M6 Transmission Swap to 2.66 - it's awesome! Long thread with pics

The new transmission is in! The 2.66 1st gear is amazing! It’s a totally different car and it is a travesty that it did not come this way from the factory.

I recently finished the install of a new RPM Transmissions (from Anderson, IN) “level 6” TR-6060 with modified gear ratios including the 2.66 1st gear. It is so much better to drive, more consistent to launch and roll away from a stop sign, and ultimately more fun around town. I no longer have to think or concentrate on perfect footwork to get the car moving, it just goes. I will update this thread if/when I go to the track with this new setup.

The car is a stock-power 2018 WBHC M6. Minor mods include Barton shifter, MT ET streets, LMI intake, and a few other small cosmetic things. The best pass I ever got on the stock transmission was 11.48 @ 120.8mph.



This is a long thread all about the negative effects of the OEM M6 hellcat gearing, my journey to discovering the solution, and details of how I made the swap. I appreciate any comments or feedback!

For all M6 hellcat drivers, we are intimately familiar with the “crazy tall” 1st gear ratio in the TR-6060 – which may be OK in the Viper (the only other car in the whole industry to use the TR-6060 with the 2.26 1st gear), however for the heavy Hellcat it just plain sucks. Whether you’re puttering around town, or trying to get a decent 60-ft time at the track, the practical effect of the 2.26 1st gear is very low torque at the wheels when the car starts to move from a dead stop. Yeah it’s fun going 60mph in 1st gear, but if you’re constantly bogging or over-revving to get moving, then you’re already disappointed before you even get to 10mph.

I noticed it immediately when driving my new 2018 WBHC off the dealer lot for the first time – why am I constantly over-revving or bogging when driving away from a stop sign? Why can’t I just have smooth and easy starts? I’ve been driving manuals for all of my 23 driving years – German cars, Japanese, Corvettes, small trucks, 2500 trucks, shifting with my left hand in the UK, Nürburgring track car rental, even an 8-seater Mercedes sprinter van in Spain – and never had this issue before driving the M6 hellcat.



At first I thought I needed more seat time with the hellcat, and I would eventually “learn” how to drive it properly. Well… 4 years, 12k miles and 40 track passes later didn’t help – I still struggle to get consistent starts (at the track or on the street) and it really takes away from the driving experience in my opinion.

The issue was never more apparent than when I raced an M6 392 Challenger at the drag strip a couple years back (both cars were stock with 20” drag radials) – we both got a decent launch and were dead-even on the 60-ft times. I obviously pulled away on the top end, but we were evenly matched on the low end. The reason is simple – although the 392 has roughly 30% less HP/TQ, it’s 1st gear has a 30% torque advantage. Of course there are other factors that help the 392 like weight and axle ratio – but that gearing advantage off-the-line is huge.



So why would Dodge put such a crappy 1st gear set in an otherwise perfectly spectacular car? I don’t know for sure, but there are a few theories. I speculate that it’s the same reason you can’t get a M6 with the Redeye challenger – the 650 ft-lb torque output of the “base” hellcat engine matches the maximum rating the TR-6060 in factory form. First gear is the highest load case from a torque perspective on a transmission (not counting the reverse gear, but who’s doing hard launches in reverse??). Tremec does not offer a higher torque rated TR-6060, and that also drives the requirement to pair the most torquey engines with the numerically lowest gearset offering – and Dodge isn’t about to develop their own transmission or modify the Tremec on a relatively low volume option of a mass produced vehicle, it doesn’t make financial sense in my opinion. Forum user @charger440rt summarized it nicely in this thread: “I assume Dodge went with a 2.26 in 1st to increase the torque capacity on the transmission, as the choice of 1st gear ratio is directly proportional to what a trans can ultimately withstand.” That explanation makes sense to me, however it doesn’t explain how the 2.90 reverse gear is OK, or the 2.66 1st gear in the 2013 Mustang GT-500 with a 625 lb-ft engine, so maybe we’ll never know for sure why the M6 Hellcat came with the 2.26 1st gear set. Some people also speculate that Dodge wanted the best 0-60ft time possible for “magazine racing bragging rights”, however that doesn’t make much sense to me because the A8’s can smoke the M6 0-60 time in factory form and they have a much lower (numerically higher) final drive ratio in 1st gear. Maybe Dodge was just lazy and “copy & pasted” the same transmission from the Viper and couldn’t be bothered to research / optimize the gear ratios for the different platform? Perhaps that last theory is the most plausible as they obviously know that manual transmissions are on their way out forever, so why waste any more development time on them?



So why go through all the hassle of changing the transmission gearing when traditional wisdom says you can just swap out the rear axle ratio to get better gearing? That doesn’t help you much on the M6 hellcat for a couple reasons. Firstly, the M6 hellcat already comes with a 3.70 axle ratio, and the only other option for stock replacement is 3.90 – that’s not going to move the needle far enough. Sure you can go custom 9” setup, but that’s not a good or cheap option for those of us who want to keep the car “stockish”. Even if you manage to get a 4.3 or higher rear end, cruising at 80mph in 6th gear would result in 2500+ RPMs because the 6th gear ratio is also poorly spec’d in the M6 hellcat in my opinion (it should be numerically lower). Of course we’re not trying to optimize gas mileage in these cars, however I would like to be able to drive on the freeway for long distances without unnecessarily running that big powerplant at 2500+ RPMs continuously. So that’s why the only realistic option to improve the driving experience is changing the transmission gearing itself.

And about that 0.63 6th gear ratio in the M6 hellcat… it is a very strange choice when you consider that the M6 392 cars come with the 0.50 6th gear ratio. The 0.50 is better for cruising on the freeway, why put a numerically lower 6th gear in a car with more than 200hp less output? Shouldn’t that be the other-way-around? More evidence of why Dodge perhaps just slapped in the Viper trans into the M6 hellcat without even thinking about the gearing. To be clear, these gearing choices make perfect sense for the Viper – a close ratio trans optimized for high-speed road racing in a lightweight sports car – what could be better? It just doesn’t make any sense for the heavy muscle car hellcat.



In the below charts you can see the numbers – OEM gearing and shift RPMs for the Hellcat M6, Hellcat A8, and Scat Pack M6. On the A8 Hellcat and M6 Scat Pack, the final drive numbers are comparable – around 12 for 1st gear, and under 2.0 for the top gear. This gives the best spread for a heavy muscle car – lots of torque off the line, and low RPMs for highway cruising. The M6 hellcat numbers are much different – final drives of 8.36 and 2.33 for lowest and highest gears respectively. For a practical comparison, try driving and launching an A8 hellcat or M6 scat pack while always starting in 2nd gear… that’s what the M6 hellcat acts like in 1st gear!



For my new transmission from RPM, you can see the chart at below left. The new 1st gear final drive is much lower (numerically higher), and the top gear final drive ratio is closer to the A8 cars. The chart on the right shows how I can get just a little more by going to a 3.9 rear end – I’m thinking that this might be the perfect sweet spot for mostly street driving.



I found inspiration for this project based on reading the experiences of forum users @catdodgehell and @16GoManGoHC , among others who blazed this trail and showed us it's possible to lay down low 10s and high 9s, bringing parity with similarly-modded A8 cars. I don’t know if I’ll spend much time at the track with the new tranny, I opted for a new factory clutch (instead of aftermarket) and I don’t really want to be upgrading axles and the driveshaft. With the new built trans and 2.66 first gear, I image the weak point in the system is now the driveshaft? Not sure about that.

I initially wanted to build the transmission myself with upgraded components. I ended up ruling out this option because of how hard it is to find detailed instructions or writeups online. I taught myself how to build LS racing engines with the vast information available online, however the same level of detail is clearly not available or publicized for performance TR-6060 transmissions. In the few instances where I could find apparently reliable information, it was for Corvette or Camaro versions of the TR-6060. For this reason, I ended up ordering a new “level 6” built transmission from RPM transmissions in Anderson, IN. I paid full price and plan to keep my core transmission as backup, sacrificing the $2500 core charge in the process. Here is where my factory transmission will live long term: