i got an 04 f150 lariat 5.4 4x4 with 3 73 gears with 35 inch tires.. my question is, is there a formula to get the overall gear ratio about the same as stock. like what size gear will i need to accommodate the 35 inch tires.. i searched around and say like 4.56 gear but would like sec opinions.


i bought a gryphon last week and i plan on getten some custom tunes in the up coming weeks and also. id like to get the gears and electric fans to help out with performance and get a lil mpgs back.

what i got so far is K&N cold air intake
magnaflow cat back
leveling kit
35 inch tires
soon to be gryphon.

thanks guys!

 

Calculator:

http://4lo.com/calc/gearratio.htm

Use 0.7 for transmission ratio and 1 for transfer case ratio. You will need to know the height of the stock tires.

 

perfect! thanks

 

It's not quite as simple as that, since most just use the assumed total outside diameter of an uninflated tire. If you use the revolution per mile data, you can get a much better estimate. All manufacturers publish the data and what it means is the number of times a tire makes a full rotation in a given mile. The tires that came on my 4x4 were 685 rev/mi. (according to tire rack for BFG 265-60/18), which equates to a "loaded radius" of 14.72". If you take half of the claimed inflated diameter it would be 15.3". The larger the sidewall, the bigger the differences usually are, so this is why the rev per mile data is more accurate.

If I look up the rev per mile data for a 325-65/18 BFG all terrain, I find rev per mile data of 601 and an inflated diameter of 34.5". Applying the same math to the rev/mi. data I get a loaded radius of 16.78" (vs. 17.3" inflated). To compensate you would take your existing ratio and divide it by the old tire radius multiplied by the new tire radius (3.73/14.72x16.78 = 4.25).

Available ratios are 4.10 and 4.56. Unless someone started making reverse cut 4.30s. you'll have to pick one or the other. 4.10s are only sightly closer, but 4.56s would not be an unreasonable compromise (7% increase in rpm vs. stock). If you assume you are at 2,100 rpm at 70 mph, the 7% increase would only put you at 2,247 rpm at the same speed. Likewise 4.10 would drop you down to 2,025 rpm. With gas getting up there, I'd probably chose 4.10s.

If you just took the uninflated tire diameters for the calculation you would get (3.73/30.6*34.6 = 4.23. It's not that different, but we are using precise measurements stated by the manufacturer as opposed to rounded off whole sizes. (3.73/30*35 = 4.35).

More information than you probably wanted to know, but the examples I gave would have resulted in bigger differences with a 15" rims or larger tires (i.e. larger side-walls) for actual vs. assumed diameters.

Now keep this in mind too. If you're doing this to pick up mpg, a 1 mph increase is going to take you 100,000 miles to pay for that gear swap at $4/gal. gas. A 2mpg increase will take 50,000 miles. Sometimes the things we do for gas mileage are cost beneficial. Now if you want better drivability and performance, that's a whole different reason.

 

thanks you windsor for breaken it down for me..

im thinken im ganna do the 4 10s... and im not doing it all for mpgs i really wanna do it to lessen the strain on the trans and have better take off speed... i just wanna get everything back in check.

 

3.73's to 4.10's are not worth the expense. Either get 4.56's or do nothing.

 

well im not sure if im keeping the 35s next time i buy tires i might get 33s since they r cheaper and i dont offroad much.

 

I have a 2007 FX4, installs 4.56 and I must say that the difference in the response is truly significant, very positive and excellent in my opinion, the only thing in the country where I live the cost of petrol is very cheap and not a problem for us, but I can add the following, if you just want to keep performance as close to OEM go with 4.10, but in the future want to go with 37 ", please go to 4.56, not regret.

 

what size tire and how much mpg did you drop?

thanks