Tires
 

going from a 20" 60 series to a 20" 70 series . I know its a taller tire.
How will this affect my speedometer???

Same width? That number is a percentage of the width in millimeters - if the same width tire is used I don't think you will even have a notable difference

Height will effect your speedo and mileometer. Width not so much.

I have a f 150.
Stock was 265/70r20
I put stock f250 Michelin tires
275/65r20
My speedometer is off about 4 miles off.
After 60 it increases
At 65 I'm doing 71
At 75 I'm doing 82

The number he refers to is directly attached to the width when determining height of the tire.

You will be going just under 4 mph faster than what your speedometer says.

Yup

Go to the tire manufacturer's website and look up the specs on the tires you have now and compare the overall diameter to the specs of the tires you are getting. Even if two different brands are the same size, the actual size may differ a lot. I have seen some 33" tires that were closer to 32" and others that were closer to 34". Once you have the exact specs you can find an online speedometer correction calculator. I think interco has one on their site.

i went from a 265 to a 285 and it averages about 3-4 mph. less at lower speeds and more at higher speeds. at 65, my speedo shows 68 or 69. google it and you'll get more info than you can cypher!

An even better way to check your speed after you get the tires is with a GPS.

260 to 270 maybe that will make it clearer