Axle ratio vs Wheel size
 

I know there are a lot of other posts discussing thissituation, but I didn’t want to hijack someone else’s specific posts.


I have been readingall of the posts pertaining to selecting the proper Axle Ratio on a new F250Gasser.The general consensus seems to beto go with the 4.30 E-Locker rear end, if you are going to be towing, but thenthere are several posts that start briefly discussing different configurationand the MPG debates. Some posts from youknowledgeable folks allude to the fact that effective final drive ratio of thetruck is dependent on the wheel/tire size as well as the axle ratio. The pieceof the puzzle mentioned in passing or missing entirely is wheel sizes and theireffect on performance and MPG.


From the posts I haveread here and elsewhere, my assumptions on the situation are:


4.30 axle with 17” wheels = highest torque output to the groundwith lowest MPG.
3.73 axle with 20” wheels = least amount of torque outputbut highest MPG.


With all other combinations somewhere in between the above.

My questions are:
How much effect doeswheel size actually have on the final drive ratio?


What would the best combination of Axle ratioand tire size be for towing a 10,000 trailer, to give the best combination ofperformance and MPG?


How do you calculatethe effective drive ratio for a specific axle/wheel size combination, and howdoes it effect RPM at 65 MPH?

Unfortunately, the information you read regarding wheel size is absolutely incorrect. Not sure who told you that.

Wheel size has nothing to do with final drive ratio. It's the overall tire diameter that will help determine that, and you can run the same tire diameter on a 20" wheel that you can run on a 17".

So let's say you're comparing 17" wheels sporting LT285/70R17 tires, and 20" wheels wrapped in LT275/60R20. Both tires will have an overall diameter of 32.7" - 32.9", depending on manufacturer. Basically they are identical sizes. So the overall revs per mile will be the same, meaning the final drive ratio for each set is identical if you were to swap them back and forth on the same truck.

Basically order whatever wheel size you want. What you'll need to do is decide the rear gear ratio when ordering, as that's far more expensive to change later. Higher numerical/lower ratio will improve towing performance but reduce fuel economy. Lower numerical/higher ratio will increase economy but may make a noticeable difference in towing. If possible, I would suggest a test drive of similar trucks with different gear ratios to see for yourself. However, 10,000 lbs is nothing to sneeze at, so the 4.30 might be the more optimum choice, depending on how often you plan to tow that heavy.

Yes it's correct that the tire diameter is the variable in this issue. But I think I know what the OP is getting at. From what I recall when I bought my 14, all the trucks on the lot with 20 inch wheels had taller tires. They are almost 35s. A truck with 17s have smaller tires, around 32 inch. Off the lot a truck with 4.30s and 17 inch wheels will pull harder than a truck with 3.73s and 20 inch wheels. The thing to look at is what each truck is rated to tow. Choose a truck that meets the weight you want to pull. The difference in true final drive is not as noticeable between tire size as it is between gear ratios.

Giving all the variables, if you know what rpm you want to run at x speed there are calculators out their to help.

Here is one Gear Calculators Courtesy of Richmond Gears

For a 10k trailer and a 6.2 that makes it's power in the upper rpms the 4.30 is a no brainer. Your fuel costs towing with the 3.73's would probably be higher vs the 4.30. 17" wheels are a more economical choice down the line when you need to buy new tires.

Thank You.
 

Gentlemen, Thank you for the information!


I have been lurking the local dealer and they have a wide range of Wheel/Tire sizes, so from your responses I assume it is purely an aesthetic choice vise a performance issue.




Pocket: I was trying to find a local 6.2 with 4.30 rear end to test drive but the nearest one is 2 1/2 hours away. The local dealer won't bring one in from another outside dealer unless you are going to buy, so I will need to drive to the other dealer to test drive one. The only 6.2 gasser the local dealer has on the lot is a Reg. Cab with 3.73 rear end.


Thanks again for the info.