I have a 2021 Chevy 2500 came factory with 275/65/20s gong to change to my spare tire size which is 275/70/18. Any issue with speedometer or anything else. Thanks

 

Please read the bright red text at the top of the page. Thanks.

 

ABS, traction control, steering, shifting, speedo/odo, fuel economy, acceleration ... all will be affected.
It's generally recommended to stay within 3% of the OEM size.

 

Thanks. I have read that before about the 3% tire size change. On your chart you had put 270/70/18. My spare tire is 275/70/18 which puts it at 2.69%.

 

Welcome to the forum.

 

Thank you

 

No, it won't.
275/70 is a smaller by 1 inch.
Speedometer reading will 2.6% higher than actual speed

 

My math indicates that the new spare would be 0.2" taller which would probably be inconsequential in the speedometer reading.

Edit: another problem besides speedometer reading with tires of different ODs is that it puts more wear and tear on the differential.

 

Show your math.

275mm wide tire x 65% profile = 179 mm height (7 inches) or 14" (top + bottom) + 20" wheel = 34 inches total height
275mm wide tire x 70% profile = 193 mm height (7.6 inches) or 15" + 18" wheel = 33 inches total height

 

You are correct. I miswrote the OEM tire size at 275/60 instead of 275/65. That would make a difference both in speedometer reading and wear and tear on both differentials.