Originally Posted by Ryno

You are correct. Tire size and axle ratio go hand in hand. When you change your tire size you are effectively changing the overall gear ratio of your axles... That's why Jeep pairs 29" tires with 3.21 gears, and 32" tires with 3.73s or 4.10s, without a significant difference in fuel economy.

Given the same engine RPMs, lower gears will move your vehicle slower than higher gears.

If you took two identical Jeeps--except one with 4.10s and the other with 3.73s--and set them on cruise with the engine RPMs at 2000, the Jeep with 4.10s would be traveling slower than the Jeep with 3.73s. Less distance traveled at the same engine RPM = lower MPGs. Put bigger tires on the 4.10 Jeep, though, and it will be closer to the 3.73 Jeep's speed at the same RPM.

If rolling resistance and wind resistance stayed constant, you would probably see a slight increase in fuel economy when upsizing to 33" tires, because you are traveling more distance at lower RPMs (if the change is not enough to cause more downshifting). However, our world is not that ideal.



I'll be the one reassuring voice. 33s and 3.73s won't be much different than stock as far as power goes. Just go for it.

Originally Posted by Berra

I have auto and 4.10 Changed from stock tires to Cooper STT 285/70 R17. I measured the circumference on one of the stockers and compared it witn the circumference on the Cooper STT a difference of approx 3,5%.
This means that I get 3,5% further than the cars odometer shows.
I have a scangauge installed on wich I can read my MPG ( or really L/100km as thats how welike it here)
On my "testtrip" I usually get 11,9 L/100km, with the new setup I get around
12,3- 12,5L/100km. 12,3 - 3,5% is 11,86 so virtually the same not bad for a MT tire. My test trip is from town to home, not a very long trip but as I make it frequently I use it as reference.