Hey,
Im pretty new to owning a Runner, and I noticed mine has 30" tires on it now.

My questions.
What was the stock size for a 98 SR5 V6? 265?

How much difference in diameter is this over stock, and how will that affect my speedo, and my odometer.

Yeah, Im the guy always asking about gas mileage. I just wondered if maybe Im getting better mileage than I think?
If the tires I have are a larger diameter than stock, and say, slow down my odometer by 5% (pulling that # out of my butt), then,
if it tells me Im getting 18 mpg, then actually I would be getting 19mpg? Is my reasoning correct here?

 

Check the size of your spare, that should tell you what size came stock.

 

Next time you are cruising the highway check your odometer against mile markers. When I went from 28s to 31s My odometer would only measure 9/10ths for every mile marker I passed. My speedometer was also off by 10%. Then I regeared and it was just about right, then I got bigger tires again. Now I figure i'm about 4% off.

Timing mile markers at 60 MPH will also give you an idea of how far off your speedo and odometer are.

Your stock tire size should be on a plate in the driver door frame, in the owner's manual and as mentioned, on the spare.