I'm starting to wonder if the Duratec needs smaller gearing. I had 31's on my truck for a bit which reduced the effective gearing down to 3.00 or lower. It really wasn't much slower than with the smaller tires but if you shifted wrong it wasn't so forgiving. I had to rev it more which hit more of the sweet spot of the torque curve, if you have a duratec you know how it likes to rev. I took them off because of tire problems but i'm planning on replacing them with 235/75's eventually. That size would reduce the gearing down to the ~3.55~ area. As far as gas mileage it didn't go down much with the 31's 1-2mpg if any at all, I figured in the difference roughly.

Thoughts?

What was your stock tire & gear size, 225x70-15 like mine? If so your math is a little off. A 225x70-15 is 27.4" diameter, by going to a 31x10.5x15 you effectively made your 4.10:1 (assumed since most 2.3l come 4.10) axle drive like a 3.62, not 3.00 or lower. Putting 235x75-15's would put you at 28.9", making the axle feel like it's got 3.89 ratio gears.

http://www.4lo.com/images/geartire-formula.gif

To convert metric to inch sizes use the following formula:

http://www.4lo.com/images/gearmetric-formula.gif

You will have to adjust your computer, as your speedo/odo, plus other speed sensing features like your ABS will be inaccurate too.

http://www.4lo.com/calc/geartire.htm

Yeah the 225/7015's came stock on my XL. It also came with the 3.73's which I should of stated. Would of been nice to have 4.10's though.. thought about upgrading but I may not now. If I get the 235/75' and like them i'll have the computer updated. It was about 5mph with the 31's.

That just changes it a little - you were at 3.30 with the 31's on there. By bumping it down to the 235's, you'd be at 3.54, still a bit low for the duratec's torque curve. 4.10 is a much better gear choice if you want bigger diameter tires on there!

With the 31's your speed would have been 67.8 when the speedo read 60 mph. With the new tires it'll be more like 63.3 when it reads 60 mph.

Wow, I can only imagine how much pick-up your truck does NOT have. Mine has 3.73s and it drags butt unless you shift beyond 3k. I shift at 2300-2750, so people are always riding my ass. Oh well, I get killer gas mileage.

i had 33s on my 01 duratec with stock 4.10s and a auto

I have a 4.10 rear on my 01 supercab. I replaced the stock tires with slightly bigger 235/75R-15's. I did not notice any decrease in acceleration performance.

I did not adjust my computer. There is about a 5% difference in reported speed and milage that I keep in mind while driving and when calculating gas milage. I figure the bigger tires will keep the odometer reading a little lower than it otherwise would be. That could help with resale value.:naughty:

I get 20-21 mpg around town and 24-26 on the highway. And yes, I do have a lead foot!:thumbsup: