2018 Mustang GT Steeda Drag Car, Part 3: Rear Gear Ratio

December 18, 2017 3:09 pm

As we’ve mentioned, we want to make an example of this 2018 Fastback. We want it to go fast and we want to do it on a modest budget. This car is going to be a demon on the drag strip, but a family-hauler on the street. After a day of Test & Tune at South Georgia Motor Sports Park, along with a week of part-development in Valdosta, GA, our silver 2018 made its way back to South Florida, just in time to miss the Georgia snow!

With its better looking, red, road racing friend (above) drawing a lot of attention, the silver Mustang craved some kind of upgrade. Being the good owners we are, we listened to the car and began by adjusting the rear gear ratio. The car is a 10R80 automatic with a factory gear ratio of 3.15, but we wanted to gain quicker access to the powerband, especially since it’s a drag car with 10 forward gears! “Gear ratio,” simply refers to the number of times the driveshaft needs to rotate for the wheels to turn once. A higher gear ratio gives the engine increased leverage and thus quicker acceleration. Ultimately, altering the ratio simply means manipulating your available torque. Higher ratios sacrifice a bit of top speed and gas mileage, but the car is quicker off the line. With all of the above to consider, we settled on a 4.09 ratio for the rear gears on this one. This was chosen by going over the car’s powerband and analyzing where we think it will hit top speed once it is all said and done. While a 3.73 gear ratio might be better suited for the car right at this moment, we have goals of this car running in the quarter-mile with speeds in the mid- to upper-120s and we know the 4.09 gear will be perfect to grow into.

So as the 55°F (in South Florida?!) morning air slowly warmed, one of our mechanics, Steve, put the car on the lift, prepping it for the switch. With a bit of sweet talking, Steve got the rear differential drained and out of the car in no time. Despite it being a job that requires incredible precision, this rear diff was no match for Steve. The gear ratio had been changed and the diff put back in the car faster than this article was written. With the car back in one piece, we’re just waiting for new wheels and tires to come in before moving on to the suspension and exhaust systems.

Our goal with this project is to showcase how fast you can go in the quarter mile, without breaking the bank. We want this to be the car that you can drive every day to work and back, then take to the strip on Friday night and put down impressive numbers. We’ll accomplish this with simple bolt-on parts, made right here in the USA! Stay tuned for more updates on our 2018 Mustang GT drag build.