12 inch tires and mileage?

[Ogrekhan]

Hey all,

I have often wondered what effect switching up from 12 to 13 or 14 inch tires has on mileage. I've been told it effects final ratios and therefore will change mileage/performance. Anyone have experience with this? 12s are spendy now and harder to find so I've been thinking about a swap.

Thanks Jeremy.S

[GenevaDirt]

things should stay the same if you keep the same diameter. I have used larger tires and with size comes lower gas mileage and sluggish performance unless you mod the engine

[shawnw]

I always thought that larger diameter tires would give you slighty better mileage on the highway or at speed. I know that bigger, heavier tires take a little more effort to bring up to speed but once you're there dont they help keep the revs down even a bit?

[GenevaDirt]

revs are down....but unless you drive on a long trip or something then you most likely will not notice any increase in mileage. Not even staying the same. If you have stop and go conditions along your trip....it really eats up the gas more then you would think.

[TominMO]

Mileage

I always thought that larger diameter tires would give you slighty better mileage on the highway or at speed. I know that bigger, heavier tires take a little more effort to bring up to speed but once you're there dont they help keep the revs down even a bit?

Correct. You will get lower city mileage with heavier tires, even if they are a little taller, due to the extra effort by the motor to bring them up to cruising speed. But on the highway, the weight doesn't really matter; the taller gearing provided by taller-diameter tires will give you better mileage, since the motor is cruising at slower revs. You can do little hypermiling tricks for better mileage like coasting whenever possible, shutting off the motor if a stoplight is just turning red on you, etc. Doing things like this, but not being super-fanatical about it, my Subaru Loyale 4WD station wagon would average 32 MPG city.

Bear in mind that heavier tires/wheels will require longer stopping distances for the stock brakes, so drive accordingly.

Some tire diameters:
155/80-12....21.8"
175/70-13...22.7"
165/80-13...23.4"
185/60-14...22.8"
The biggest difference here from the 155/80-12 is 1.6", which is only 7%. Much more important to mileage is the weight of your right foot. But as a good general upgrade for handling, better quality, and availability I'd recommend the 13" Geo/Chevy Metro wheels with the 175/70-13s, Kumhos for example.

[spiko]

I had 13" snow tires with some heavy rims, the best I got was just under 32 MPG. I sold the car with the stock tires, they're reporting 35 MPG whipping on it in the city and it still needs a little TLC.

[jglutz]

I always thought that larger diameter tires would give you slighty better mileage on the highway or at speed. I know that bigger, heavier tires take a little more effort to bring up to speed but once you're there dont they help keep the revs down even a bit?

Typically..thats it in a nutshell.