Gear Ratios & taller tires

[JCHH]

Does anyone have any recommendtaions for a gear ratio change when running 37" tires? I've got an 08 F-250 4x4,6.4L & 3.73's, obviously towing kind of sucks right now especially when getting 6mpg.

I previously owned a 2002 with the 7.3L & Banks 60hp chip, I was running 35" with 4:30's the towing was excellent and was getting 10mpg when loaded and about 14-16 mpg with no trailer.

I feel there was a greater power loss with the 7.3L/ 35" tires over the 6.4L/ 37" tires. I'm thinking of having either 4:30's or 4:56's installed; has anyone else ran this setup? -thanks

[mikey2000]

Here is a link to the "Ideal" ration chart. I would do the 4:30s.

4Lo.com :: Gear Ratio & Tire Size Chart

[dt75flh]

Im running 325-60-20 tires on the stock rims (f350 srw CCLB 6.4 auto 2.5 level kit )there about 35.5 tall....i changed out to 4.10 gears....raised my rpm 200 at 75 mph.....gps verified it corrected my speedo to read 2mph fast at 80mph (speedo reads 80 im really going 82)....if your gonna run 37" tires i believe 4.30 gears would put you back to stock..the diff between 4.10 amd 4.30's was another 100 rpm at speed....also measure your tires mounted to get a (real) height as some 37" tires are 36 or 36.5"

4.56 would put you even higher rpm wise if you drive alot of highway...if your towing heavy i prob would stay away from 4.56 cause the deeper you the gears the smaller the pinon.

also i used prescion gears and the new 08 trucks are so quiet inside i could hear a little whine. most after market gears all make gears for the rear 10.25 but not the 10.5 rear end.. the use a thicker pinon spacer to make up the diff....I changed the rear back out after 3k miles and went with ford factory for the rear which cost an extra 200.00 but there worth it. plus changed out and put factory bearings back in the rear as the koyo ones that came with the kit i had 1 go bad. the bearing kit had 1 timkin oinon bearing but the carriers were koyos? ...

hope this helps.

[dt75flh]

Oh yeah and you can get the 4.30's from ford also

[JCHH]

Thanks for the info. The chart and the equations at the bottom were helpful. I got a quote from a local offroad shop and they told me that they installed 4.56's on an '08 with Traction control and for some reason the truck wouldn't do over 40mph until the TCS was turned off Has anyone else heard of this?

[spence13e]

minor

how do you measure the height of your tires??? because it's not too hard to get some variations...

[dt75flh]

easiest way is to fill the tires to your road pressure....straight edge (ruler) across the tread the measure from the ground up....

some manufactures give you unloaded height check the web for your brand tires and go to spec's page.....but ive run into tires that were supposedly a certain height but when measured were off by as much as 1 1/2- 2 inches

[85e150six4mtod]

Originally Posted by JCHH

Thanks for the info. The chart and the equations at the bottom were helpful. I got a quote from a local offroad shop and they told me that they installed 4.56's on an '08 with Traction control and for some reason the truck wouldn't do over 40mph until the TCS was turned off Has anyone else heard of this?

Makes sense, as unless you program the ecu for the new speeds, it is going to read the old MPH at the New RPM and think a wheel is spinning.

[dt75flh]

And fyi the mpg has been beat to death on other threads ...but my prior 2002 7.3 4x4 f350 4" lift running 35" tires and 4.10 gears w/ banks power pack got better mileage than my 2008....

with the new emissions / dual egr's/ DPF and regen programing and ULSD fuel the mileage on most of the new 2008's and above are gonna suck....

havent heard about issues with the 4.56 gears and the tcs...but since you can get a ford f350 dually with either 4.10 or 4.30 gears you should be able to have ford dealer program the computer to tell it the new gear size. since the 4.56 arent an option that might be hard to do