As we mentioned last week, the Hyundai Genesis coupe was sold with a variety of final drive ratios due to the different engine and transmission options that were available. We’d previously had an opportunity to tinker with this aspect of Project Hoondy’s performance during our Cusco RS build, but due to a lack of knowledge we didn’t take advantage of it at the time. Now that we were set to tear into the rear end of the car once more, we were determined to have a go at the rear gears.
For anyone completely unfamiliar with final drive ratios and their impact on automotive performance, check out this article from Hot Rod Network.
When it comes to the tuning goals of Project Hoondy, we’re all about low-speed acceleration and drifting. For us, these things are what make a driver’s car really fun. Your milage may vary, but in our case we would be best served by a “shorter” (numerically higher) final drive ratio.
This desire stood in stark contrast to the final drive ratio that Project Hoondy came with as standard. It had a V6 engine and manual transmission, therefore it also had the tallest final drive ratio available for the platform: 3.538.
The complete set of final drive options that were available on the Genesis Coupe are as follows:
- 3.538 (V6 6MT cars from 2010-2013+ and I4 6MT cars from 2013+)
- 3.727 (V6 6AT cars from 2010-2012)
- 3.909 (I4 6MT cars from 2010-2012 and I4 5AT cars from 2010-2012)
- 4.101 (I4 5AT cars from early 2010, low production volumes)
- 4.181 (I4 8AT cars from 2013+ and V6 8AT cars from 2013+)
Due to all the engine and transmission combos that came and went in a short period, there are a pretty wide set of options available. We knew we wanted to go with a shorter set of rear gears, but exactly how short was a tricky question to answer.
Happily, GenCoupe forum super-user Snoopy had meticulously constructed an extremely handy .xml file that would calculate a speed vs engine RPM table based on all the significant variables related to transmission gearing and final drive along with rear tire height.
So let’s use this handy tool to have a look at the gearing for Project Hoondy in stock form with 3.538 rear gears:
You can tell this is very tall gearing straight away. The car is geared to do more than 180MPH in sixth gear! This is great for highway cruising and is the main reason why the car always got surprisingly good fuel economy at and above 80MPH. It’s also not bad for 0-60 time as the car reaches 60MPH just as it starts to run out of RPM’s in second gear.
Overall though, the 3.538 rear gears are terrible for acceleration below 100MPH, with poor surge performance in third, fourth, and fifth gear. Fifth gear in particular is near useless with the 3.538 final drive ratio. It’s too tall for good acceleration at legal road speeds, but shorter than sixth gear so not useful for high-speed cruising either. We found ourselves perpetually skipping fifth gear in day-to-day driving, using first through fourth to accelerate and sixth to cruise. This always seemed like a waste of performance potential.
So now let’s have a look at 3.727:
This is the awkward middle child of the options. It would result in minor acceleration gains, but nothing profound. Highway cruising speeds aren’t drastically affected and 5th gear is still pretty lonely up there. The one bright spot in the performance potential of this option is possibly the best 0-60 MPH time. 60MPH is just barely achieved at the tippy top of second gear, using the most of the engine performance without losing time to an additional gear-shift. 0-60 performance is just a number though, and doesn’t actually have that much bearing on real-world performance characteristics. We think surge performance, say 30-70MPH or 50-100MPH, is a lot more significant.
Which brings us to 3.909:
This, we think, is the best all-around option for a driving enthusiast of the Genesis Coupe. You get some great surge performance with a solid 30-70MPH pull using second and third gear as well as 50-100MPH using third and fourth gear. Fifth gear becomes relevant as well for decent acceleration above 60MPH.
Despite all these acceleration gains, highway cruising potential isn’t completely destroyed with the car doing more than 75MPH at 3000RPM in sixth gear. Not great for cruising at 90MPH, but perfectly livable. This option does require you to shift into third gear to complete the 0-60 sprint, but we think that matters little in the real world.
We’re going to skip over the 4.101 final drive option because it is so rare and head straight to the big daddy 4.181:
Now this is starting to get silly. You need to accelerate deep into third gear just to hit 60MPH. Surge performance is even nuttier than with 3.909. 0-60MPH performance is probably better as well since this has the same penalty of needing to upshift into third gear, but has harder acceleration along the way. Interestingly for performance metrics, 0-60MPH isn’t great but the 4.181 option offers up a spanking quick 0-100MPH ending at the top of fourth gear. Fifth gear is brought into full relevance as well with a nice RPM spread from 60-120MPH.
Things get pretty grim when we turn our attention to the top of the chart though. Sixth gear has become desperately short, with the car barely getting over 70MPH at 3000RPM. You can forget about economically cruising at or above 80MPH. In order to turn over decent highway fuel economy with this option, you need to be willing to sit in the slow lane with the semi trucks doing high 60's to low 70's.
We think the logical choice here is pretty obvious. 3.538 is just too tall, 3.727 is the odd man out, 4.181 is cool but crazy, and 3.909 is the clear Goldilocks compromise between acceleration and high-speed cruising. Any logical, reasonable car enthusiast would go with 3.909 rear gears and be happily secure in their choice.
Longtime readers of Project Hoondy should know where this is going.
As much as we try to base our tuning decisions on empirical evidence and logical reasoning, we do have a flair for the dramatic. We tend to veer towards the extreme side of the tuning options available to us, and this case ended up being no different. We deeply craved axle-twisting, tire-shredding, neck-snapping acceleration and we were willing to make sacrifices to have it. In addition, our taste for amateur drift events had grown into a full-blown hobby. High-speed efficiency just has no relevance in that particular motorsport, which made us all the more disinclined to consider it.
So it was with a manic gleam in our eye that we purchased a complete 4.181 differential “pumpkin” derived from a wrecked low-mile 2014 Genesis Coupe V6 8AT. This decision would result in the maximum possible twisting force delivered to to our rear tires which meant a punishing amount of stress for the rear axle assembly. We would need a bullet-proof rear end to stand up to repeated drifts, clutch-kicks, and all-around hooniganry.
We had a great foundation for said rear end in the form of our modified Genesis Sedan subframe, but it would take more than that to contain the fury of the mighty 4.181 rear gears. Tune in next week to see how we decided to round out our selection of parts here on Project Hoondy!