Re-gear 2023 SR 2.7 2WD. I have questions...

I made an honest attempt to search. There's a lot of confusion and conflicting info (2005-15 diffs in the 3rd gen section...). Lots of tire size requirements adding a variable I'm not concerned with. I'll run 245/75r16 happily.

Anyway.

I realized I've owned a base 4 cylinder for the last 4 generations ('93, '98, '12 and now '23). That base engine is why I bought this truck. Another old-man truck for the next 20 years.

I've been through 4 iterations of transmissions too. This 6-speed auto is definitely the worst.

I've been driving it in S allowing it to 4th gear on two-lane Missouri state highways and county roads. 45-65mph, and it's like my old trucks. Hums right along in it's 2500-3500 sweet spot.

4-lane highways and interstates are a different story, as everybody knows. I tend to bump it to 5th and let it run 2500 at 77. Seems it should do that in 6th. I'm looking at re-gearing the diff and dial this truck to my region and purposes.

I looked and the door code is A07A.

Best I can tell that's an 8" with 3.90 gearing, but I'm not sure that's right. Some sites insist all 2016 and up are 8.75" (then there's 8.2 and 8.4 stuff...). Some insist all 2WD autos get 4.30

WTF is my diff?

I'm considering 4.88, 5.29 and (seriously) 5.71.

Anybody got an opinion?

 

Your truck should have 4.30 gearing, and the 8" rear diff. All autos besides TRD OR and Pro are the 8" rear diff. All 2.7L autos are 4.30. All manuals are the 8.75" diff, and the ratios for them depend on 6 cyl or 4 cyl.

A 245/75R16 tire ends up around 29.8" tall typically.

4.30/70MPH/6th = 1966 rpm
4.88/70MPH/6th = 2032 rpm
5.29/70MPH/6th = 2419 rpm
5.71/70MPH/6th = 2611 rpm

To me, the 5.71 is the only ratio that truly sounds good for a 4 cylinder in 6th gear, but even then 2600 rpm isn't much for a 4 cylinder in a heavy truck. But, 5th with 5.71 would be 3106 at the same speed, so I imagine overall it would perform well if it uses 5th and 6th.

The only downside I can see to the 4 cylinder and gearing super low, is that the 4 cylinder has a redline of 5500 rpm (or close to that). With 5.71 gears, your passing gear will undoubtedly become 4th, because 3rd gear would now max out at 57-58 mph. 4th would be able to go up to 85 mph, and with 5.71 you'd be able to achieve the most possible torque in 4th. In stock form, your 3rd gear can only go up to 75 anyways, so being able to now use 4th with nearly the torque of existing 3rd and still take it up to 85 vs 75 would be beneficial I think. My best guess is that as a 2.7L owner you aren't doing super high speed passes really anyways, so max speed per gear probably doesn't matter that much. I only mention this stuff because it is something to consider when factoring how you use the truck vs what gears are right for you to choose.

There is also the noise factor, you might get a bit of driveline noise with 5.71s, but maybe not. Probably wouldn't matter much if you did anyways.

Overall the 5.71 is probably ideal all around for simple maximum torque increase. I bumped my manual from 4.30 to 5.29 and absolutely love it. I only stepped the tires up from 30.6" to 32.1", and I ran on the 30.6" tires with the 5.29 gears for about 4 months and probably 10K miles. It was great. You would have a drastic performance increase going from 4.30 to 5.71.

You'll probably need an edited tune to raise your governed speed back up. That is easy to do through a tuner and you can still stay on the stock tune or whatever tune you like with that adjustment made.

 

This is an excellent write up!!! Very well said.

 

Thanks.

I confirmed the door-sticker decoders were wrong, and that my truck is 4.30 like it's supposed to be. It's also (as an SR with Utility package) not supposed to have intermittent wipers - yet there they are. Go figure...

I built a spreadsheet with all the engine RPM's (I get 30.5" for factory tire size) in all the gearings. 5.71's make sense for a utility truck that's a companion to a Kubota B series, and not an interstate runner; which is what this truck is intended for.

Problem really isn't the engine RPM's, but the driveshaft RPMs.

70/80/100 MPH spins the driveshaft at these speeds:

4.30 - 3371/3791/4793
5.29 - 4081/4664/5830
5.71 - 4405/5034/6293

70-80 covers most highway speeds, and 100 is an arbitrary "terminal speed" benchmark. Much more and you exceed the tire speed rating or (I assume) driveshaft critical speed.

This is probably why folks are getting speed limited when they regear. While the speedo is based off the wheel speed sensors, I suspect the ECU is also using transmission output shaft speed. I'm going to guess that 4800-ish is the programmed limit.

 

Yes, the governor on automatics is driven off of a speed sensor in I believe the transmission which measures driveshaft speed. The regear ups the driveshaft speed and so it cuts you off earlier. The manuals only have speed data from the wheels, so no issues on those. Autos will definitely be cut off early (proportionally to the gear reduction used), and the only way to raise that back up is to retune or to add larger tires, which defeats the purpose.

The engine rpm isn’t going to be too much no matter what you choose, as you’ll never outrev a 4 cylinder with such a tall 0.58 OD2 ratio.

Driveshaft rpm will be high as you stated, but it shouldn’t be a problem. 5.29 and 32’s mine doing about 4500 at 80. I previously ran 47-4800 at 80. As long as the gears are setup well and bearings preloaded properly, should be fine. Just adjust the speedo accordingly with the tune and all should be well.

 

I agree the engine RPMs are manageable. I'm not sure the driveshaft RPMs are. Is the 4800-ish governed speed there because it's reaching driveshaft critical speed?

There's a lot of measurement and math, and wall thickness of the shaft is crucial to calculating critical speed. The internet apparently doesn't know that answer, and I'm not going to cut mine open to find out...

The shorter shafts with carrier bearing hypothetically increase critical speed, but the carrier and u-joints are another variable to be considered.

 

I’ll dig more into it myself when I get a chance but if I recall, the autos are all governed to 112mph by their driveshaft speed. I imagine the 3.90 autos (V6) are programmed to cut off at whatever driveline rpm equals 112mph. Whereas 4.30 autos (I4) trucks are programmed to cut off at the higher driveline speed that equals 112mph.

I doubt it’s related to driveshaft critical speed, but maybe. I don’t know for sure. I do know manuals don’t have that same limit based on driveshaft speed because I’ve taken my 5.29s up to 99 mph. It was still smooth as silk and for me 99 mph now is more driveshaft speed than 112 would have been with stock 4.30s.

I do agree that you have the longest driveline combination and so you may have a lesser critical speed than others. I have my doubts that that’s why the speed limiter is set where it is. I do know that 5.71 is 1.32x the gear that 4.30 is, and so without making any adjustments, you would be limited to around 84 mph accordingly vs the stock 112 setting, unless you got a tune file that overwrites that.

 

I’ve run a bit more numbers. I’m assuming the top speed is 112mph from factory which is what I’ve seen in numerous locations here.

Taking a stock SR:
4.30 Gears
245/75R16 (682 revs/mile) - 29.57”

70/80/100/112
3420/3909/4880/5472

So I’m going to say 5472 is the max speed, not 4800 because the speed governor as far as I am aware, is not at 100mph, but at 112 according to forum folk.

Now, SR with 5.71 and same 29.5” tires:

70/80/100/112
4542/5191/6488/7267

5472 (the stock limited speed @ 112) = 84 mph

Again, I don’t know what the critical shaft speeds are. I need the length and the size of the tubing to know that. I do think that the longer the driveshaft, the more they lengthen the first half? I think the rear half is mostly the same length on most trucks, to keep it from being too crazy long on the extra long ones like 2WD or CCLB. But I need measurements from different trucks to say that with certainty also.

Driveline speeds are certainly a valid concern. I can say with certainty that a short wheelbase 4WD with no lift has absolutely 0 issues running any driveline rpm up to 100 mph with 5.29 and 30.5” tires. I can’t promise the longer driveshaft trucks will have the same result with their longer shaft and the smaller tires and even deeper gears. Need some testing to determine that. If you could tune yourself to remove/raise your governor and find some open road to go above 112, maybe you could figure it out yourself lol.

 

You could also just look for 2WD owners that did 5.29s on small tires. I bet it would be perfectly fine on a SWB, because the carrier bearing should be mounted at the same location on the frame for all SWB trucks and the front driveshaft half should be longer to make up the 2WD vs 4WD difference. I would think the rear half of driveshaft would be the one to reach critical speed first, so 5.29 should be fine on both SWB 2WD and 4WD driveshafts.

But it would be good to confirm that by finding experiences of others who are 2WD with 5.29 and smallish tires and see what speeds they drive without problem.

 

Are you doing this just to run one gear higher at speed? Seems like a waste of money. Unless I'm misunderstanding what you're saying.

 

I'm curious, how did you confirm the axle code is wrong?

 

I bought that truck because it's a stout little block that will run 3000 rpms pretty much indefinitely. Same reason I have a little Kubota for other tasks. This one will tote the odd load of wood, hay bale, jug of bar oil, and maybe a couple of coffee cans with bent nails or dogfood. I've got 300' of overgrown fence to clear out and take down.

They've strangled the engine, which is tolerable; but they've really got the gearing off to squeeze every last bit of life out of it. Yeah, I wish there were a manual option too...

I just want to gear it down like they've been since the 70's - which is why they were so useful. Easiest way to do that is with the diff, which is a normal and routine choice when buying a real, full-sized truck.

Useful is never a waste of money.

 

There's another thread in the 3rd gen section that has axle codes. The "decoder" is wrong, based on my tag. It says the '07' part is 3.90 gearing. Clearly (comparing gps speed and rpm) my truck is 4.30

Not a big deal, but the errata is why there are multiple redundant threads asking the same question... lol

Axle speed is something that's very important for every owner to consider though - especially when you're worried about (or getting) vibration. Go to Spicer's calculator and compare the variety of materials and whatnot. I just don't know the specs on the Toyota. My Frontier used a Spicer part number. Toyota makes their own or has a Japanese manufacturer? No idea.

You don't want an axle flopping around though, and you have to think about if the U-joints and carrier bearing (if present) is up to the speeds.

 

I have some minor disagreement with some numbers, but I'm ok with spitball ranges to start.

Using your numbers 70-80mph you're in the 2500-2900 engine RPM ballpark in 6th. Kickdown to 5th puts you at 3 to 3 and a half. That little motor is humming perfectly where it likes to make power. It's got loads of pasture torque to boot, which is what it will spend most of its time doing.

There's still the problem of running the interstate and having it cut out at 80-whatever the numbers work out to. It's potentially a valid trade off, just like the slight mpg hit.

But there's the issue of safe or not, and I have no idea. I can do the math, but I don't have the specs to run the formula on (and there's bearings to consider). It could all just as easily be in a perfectly safe range, but the fact there's a cut off at all should prompt the question.

 

The numbers I’ve come up with are based on taking the revs/mile of a tire (I use Toyo) and dividing that out to its diameter. That measurement has always been perfectly accurate for my use when calculating rpm. One thing for certain is your tire will not come out to 30.5” on the truck. So that’s why I’m using closer to 29.5”.

From everything I’ve seen here on the forum, factory cutoff is 112 miles per hour on all automatics. For you, with stock tires, that turns out to be somewhere around 5400-5500 rpm. That makes that rpm a known safe point. 4800 is closer to 100 mph. I do not know of any factory cutoffs at 100 mph. But, I also have not driven any automatics so I can’t say for sure. I’m just going off of what people say here.

Now, your cutoff WILL be reduced. That is a given. But like I’ve said, there is a fix for that. If you like the stock engine tune, you can have someone take the stock tune file and edit it to a higher mph where the cutoff happens, so that you can still drive above that speed. Then they just flash that to your truck and all of a sudden your truck magically lets you drive faster.

Of course, the cutoff with regear will happen at the point where your driveshaft would have spun the fastest in stock form, aka that ~5400 rpm point, which will be somewhere around 84mph with 5.71 gears. Whether it’s safe for the driveshaft to go beyond that point is an unknown. So while removing the truck’s ECU governor (or raising it) fixes the computerized hindrance of going faster, it’s still unknown whether or not you’re exceeding the critical speed of your driveshaft.

All I can say to that point is that the factory 112 mph cutoff (if it exists on a manual transmission) would have kicked in at the same 5400 rpm that yours does, because we both have/had 4.30 gears and very close stock tire sizes. Since my regear, I have had my driveline up to 5600-5700 rpm and it was still perfect, no vibes no noise. But like we’ve discussed you have a longer driveshaft than I do.

That said, the longest points of either of our driveshafts should be the same. I believe you have a longer distance to the carrier bearing from the transmission than I do, and I believe our carrier bearing to axle distance is about the same or close. I’d be happy to take some measurements to compare with yours if that would help figure out where in each of our driveshafts the segment lengths differ. That might put your critical speed concerns to rest, or at least help start to determine a safe point. I doubt you’ll be hauling down the highway at 85 mph+ regularly, but I can see why you’d want to make sure it’s an option to do that if you ever needed to for some reason.

Edit: got bored and went to measure mine. ~27” from tcase flange to center of u-joint after carrier bearing.

~40-41” from carrier bearing u-joint to rear axle flange.

I would think calculating critical speed you’d focus on the longer piece, not the entire driveshaft length.

For reference, the info I’ve found says our driveshafts are 3” X 0.083” tube. 1330 u-joints. I come up with a critical speed of 5000 for either of the two measurements.

However, the truck lets it go faster than that anyways. There may be something better or more predictable about Toyota shafts that makes them more reliable at the critical speed than others. I dunno. I just know mine spins fast and I’ve got nearly 20K of that and it’s been nothing but great now.

 

Have you tried turning rear wheel 2 full turns and count driveshaft turns? I did and mine is close to 3 3/4 turns than to 4. My sticker states 04 - 3.72.

 

Not that big of a deal for me. The sticker says what it says. The "decoder" says what it thinks it means. Reality is still what it is, and mine's a 4.30. Happens to match with the Toyota published spec as well, strangely enough.

I'll probably end up posting a chart (or part of it) and folks will necro this forever, endlessly arguing tire diameter significance...

 

Cool. I use Every tire will vary slightly, and then there's inflation differences and whatnot. I get 30.5 and 30.6 for the alternate 265/70 16

I'm not going to quibble over the numbers, but I'm not going to recalculate my chart - so we'll just have to each understand we're in the same ballpark.

The numbers are good stuff. the 1330 is a Spicer part number. My Frontier's drive shaft was a Spicer part number, for that matter. I'll play with Dana's AR 'partfinder' tomorrow in daylight.

Just plugging those numbers into Spicer's calculator (3x.083 with 27" lengths) output a 6000rpm critical speed.

 

Just count teeth and let us know when pull the diff out

 

Hah don’t worry, I’m not looking to quibble either. It won’t make enough difference to matter.

Yeah I had 2 frontiers and they had all spicer parts including the diffs. I’m not sure about the Toyota, it seems to use spicer u-joints but not sure about the shaft. I want to say the center support bearing is a spicer part.

I think the best way to make this decision is decide what speed you want to be able to travel and call that good and settle on whatever gets you there. There are also some 5.71 pinion strength concerns (that is an ancient ratio option especially on the 8” diff) and most of them broke while wheeling. So probably not a problem, but something to consider.

5.29 will not quite get you to an awesome 6th gear, but it might be a better choice overall depending on speed you want to be able to go.

You could consider stepping up the tires to 265/75 to help the 5.71s slow things down a tiny bit if you went that route.