i got a 95 blazer 4wd 4door. factory size tire is 235 70 15. i got 235 75 15. Am i losing mpg with having the bigger tire size. Should i go back to 70 or stay with 75?

 

I have 75's on mine also, stock for mine is also 70's but I have noticed I am getting better mpg's with the 75's, maybe since there a bit narrower cutting less air and I little taller giving me a little lower rpm and running on the flat that helps a bit, not too noticeable there is not much difference runnin in the hills around here either and I average 21-22 mpg on the open road

 

going from the stock 70's to 75's. the gauge will be a little off

 

The 75's were available as an option on a lot of Blazers, so no it wont harm anything.

The size difference between the 70's and 75's is small enough that the gauge wont need calibrating. 3.2% difference.

 

You should not really have noticed any change as already stated. They were both offered by the factory. I have been running 30x9.5's for years and have not really noticed much change.