you can do a quick MPG differential by multiplying your trucks MPG readout. For example 15mpg. Times .034 or 3.4 %. For about .5 MPG difference. The ratio applies to speed and distance. It’s takes 15 miles to gain .51 more miles.
I dont worry too much about my speed because at 75 it’s still only off by 2.5 mph.

 

My tundra was off by about 1mph with stock duelers, I upped to 275/70 and speedo is dead on.

 

Yep, I noticed right after I purchased my Speedo would say 70 and GPS app showed 68