Just put some new factory 275/65 R18s on my '07 4.6 in place of the stock 255/65 R17s. Will I see a decrease at all in fuel economy and acceleration on an already weak motor? Thanks.

 

Um ya a little bit.....you went from a 30" tire to a 32" tire....

 

yup

 

the added weight will be a big factor in your fuel economy.
yes.

 

Not enough weight to make a noticeable difference, all in tire size.

 

Two thing in play here - diameter and weight ( okay okay ... and where the weight is concentrated, static weight, and inertial effects, lol)

I'll say it again - rotational inertia increases with the square of the speed.

It does not matter if the weight increase is due to the tire or the wheel, or both.

Refer to:

http://www.mazda3forums.com/index.php?topic=10243.0

http://www.tirerack.com/wheels/tech/...jsp?techid=108

Only a couple examples, of hundreds of sources of info on this topic. And that was just classic 'plus' sizing, where the overall diameter stayed roughly the same.

Go intentionally larger in diameter (say from stock 32" to 35" ) and the problem becomes MUCH worse - now the mass is really concentrated far from center. Affecting everything - performance, mileage - including braking, as you have just reduced yer brakes' effective leverage.

There's more - Unsprung weight also impacts yer suspension calibration. And yer effective gearingi changes - for the worse.

Not stopping you - just be aware of the consequences - nothing comes without a price.

MGD