380 miles on a tank? That's about 15 more then I have ever gotten.

What's the difference in our trucks? I have a 01 4x4 with a 25 gallon tank. I generally fill up 15-25 miles after my low fuel light comes on and put it 21.5-23.00 gallons.

David

 

Hello david.chaney,

The bottom line is this, your truck most likely has stock 3.55 gears, for a 33" tire, like a 285/70-17 they are to tall (smaller number is a taller gear), to fix your gas milage and other issues, you need to get smaller gears, a set of 4.10 gears would put you about back to what you started with from the factory. You would also need to get a Superchip or Hypertech programmer to completely recalibrate your speed and trany. I am holding off on the same tire upgrade for that very reason, it's a lot of money for all that stuff.

 

Yes it is.

I just got off the pnone with my local Ford house and they want 90.00 to re-calibrate my speedo, and a local 4 wheel drive shop wants 400.00 for the program.

I think that I'm going to have Ford do the speedo and hold off on the gear change.

 

david.chaney,

You may want to ask Ford if this is true, I think the dealer re-calibration will be re-set back to stock if you disconnect your battery. If you purchase the programmer you can reset it your self any time.

 

Hmmm. Thanks for that tidbit of information. I'll run it by them.

BTW, how does the programmer work? What and how does one hook it up?

Thanks

 

It is supposed to be fairly easy to use. Here is a link to a Superchips Tuner group buy, Troyers is supplying the Tuners. Read up on it there is allot of info. there. They are $285.00 shipped! http://www.fordtruckworld.com/forum...c.ten?id=189418

 

I disagree. At highway speed, your torque converter is locked up, effectively giving you a solid drivetrain just like a if you had a manual transmission.

With larger diameter tires the speedometer reads slow because the wheels, driveshaft, and engine are all turning slower, thus RPM goes DOWN.

If you go from stock tires to HUGE tires, of course it will take more power (and RPM) to get them moving, but once the torque converter locks up, the RPM has to be less than the stock tires. With huge tires (more friction), its going to take more throttle to maintain that RPM, and that's where any actual milage loss comes from.

Going up one tire size without significantly increasing the width of the tire should have an almost undetectable affect on actual milage. The weight difference between a 265 and a 285 can't be anymore than an extra passenger, and the amount of rubber in contact with the road won't increase that much.

 

Arrg!

It takes an increase of power (NOT RPMs) to get the taller tires/gears rolling, especially under increased load.

With the TC locked, engine RPM is directly proportional to driveshaft speed to the final drive ratio. RPM indicated by the tach will NOT increase above what the direct drive would allow.

Moral: Taller tires will require more power to get rolling. Once rolling, the power the engine must provide to maintain a give speed will depend on numerous variables. Note that the engine RPMs will be lower than for a taller set of tires/gears at a given road speed versus a shorter tire/gear set.

There will be a crossover point at which the reduced engine speed will eventually require more fuel to maintain a given speed than it would have for a shorter tire. You can bet that the Ford engineers figured all this out using their mathematical models and wind tunnel testing before settling on supported tires, rear axle ratios, and transmission gear ratios.

Steve

 

Bigger tires=more resistance=more engine work=less MPG

 

projectSHO89
You essentially said the same thing I was saying except for one point I diagree with:

Yes it takes an increase in power (which is what I stated), but your RPM will also go up as a result of that extra power. When starting from a stop with taller tires/gears, RPM will be higher than it was with the stock tires/gears because your torque converter is going to slip more. (remember, we're talking pre TC lock-up) That is by design to get your engine in an RPM range that produces more HP. When the torque converter locks up at highway speed, yes the RPM will then be lower than it was with the stock tires/gears and it will take more power to keep it there. (which is also what I said)

Try taking off from a stop facing uphill (which would exaggerate the extra load bigger tires would place on the drivetrain) and tell me you RPM doesn't go higher than it would if you were on level ground. Even in a manual transmission, you would have to slip the clutch a little more on take-off to keep from lugging the engine under the extra load which equals higher Power AND RPM to get moving. Once the clutch is fully engaged, (just like the TC locked up) RPM will drop.

 

I know this is an old thread but here is what I have found. I had toyo all terrains size 255 75 r 16.
Putin new cooper American prospector tires size 285 75 r 16.
Triton 4.6 liter v-8 and supercab body style with shift on the fly.
Got mostly 14 to occasional 15 mph. With 3:55 gear ratio.
After new tires 12. If I had known this would happen I never would have got the bigger tires. Tread pattern was very similar also.
TT[/QUOTE]

 

Your larger tires also have thrown off the speedometer/odometer. You are actually traveling more miles than indicated.

 

Hmmm well I get about the same mpg with 285 75 16 mts as I did with stock tires which were 235 70 16's, that's about a 12% increase in tire size. I usually average 15.5-16 mpg on the highway and 12.5-13 in town, about 14 mixed. I have a 2000 4x4 super cab with a 5.4 and wait for it........ 3.31 gears! The jump to bigger tires did make the truck a little but more sluggish off the line, but other than that there isn't much difference and the truck looks wayyyy better.

 

you will see better milage on the highway at steady speed, but worse in the city because the final rear-end ratio.

That rolling resistance stuff is negligible.