Think of Changing Tire Size Like Changing Gearing

[Blown]

I went from 265/75r16 AT's to 255/70/R16 highway tires and picked-up some power. There is less rollng resistance with a highway tread pattern, they are slightly narrower and smaller in diameter. It was a change from 31.6" diameter to 30.1. That is like changing gearing by 4.7%. That's another way to look at it, 4.10's become 4.19's. That results in more torque to the ground! I also get a little bit better mileage.

Go the other way from 225/70R15 to a 265/75R16 or 27.4" diameter tire to a 31.6" tire, a 13.2% change in diameter. Your 3.73's are now more like 3.23's and not likely to drive your rig effectively. Ford knew what they were doing in selecting gearing and tire size for each engine and truck combo for efficiency. Adding large tires without a gearing change just won't allow the engine to work in its operating RPM range and reduces efficiency. Assuming stock power, 265's with 3.73 would require a gearing change to 4.22. That roughly 13% less torque to the ground is noticeable! The gears available, 4.10's would be slightly higher than needed, 4.30's slightly lower, and 4.56's considerable lower (but may be necessary for large heavy tires). Even if you get the gearing right, mileage will suffer because you are turning a larger mass.
This site has a great tire size calulator: http://tire-size-conversion.com/tire-size-calculator/

Tire width, inflation and tread type effect rolling resistance. Rim material and size have effects too.

I know I haven't covered it all, please add your comments, I posted cause looking at the percentage change in gearing helped me see the effect better, the light bulb popped on when I looked at it this way.

 

[scotts90ranger]

Remember you're also going 4.7% slower on your speedo, and that much less miles on your odometer, so your mileage might be the same...

 

[Big Jim M]

Not to mention the vehicle "thinks" it is going UPHILL all day long! Not only power suffers but also mileage goes into the creek!
THEN there is the braking! Them shoes were NOT put on there to STOP huge , heavy meats from spinning. Be prepared to brake longer to achieve a stop. And no, racing brakes will not help either. If anything racing brakes will prolong the stopping.

Our point here is to actually think about what you are doing before changing tire height/width on your ride. Just because you like the "looks" doesn't mean you have helped the vehicle! In most cases you have detracted from the designers intent and engineering.
Big Jim

 

[alwaysFlOoReD]

Tire width, inflation and tread type effect rolling resistance. Rim material and size have effects too.

turning a larger mass.

Especially in town, stop and go driving.

Be prepared to brake longer

very important to remember

 

[Blown]

Good replies!

(I double posted)

 

[RonD]

On '01 the speedo is probably adjusted by the GEM(generic electronic module) not the EEC.
I know you can reprogram GEM for tires size just not sure if that programmer is available to non-Ford dealers.

EDIT:
I did find this post: http://www.explorerforum.com/forums/showthread.php?t=146122

 

[Blown]

Remember you're also going 4.7% slower on your speedo, and that much less miles on your odometer, so your mileage might be the same...

Good points made here guys!


I do reduce my mileage and speed by that 4.7% as well and, tuning with a Tweecer, I increased the shift points by 5%.

 

[Blown]

I see a Scaler in the PEAV4 strategy of the EEC I am running which is labelled " Tire Revs per Mile".