Tires: tire soze, tire placard, inflation pressure

Question
if i put a 265/70/15 on my truck instead of a 265/75/15 what would that do to the truck? would that mess up my speedometer or anything else

Answer
Billy,

That would be going down in load carrying capacity - and there are a couple of scenarios where this is incredibly dangerous.

First, did you truck take LT265/75R16's ?  

On every vehicle sold in the US, there is a sticker – commonly called the vehicle tire placard - that lists the original tire size and the proper pressure for that size.  The placard is usually located on a doorpost or in the glove box – but sometimes it is located in the trunk or on the fuel filler door.

BTW, it doesn't matter who makes the tire or what pressure is listed on the tire's sidewall, if the tire size is the same as the placard, then the pressure listed on placard is also appropriate.

Some trucks use tires with the letters "LT" in front of the tire size - and some trucks use tires with the letter "P" in front of the tire size - and some trucks use tires with the letter "C" after the tire size.  You should not use "P"'s in place of "LT"'s and vice versa - and it is possible that is the situation here.

So look at your vehicle placard.  If it says LT  in front of the tire size then you need to fix that IMMEDIATELY.  If not post back what the tire placard says about inflation pressure.