My rig new had 265/70/16's. The tires were upgraded by the previous owner to 265/75/16's. I have been dividing my accumulated mileage per tank by the number of gallons used. I believe my odometer is slightly off due to the slight increase in tire size. My question is: How much is it off?

I will add the % increase to my excel spreadsheets to "correct" the equation.

I suck at math, and I really suck at story problems. Please

 

Why not check it with GPS?

 

Because I dont have GPS.

 

Ok, I will take a stab at it.

original tires:
radius wheel = 265mm*%70= 185.5 mm or 7.3 in of sidewall + 8 inches or rim = 15.3 in
cirumference is 2*pie*radius = 96.08 inches

New tires:
radius wheel = 265mm*%75= 198.75 mm or 7.8 in of sidewall + 8 inches or rim = 15.8 in
cirumference is 2*pie*radius = 99.22 inches

so I get a difference of 99.22/96.08 = 1.032 or 3.2% larger

 

Really? That much? Wow...anybody else?

 

Sorry, needed to recheck my math!
Made a calculation mistake.

Should be about 3.2%

 

Thanks Jamie. So I should add 3% to my excel spreadsheet right?

 

yup... about 3.5% difference... multiply your mileage calculation by 1.035

 

Exactly!
Hey, engineering does come in hand after all.

 

Awesome, thanks a ton guys. I incorporated it into my spreadsheet.

 

Just a by the way, I used 25.4 for converting mm to inches and 3.14 as pie.
I don't think you need to be any more precise than that tho.

 

The toyota odometer and speedometer aren't synchronized exactly either - or so I've read... Supposedly, with 265/70's the speedo is off by about 3-4% (you're going slower than it says) and the odo is dead on. With 265/75's, the speedo is now DEAD on and the odo is reporting 3-4% less distance than you actually traveled...

 

How much do you subtract, and at what rate, for the wear of the tires themselves? They aren't the same diam when they're worn as when new. In other words, it ain't accurate to begin with so all this math is like tuning up the band while the ship is sinking - fruitless.

 

You are right, they do change diameter but it is so small relative to the size of the tire that it becomes negligible.

bamachem, has a good point. You need to know what tires came stock on the vehicle to calculate the total difference between your current tires and the stock tires (assuming the odometer and speedometer were dead on when new). Otherwise the best you can do is calculate the relative difference between the MPG of the old and new tires.

 

WTF

Post size must be 15 characters...

 

I figure the overall diam reduces by at least an inch (1/2 inch or more all the way around) - sounds ligible to me. (is that the opposite of negligible?)

 

good one!

I suppose if you go from brand new monster mudders to totally bald nobbies you would see a decent difference. It would take quite a while to show tho. You might be looking at a max difference of like 1-2% at the worst case.

 

This is a handy tool..

http://www.miata.net/garage/tirecalc.html

 

I use 3% increase average, got that tip from Dr. Z.

Mick

 

doesn't matter, both divide out since all you care about it a ratio between old and new.