acdii

Keep the wheels you have, you wont gain anything. Going down a size will give you taller gears. Going to larger wheels will effectively give you shorter gears.

If you are thinking of changing tire sizes then go with 3.55 gears and 18" wheels. This should give you good towing ability without sacrificing regular drivability. If you stay with 17" then go with 3.73 gears.

msgtord

Your backwards in your thinking. Or I'm misunderstanding something. A tall gear would be a 3.31. A shorter gear would be the 4.10. The 17 inch would be for more payload and pulling power. It is why Ford puts 17 inch on all vehicles with the heavy payload packages, including the Superduty.

PerryB

Yeah, someone's going to have to explain that one to me as well. Also, one of the reasons for increasing wheel diameters is increasing brake size. Will 16's even fit?

jp360cj

I don't believe 16" wheels fit over the brakes. Also, wheel size is irrelevant to gearing. However, going to a smaller diameter tire would effectively be the same a lowering the rear end gear. Even if your stock tires are 32", you'd have to go down to about a 29.5" tire be equivalent to a 3.31 to 3.55 swap.

You would probably notice the 3.31 to 3.55 change more in the lower transmission gears than you would on the highway. But if you can get it cheap enough, then it wouldn't be a bad way to experiment (assuming you are looking for better towing performance). If you are just looking for the gain in GCWR, you are most likely barking up the wrong tree. Can you even hit your current GCWR without exceeding GVWR?

acdii

With 3.15 that I had, going from a 20" to a 17" raised my cruise RPM up 400RPM. Going from 17" with 3.55 to an 18" with 3.73 would be more efficient yet give better towing gains.

The overall diameter of the tires is not 1:1 between wheel sizes. Close, but not perfect. You can get a much better LT tire in 18" size than you can in 17". More options. A 1/2" difference in tire diameter can be quite a difference in the RPM range. It is all about RPM's when it comes to either towing, or pushing it as fast as it can go. With towing you want to get the engine into it's torque as soon as possible, so raising up the engine RPM's at a given tire rotation is the key. A tall geared truck would be lugging where a short geared one will be in its power band.

IF all the truck did was tow, then pop 4.10 and small wheels on and go for it, but if you do more daily driving, and occasional towing, you want to compromise by putting a gear and tire combination that gives you best of both. 3.55 with 17"s or 3.73 with 18"s gives two of the best options for pulling power and fuel economy. They both put the 60 MPH RPM right around 1600-1800 RPM.

kmlacroix

16" probably will not fit, unless you do a crazy offset.

BRUTE448

i got 3.73 gears as i knew i was going to upsize tires. i had the factory 20's. i sold them and got the FX4 18's off craigs list. when those tire wore out i got 275-70-18 BFG's. this reduced my effecit gearing to about 3.60 or so. I knew that was going to happen and thats what i planned for when i got that exact truck vs getting any trcuk on the lot (mainly 3.55 gears) and slapping 35"'s on it.

Dirttracker18

Let's clarify a few things here that are either incorrect or misunderstood.

Changing wheel size has ZERO effect on towing ability. It is the overall tire diameter that makes any change, not the wheel size. You can have the same size tire diameter in multiple wheel sizes so that is irrelevant. If you changed wheel size and saw a change in RPM then you changed tire diameter as well and that is where the difference lies, not in the wheel size.

Changing to a smaller diameter tire can give you a better towing ability and jumping up to a larger one will make towing more difficult.

Next, your assumptions and descriptions of what changes are being made are partially correct at best.

Gearing is not so much about RPM range, that is actually only a small change and and often misunderstood and misquoted issue. While the change in RPM does make a difference and get you into more torque, the big change is mechanical advantage.

Adding shorter gears (higher numerically) is like using a longer lever. A good comparison is when you are attempting to remove lug nuts and you get a longer breaker bar or add a length of pipe to the bar. By going deeper with the gears (again higher numerically) you are giving your engine a longer bar to work with and thereby make better use of the available torque. The small increase in RPM is only a small side benefit.

Now adding larger diameter tires is taking away some length of the bar or using a shorter bar. In essence you are taking away some gear. Add to this the increased weight of the tire (this then requires a longer lever again) and you can start to see the issue. On the flip side a smaller diameter tire is like giving the engine a little longer lever (or slightly deeper gears).

Finally lets look at your assertion that a 1/2" difference in tire size is significant.

A typical change for our trucks is to go from a 275/65-18 tire to a 275/70-18

275/65-18 = 31.1" diameter
275/70-18 = 33.2" diameter

That puts the tire at slightly over 1" total difference in diameter.

Using an effective gear ratio calculator the effective change, assuming we started with a 3.55 gear is effectively down to a 3.43. While statistically different it is likely not enough for most people to notice the difference assuming the tires weigh the same.

If we look at a situation similar to what your's was, changing wheel size we can see what the difference would be. As you noted, it is difficult to get exactly the same size tire diameter when changing wheel sizes.

275/55-20 = 31.9"
275/50-20 = 30.8"
275/65-18 = 32.1"

There is a difference between them and that will show up in the tach and the speedo.

acdii

Actually it is the tire diameter I was pointing at. I just compared(didn't have time earlier) the OEM 17 to 18 and the overall diameter is practically nothing, but 20 vs 17 is quite large, 4.61% difference. so based on that is what I was referring to. So going to 3.73 and 20's would compare better to 17 and 3.55 to get the same torque advantage and fuel economy. Skip the 18's they are irrelevant now.



245/75R17 w 3.55:1 gears = 275/55R20 w 3.71 gears. based on a gear ratio calculator. This is what I was trying to point out.

Now if you put 3.73 gears on with 17" wheels, you can go to a 3.90 ratio with the 20's. You MPG will suffer though going that low as a daily driver.

Dirttracker18

Again, you are talking wheels size here which is irrelevant.

Tire diameter is the only thing that matters.

A person could go from a 31" tire on a 20" wheel to a 35" tire on a 18" wheel or vise versa.

Focus on overall diameter, the only reason to change wheel size would be that you can not get a small enough sidewall to get to the size you want.

Your example uses two completely different sized tires in both diameter and width. Apples to oranges regardless of wheel size.