Ok, just recently changed to my second set of aftermarket wheels and tires for my 2002 Supercrew FX4. Went from the stock 265/70-17 to a 33/12.5/17 tire on a 17x8 wheel. Now I am running 305/55/20's on a 20x10 wheel. Since making the most recent change I have lost almost a full mile a gallon. I never changed my speedo so I know there is some inaccuracy there but the change is from the 33's to the 305/55's which are still 33's. I did crank the torsion bars approximately 3/4". So I'm a little confused why it dropped so much. I was looking at gear forums and came across the codes that identify which rear axle you have. I have always thought I had the 3.73LS since I have the FX4, but to my surprise I only have the 3.55LS.

So now my question is, are gears the problem, did the bigger wheels and added width contribute to the loss of gas mileage? Should I change my gears and if so, what should I go with? I read on one of the posts how to calculate which ones to move to, to get stock power back. According to those calculations I would go to the 3.73. Does all this make sense and sound right? Please help. Thanks in advance.

 

If you go to 3.73 gears from 3.55s your mileage will get worse as you will be turning a higher RPM for the same speed driven. With these slightly higher RPMs you might notice a small increase in power at lower speeds.

Your overall tire height and width is fairly similar on your last setup so I do not know why your mileage would drop that much.

 

what did the 17x8's weight and what do the 20x10's weight? My only guess would be a weight issue!

 

3.73s will likely give better fuel econonmy around town and on secondary roads. on the highway you theoretically should have better mileage with 3.55s.

 

You need to multiple the miles driven by the % increase between the old tires and new ones to get accurate mileage.

Around 4-6% (depending on brand) I think for your increase. So if you go 340 miles on a tank, multiply that by 1.04 to get 353.6. If that was a 26 gal fill up it would be 13.07 mpg vs. an adjusted 13.6 (@ 4%) or 360/13.89 @ 6%.

The reason for this is that you are turning more revolutions than before, now that you have a larger diameter tire.

 

Can you explain to me how increasing the load on the engine with the 3.73s is going to give you better fuel economy? And why this phenomenon only occurs at lower speeds?

 

Since you went to basically the same size tire I would think that the added weight from the 20" wheels is what is causing the loss of 1 mpg.
With that being said it usually isnt recommended to only go up one size when regearing. Going from a 3.55 to a 3.73 wouldn't be that noticeable and isnt worth the cost. Now if you wanted to you could go up to a 4.10 but if you do a lot of hwy driving you will loose fuel economy as your RPM's will be higher. I just swapped out my 3.55's to 4.10's but I am also running 35's on my truck. At 80 mph I am turning 2250 RPM's.

I would recommend adding a programmer to your truck. Then you can adjust for your tire size and also get a little better performance and fuel economy if you can keep your foot out of it after reprogramming it.
You can pick up a good programmer for $200-$400, where gears and install are going to cost anywhere from $1000-$1500.

Just my .02

 

It is easier on the engine to get the bigger tires going with taller gears. Yes the RPMs might be a little higher but it creates less strain on both the engine and the tranny. Every time you increase your tire size you decrease you "effective" gear ratio. If you have 3.55's with 30" stock tires and you increase to 33" your new effective ratio is 3.23. This causes the added strain to the tranny and engine when trying to accelerate or even maintain a constant cruising speed up an incline as it will gear search down shifting when it shouldnt.

 

As mentioned in one of the posts above, since you went from a 33" tire to a 33" tire, and just changed wheel diameters, it's most likely the added weight of that 20" wheel (giving you more spun weight) that is sapping your gas.

 

There is also other factors, such as air pressure. Too low a pressure on the new tire will definitely cause you to lose mpg.
Different load ranges of tires in the same size need different air pressure. On an F150, a P265/70R17 should be at around 34-35. An LT265/70R17 load range C tire should be run at 38-40 on the same vehicle. Load range D and E tires in that size would be even higher for proper inflation.
The tread width is another factor, although in this case the 12.50 and 305 are pretty close to the same, with the 305 being slightly narrower.

 

Isn't a 265/70/17 a 31.5" or 32" diameter tire?

 

Yes, I used that size as an example because you can find it in every load range there is.