I've never been good at math. Here's what Id lilke to know.

My truck came stock with 265-70-17 tires, I installed 315-70-17's. So how far off does that throw my odometer. I couldnt care less about the speedo, but I cand figure out my fuel milage correctly because my odometer is off. So I guess what I'm asking is what percentage is my odo off and once I get my gallons used, and milage (acording to the truck) where do I put that number into the equation?

Thanks yall.

 

8% at 70mph. So if you travel 200 miles (odo reading) using the new tires will be 16 miles less than actual (200 x .08 = 16). So use 216 divided by the number of gallons used for your fuel economy.

MikeyB

 

I'm horrible at math as well, but I was following you 'til you threw that 216 in there. Where did that come from?

chaikwa.

 

Nevermind... I see!

But how did you arrive at 8%?

chaikwa.

 

Mikey, I always found it easier to just multiply, 200 X 1.08 = same 216 miles.

Chak, I would bet he looked up the diameter differences.

 

Probably, but he'd still have to have some kinda formula to find the percentage of difference... I dunno how to do that either.

chaikwa.

 

I found this place a few years ago http://www.csgnetwork.com/tireinfo4calc.html Looks like a more accurate number is 1.08718 .

 

I used the calculated revolutions per mile for each tire given the tire size. There will be some variations with the number due to the actual numbers that are posted by each manufacturer.

MikeyB

 

http://www.jl4c.org/dtr/tires/tirecalc.zip

 

i would borrow a GPS unit to see exactly how long it takes you to go a mile, or use the mile markers on the highway. from the factory your truck may have not been accurate.

 

Thanks for the info yall, and thanks for doing the leg work for me MikeyB.

Good suggestions 2006Mega, I mostly drive back roads with no mile markers and dont know anybody fancy enough to have a gps, but deffinatly things I'll keep in mind if the opportunity comes about.

 

Here is another way to look at it.

a 265/70/17 should be about 31.6" tall.

A 315/70/17 should be about 34.4" tall

Divide 31.6 by 34.4 and you get .9186046,round up an your at 92%

A 265 is 92% of a 315. There is your 8% difference.

Thats about as close as you'll come. Depending on manufacturer,a 265 may be a bit taller or shorter than 31.6"

Same thing with 315s.

 

and to further expand on this, 265/70/17 or whatever your tire size is can be turned into a tire diameter this way...

17 is the wheel diameter in inches... 265 is the tire width(on a correct width wheel) in millimeters(confused yet).... 70 is the percent of the wheel width that is the sidewall height... so a 265 tire has a sidewall height of 185.5 mm or 7.30315 inches... you have 17 inches + 2 x 7.30315 inches = 31.60630 inches.