Tires: Changing Aspect Ratio, exact replacements, tire diameter

Question
Hi Barry,

Regarding my '99 Dodge Dakota 2WD. I currently have the OEM P255/65R15 Goodyear Eagles on the truck. Unfortunetly, exact replacements for this size are very limited and expensive. My dealer is suggesting a change to a highly rated competitor tire at P255/70R15. I've calculated this will increase the tire diameter from 28.06" to 29.06" and change the speedo reading by 1.9 MPH.

Any other factors I should consider? Will this significantly impact MPG?

Thanks,
Dave

Answer
David,

when changing tire sizes, you have consider the clearance around the tire, esepcially in the fully compressed / fully turned position.  You also have to consider the rim width.  These are both probably not a problem with the sizes specified.

But you can expect a small loss in MPG for thre reasons:

1)  OE tires generally give better MPG than replacment tires, but OE tires don't wear as well.

2)  New tires are worse for MPG than worn out tires.

3)  The slightly larger tire size.