Trying to figure out what size gears I want to go with. Figured id get yalls opinion! Its a stock 1993 5.8 ford bronco and just put a full set of 35x12.5 tires on there. Just feels like I have less power! Any thoughts?

 

A good rule of thumb is to multiply .12 by tire diameter.
.12 x 35=4.20
For mileage go with 3.90
For close to factory performance go with 4.11
For a lil more pulling power go with 4.56
For all out awesomeness that you wont be able to afford to drive much go with 5.13

 

4.56 sounds like what I want!

 

The Bronco in '93 came from Ford with 3.55's stock with an available 4.11 special order. With that in mind and the fact that tire sizes are nominal not actual, the math to calculate what the engine is actually turning (within a couple hundred rpm in the highest gear) is pretty straight forward. It looks something like this:

60 (highway speed in MPH) x 3.55 (axle ratio) x .74 (over-drive gear ratio) x 336 (this number is a constant) / 29.5 actual tire diameter (which is always slightly smaller than the nominal size)

So, the short version for a stock '93 Bronco with OEM axle gears and an automatic transmission looks like this:

60 x 3.55 x 0.74 x 336 / 29.5 = 1,795.26 rpm

To figure out what makes sense when changing tire sizes and gear ratios, simply change the numbers that represent the way your truck is set up or could be. The idea being that Ford engineers already figured out a good performance-to-fuel economy engine speed so all you are trying to do when you make changes is to keep those RPMs as close to the number above as you can. For a little more power, let the RPMs be a little higher than stock. For better fuel mileage leave them the same or just slightly lower than stock. Always remember that with an over drive transmission, you can get away with slightly taller (bigger ratio) axle gears and not suffer as much loss in mileage because the OD gearing keeps your RPMs proportionately lower in the higher gears. This leaves you with higher RPMs in lower gears where a truck needs it. Be careful going too tall though or you may be able to pull your house off its foundation but you will burn 30 gallons of fuel between the gas station and the house!

A couple of notes:

If your '93 still has its stock automatic transmission, leave the overdrive ratio in the equation. Other automatics and manual transmissions will either not need it (non-overdrive units) or the over drive ratio may be different.

The 336 number is ALWAYS the same and must ALWAYS be included in the equation.

Actual tire size can be calculated by looking up the tires on the manufacturer's website or you can simply measure from the center of a wheel hub straight down to the ground then DOUBLE that measurement. Just make sure the tire you measure is properly inflated. Don't measure across the tire because the weight of the truck displaces the air in the tire and expands the tire around the perimeter everywhere but the point where the tire touches the pavement. As mentioned, you can use the nominal size but your RPMs will calculate LOWER than they truly are.

 

i am running 4.88's and love the setup. its great around town and even better on the hwy. with the overdrive i rarely drive above 70mph and sit around 2500rpms in OD. this allows me to pass and such with out having the tranny downshift. this gear also helps when towing and of course off road. and if for any reason i go up to a 36 or 37" tire i will still have enough gearing.