First Impressions of Dyna R452 5th Gear

TL;DR - The new 5th gear is gutless, but it does seem to have improved my gas mileage significantly. Maybe.

Can installing this save you gas money?

Not more than a few hours after posting that I installed the Toyota Dyna R452 5th gear in my Toyota R150F transmission, I started getting questions about how it affected my fuel economy. At that point, I'd only driven it a few hundred miles, and of those, more than half of them shared a tank of fuel with my stock 5th gear as I'd driven across the state to do the swap.

I always get a kick out of guys who post a picture of their gauge cluster showing 35 MPG to their favorite social media platform, with some caption like, "What's your gas mileage?" Usually, it turns out that these folks have travelled 12 miles, downhill, with a tailwind. It's total garbage.

Anyway, I figured that a 1,000 mile journey from Seattle to Las Vegas for our Return to the Desert was a reasonably good first start at figuring out whether the new 5th gear in the Tacoma was going to result in a MPG boost or whether it was going to be so gutless that I'd constantly be downshifting into 4th gear, negating any gains from the higher overdrive.

In the end, it was a little of both, and probably too soon to tell whether the modification was a resounding success. Still, it was definitely enough to get some first impressions, so with after an extremely long preamble, here they are:

Pros

The swap itself was successful. This may seem like a strange one, but not knowing anyone else who'd done the swap, I was worried that the Dyna R452 5th gear might not work correctly in some way - perhaps the synchros wouldn't work, or there'd end up being some problem with the gears on the shafts, etc. - but none of those worries have come to pass.
- Shifting into gear is easy. The very first few times I shifted into 5th gear were slightly harder than my original, worn-in 5th gear, but after driving a few hundred miles, there's no distinguishable difference in shifting difficulty/ease.
- Noise is the same. There's no distinguishable difference in noise between the two 5th gears. This wasn't something I'd thought of prior to the swap, but was something Zane @Speedytech wondered about when we were doing the initial test drive.
Cruising RPMs are significantly lower. This was what I was hoping from the swap, and without a doubt, my RPMs are a lower now. At 65 MPH, I see a little more than 500 RPM reduction, and over a range of speeds, I see a ~16.8% reduction in RPMs
4Lo is a little more versatile. One of the things that bugged me about 4Lo - ever since I installed 4.88 gears - was that the max speed - or at least, the speed where the engine RPMs start to get loud/overwhelming - was around 25 MPH. Swapping 5th gear raised that speed to a little more than 30 MPH, without affecting the lower end. 4Lo- 1st, 2nd, and 3rd still reap the benefit of 4.88s, while the upper end now feels more like 4.10 gears in the diffs. It's fantastic.
Gas mileage seems like it is probably better. I've got this listed in the pros for now, because better gas mileage is a good thing, but even with 1,000 miles on the odometer with the new gears, I think I still have more to learn here. For details, see "More on Gas Mileage," below.

Cons

5th gear is gutless. I've always felt that 5th gear on my 1st gen Tacoma - even with 4.88 gears and 33" tires - has been gutless, but the R452 5th gear is even more so. Mathematically, it's ~16.8% more gutless, but I was surprised how much more gutless that feels. Anything but very slow acceleration is difficult. This is true across the entire speed range, because as speeds increase to the point where the RPMs should allow faster acceleration, wind resistance has also increased (proportionally to speed-squared), making it harder to accelerate further. It really feels like 5th gear has no power band, now.
The usable RPM overlap between 4th and 5th gear is smaller. With the original 5th gear, speeds between approximately 45-50 and 70 MPH seemed to be where I could reasonably transition between 4th and 5th gear without feeling that I was lugging 5th at the low end or overrevving 4th at the high end. With the R452 5th gear, the upper end of that does not change, since beyond 70 MPH, 4th gear still feels overreved. However, the lower end, where it doesn't seem that 5th gear is lugging, has increased to 55-60 MPH. This is not a huge deal, but simply underscores the gutlessness of the new 5th gear.
Bigger tires would be untenable. I've never seriously considered running 35" tires - despite how cool they look - because I care too much about fuel economy and I worry that with larger tires, I'd push the Tacoma in ways that would break a lot of other stuff. But, it is clear to me that with the new 5th gear and 4.88 differentials, running 35" tires would be impossible because the RPMs in 5th gear would be much too low. In order to make it work, I'd need to upgrade my differentials to 5.29s.

More on Gas Mileage

Gas mileage is always top of mind with a modification like this. As I mentioned earlier, I don't feel confident making a final call on the topic, even with just more than 1,450 miles on the odometer, but I'm happy to share the data I have so far:

5th Gear	Year	Miles	MPG	Average MPG
R150F	2018	10,787	17.34	17.48
	2019	20,383	16.96
	2020	17,695	17.50
	2021	29,529	17.80
	2022	24,403	17.61
	2023	10,662	17.35
	2024	8,061	17.64
R452	2024	1,466	18.24	18.24

Highway fuel economy over the years.

On the surface, that might seem like a clear win for the new 5th gear, with (18.24 - 17.48) / 17.48 = 4.3% fuel economy improvement, but here are two caveats:

MPG could be even better if I hadn't installed new wheels/tires. A few weeks before installing the Dyna R452 5th gear, I installed new wheels and tires. As I noted in my First Impressions - Sequioa Wheels and Kenda Klever RT Don't Fit , I added approximately 20lbs to each corner of the truck, and I could definitely feel the extra weight when starting the Tacoma from a stop. I only ran one tank of gas on the highway with that setup (16.80 mpg), so it's hard to say how they affected the overall fuel economy of the Tacoma. At the very least, it's likely that they make the 18.24 mpg number lower than it would have been, had I stuck with the lighter wheel/tire combo.
MPG could have been worse if I'd had a headwind. On my 1,000-mile trip from Seattle to Las Vegas, I believe that I had a tailwind more than I had a headwind, though I had both, and also had a crosswind part of the time. Whatever the percentages were, I feel like there were enough miles/trips in years prior to 2024 to average out the wind situation, but 2024(R452) doesn't yet have that luxury. If anything, I would guess the 2024(R452) MPG number is a little high, given what I perceived of the wind.

Do those two caveats cancel each other out? Does one play more of a role than the other? It will be hard to tell with the current set of wheels/tires installed, and I'll only ever figure out a comparable number if I revert to my previous wheel/tire setup once I wear down the Klever R/Ts - but only time will tell with regards to the wind. Naturally, I'll keep monitoring the situation, and over the coming months and years, I hope to come up with a definitive answer!