I need help with the math. I currently have a E4OD with 31 inch tires and 3:55 gears in my 94 Bronco. I do not have a tach, so I dont have a point of reference for engine speed. My question is this: If I switch from 3:55 gears to 4:11, what does that do to my Engine RPM at say 60 MPH. Will my current 3rd gear be like my future overdrive? When I'm cruising down highway in OD, she's barely breaking a sweat, but has NO POWER. It has been suggested to put 4:11s in but am wondering if OD will fall higher RPM or lower than my current 3rd gear. If anybody has a chart or can teach me the math, that'd be real cool.

 

im not sure if this will help, when i went from 3.43s to 4.11s in my Burban the OD rpms didnt change, that i noticed. all it did was give it more torque in OD. this is all just my opinion

 

you would be running higher rpm's with 4.10's than with 3.55's this chart will help you but with a auto trans there will be some slip causing the rpms to be a little higher a manual.
Gear Ratio Conversion Chart Guide [Gear Ratio Conversion Chart] - $0.00 : 3rd Strike Performance!!, Mopar, GM and Ford Performance Parts-Aftermarket Accessories!!

 

The equation is as follows:

RPM = MPH x AR (axle ratio) x ODR (overdrive ratio) x 339 / ATD (actual tire diameter) 29.5

RPM = revolutions per minute
MPH = highway speed, usually 60mph
AR = axle ratio (typically 3.55 in a stock Bronco with an 8.8 rear axle)
ODR = over-drive ratio (which in an E4OD is 0.73)
339 = Constant (this number is ALWAYS the same)
ATD = actual tire diameter not what is printed on the tire sidewall. Most tires are actually about 1.25-1.50 inches SMALLER in diameter than the nominal dimension shown on the tire. If you cannot find this information on the tire maker's website, measure from the center of the wheel hub to the edge of the tire perpendicular to the ground then double the measurement. NOTE: this requires proper tire inflation.

Or: RPM = 60mph X 3.55 (stock axle ratio) X 0.73 (OD ratio) X 339 / 29.5

This gives you: 1,786 rpm @ 60mph with 3.55 (axle ratio) and 0.73 (OD ratio) with a tire diameter of 29.5".

To get RPM's for different axle gearing simply plug in the ratio you plan on swapping to.

So: RPM = 60mph X 4.11 (axle ratio) X 0.73 (OD ratio) X 339 / 29.5

This gives you: 2,068 rpm @ 60mph with 4.11 (axle ratio) and 0.73 (OD ratio) with a tire diameter of 29.5"

To figure out how that will change when you change tire sizes simply plug in the appropriate tire diameter and recalculate.
NOTE: If your truck does NOT have overdrive, simply drop the overdrive part of the equation out and calculate with the rest of the information.

By the way, you can use the website calculators but few if any of them compensate for the actual tire diameter discrepancies. If you don't believe that makes a difference, plug in the wrong tire numbers and have a look at how different the numbers really are.

 

Charts are good, calculators are better:

Novak Conversions - Gearing & Gearing Math for Jeeps

 

Why give him a link for and send him away from this site when there is one on this site.


Automotive Calculators offer to you by Ford Trucks Enthusiasts | www.ford-trucks.com

 

This is the best one I have found and compares two setups side by side: Gear Ratio Calculator

 

I liked the side by side chart gfw1985. Thanks to all of you for your replies. Now I gotta find the right shop to do my gear swaps.

 

Thanks for that link. That's a better calculator than I thought was on here....but I like the Novak one better. Well, I DID like it better till gfw1985 posted a better one yet....

It's all good it seems....

 

that is infact a great calculator. It should be made into a sticky at top of page.

 

Glad you all like it. The drop down menu's have saved me a ton of research finding diff tranny and transfer case ratio's.