so i have a theory and I am asking for some help. I have a 2004 Lariat Scab 4x4 3.73 LSD all stock (for now). I want to lift it and put 35" tires (more like 34.6" to be more accurate) and I know that will decrease my gearing. i do mostly highway driving so I am just curious what the 35's would drop my gearing to. 3.54? lower? i know there has to be a calculation or a website that will calculate it for me, but I don't know of any. my theory is that since i do so much highway driving, the 35s might actually improve my gas mileage since it would drop the gear ratio lower than 3.73. i know the acceleration will suffer, but that would be ok if i can actually improve my overall gas mileage. i think my mileage will suffer in the city and getting the truck going from a stop, but i think highway driving will balance it out overall. can anyone help me figure out the gear ratio after the conversion? any thoughts on my theory? thanks.

 

Your theory on the highway is only partly right. The load on an engine, just not the RPMs, will determine how much gas you use. 35 inch tires will put more of a load on your engine because of the added weight. So, just because your RPMs are lower, it doesn't mean your engine isn't working as hard. In fact, it's working harder than it would be turning the smaller tires at a higher RPM. Your theory is right in that you will get more distance per revolution of tire with the 35s, and if gas consumption by the engine was the same, you'd have better MPG. Unfortunately it's not.

About the gears...the easiest way is to take the percent change in tire height, subtract it from 1, and multiply it with your current gear setup. Something like this (assuming a stock 31.6" tire):

(34.6-31.6)/31.6 = 9.5%

(1-.095)*3.73 = 3.36 gear ratio

Gear ratio you should go to: (1+.095)*3.73 = 4.08

Of course, because bigger tires weigh more, it would be smart to go up to the next size that'll fit in your axles.

You will feel a performance change if you go to 35s with stock gearing. I did and now I'm looking to go to 4.10s.

Good luck -

Mike

 

how much are the 4.10s going to run you?

 

I've seen others say about $1500...I don't really know because I haven't priced them. Just bought a house and money isn't available like it usually is

Mike