Gas Mileage correlation with change in tire Diameter

[Chedman13]

I'm upgrading my 01 QX4 with 18" wheels.

After doing all the research, the two tire diameter options for the tire brands that I want are:

1) 29"or2) 30.5"

OEM is 29.5"

I only drive city/highway and my concern is gas mileage. From other people's experience, the larger the diameter, the worse the gas mileage (someone put 31" tires and went from 19mpg highway to 16mpgh highway and calculated using a gps unit, accounting for speedo error).

So the smaller the diameter, does gas mileage, increase, decrease, or doesn't change?

I know changing diameters, creates an error in the speedo, changing shift points for my auto transmission. I'm just not sure how that affects gas mileage.

Thanks for any help.

[rsiwicki]

95% of all aftermarket wheel/tire combos weigh more and hence it takes more energy (gas) to move your vehicle at the same speed. Big fancy wheels are nice, but usually slow down the acceleration times and also increase your gas usage. Only the light weight strong high $$ forged wheels (BBS, HRE, OZ, etc.) will probably maintain same gas usage and acceleration times. Extra rotational mass kills many things. Changing your wheel diameter by 2% or less will not affect anything significantly.....61.2 vs. 60mph.

[Q45tech]

Loaded [car weight on combo] rolling diameter/circumference is the key......function of sidewall stiffnes........identical sized tires will not be the same between brands and models........ up to 1/4" x 3.14159 is legal.........just as 11/32......>>>> 2/32" is legal........9/32=0.28 x 3.14159.

A good rule of thumb over narrow rpms is that a quarter to a third of the ratio of the rpm increase will be seen in gasoline mpg changes.

2000>2100=5% so 1.25-2% decrease based on a QX4 less than 0.5 mpg

0.5 mpg is important to manufacturer taxwise ----gas guzzler tax as the decimal point may throw you into a higher significant expense bracket.Fleet averages are critical as X.499999999999 is X while X.50000000001 is X+1.