how to I figure correct odometer & MPG

I went from the stock 265 65 17 tires to 255 75 17. I realize that the speedo is off by about 4-5%. Does this mean the odometer is as well? I assume so but would I add or subtract the 5%? I tried looking it up but just couldn't figure it out.

 

Bigger tires = slower speedo and slower ODO

 

Use the tire size calculator

 

So say I my odo says I went 300 miles. How far have I gone? 315 or 285? Sorry for my ignorance just don't get if I'm adding or subtracting

 

I did. That's great for tire size and mph reading. I'm trying to figure out if I add or subtract miles to my odo. I think I add 4.7% to what my odo says.

 

If ODO says 300 you actually drove further. If you don't want to worry about any of this buy the hypertech speedo re calibrator, super easy to install yourself.

 

I'd rather not spend money to do math for me haha. I get the mph stuff and now I get the odo / mpg stuff.
I did my tank by odo reading and got 16.5mpg...if I add the 4.7% correction to my odo is actually got 17.5 mpg. That's a pretty big difference

 

The latter is correct. You add the 4.7%

 

Thanks guys.
So when so many guys say the lose so many mpg with taller tires. They may not be correct unless they are calculating this. I get that going way bigger or wider you'll lose mgp but I don't think I have lost any

 

I calculate mine correctly and I still dropped 4-5 MPG every single tank.

Pics of 255s?

 

Here you go sir. I don't think they look skinny. Kinda wish I went 245 now. Oh well.

 

Are they about the same width as the mud flaps? Looks like it.

 

I'll try and get you more and better angles tomorrow if it's not raining. We've been in a drought but finally getting some rain

 

Lemme go in garage right now

 

Slightly skinnier flaps are 10.5" at widest. 10 at the bottom. Tires are 10 I believe. Obviously hard to tell in pics because flaps are closer to camera so I measured

 

Does it matter?

 

It's nice to know when I see a sudden decrease in mpg that nothing else is wrong. And yes to me it does. If it didn't I wouldn't have posted this.

 

Obviously it did, don't come in and attempt to debate MPGs and how it shouldn't relate to trucks.

 

What size tires are you running? I also don't have the same height lift you have and I do t carry the weight in my bed that you do.

 

P265/75/16 at 41 lb per tire or so.

It was a disappointing loss of MPGs. With the new lift I haven't lost any additional efficiency which is great, at least not in the 3 tanks I've ran since they were installed. I just replicated my best MPG trip (from my town to Lake Tahoe) and in stock form I got 28 MPG. Now with a full load of adults plus 1 infant and 1 dog, and all necessary accoutrements I got 21 MPG doing the exact same drive, exactly 2 years after owning the truck.