Gear Ratios

bkg

Explorer
There are a lot of questions about gear ratios that pop up from time to time, especially when people change tire size. There's seems to be a presumption that choosing the ratio that gets the vehicle back to as close to stock as possible is the correct choice, but I think that presumption is faulty, especially on vehicles that are carrying more weight.

What is often forgotten is to compare the final drive ratio of the vehicle in question across both the automatic and manual versions available from the factory. I have three 5Vze vehicles, all with aftermarket gearing. One 3rd Gen 4Runner w/ 4.88's and 33" (equivalent) tires. 1st Gen doublecab, also with 4.88's and 33" tires. And my extracab which has 5.29's and 37's... is supercharged, SAS'd, etc but isn't daily driven.

When people hear 4.88's and 33's, there's an assumption that the vehicles are over geared, the engine is revving to high, speed is impacted, etc. But that isn't really accurate, especially when compared to their manual-equipped counterparts.

So using the RPM calculation: Axle Ratio x speed x trans ratio x 336.13 / tire diameter (where tire diameter is the nominal diameter in inches), the below charts tell a pretty good story, IMHO.

1537725702587.png

Taking a look at the data, doing a 33/4.88 combination is still geared higher than a stock R150-equipped truck. Now consider that my double cab is probably a good 600-800# heavier than stock (winch, bumpers, sliders, cap, skids, 5 full size tires) and the 5.29's start to look even more appealing as a way to help make up from the weight.

Similar for the 1GR equipped vehicles.
1537725894547.png

Point is that choosing gear ratios means more than just trying to get back to stock final drive ratio of the particular vehicle. Take a look at the other final drive ratios available via different packages and transmissions. Ideally, one would also take into account the engine's torque curve, driving habits, overall weight added, driving location... but that's adds more data points and offers more possibility for confusion, IMHO.
 

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DaveInDenver

Middle Income Semi-Redneck
LOL, I ran 5.29 with 33" BFG (so actual more like 32.4") and other than being top speed limited on the freeway I really liked having the extra under drive. I'd say 4.88 is spot on for a 33" tire on an old Toyota that's built. It would be too short for my 1GR Tacoma, which started at 3.73 on 30ish tires. I'd think 4.56 will be right on with a 33" on it.

BTW, something to think about with final gearing is the shift points are impacted. That's actually something I did not like about 5.29, even in high range it tended to rev between gears faster than stock. Where something like this might come up would be passing on a 2-lane. The rare time that I actually attempted this I found I could wind out before I complete the pass if I didn't anticipate the relative speed and my RPMs. But OTOH if you pick your speed and RPM right and didn't have to shift the thing would accelerate quick (relatively for an overweight 22R-E).
 
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bkg

Explorer
Shift points really shouldn’t be impacted if speedometer/VSS correction is utilized. To the best of my knowledge, at least.

There’s only an 8% increase in cruising rpm between a 33/5.29 combo and a stock 31/4.11/r150 combination. Again, shouldn’t impact things like top speed, per se.

The automatics are undergeared when compared to the manual equipped vehicles, likely due to the torque multiplier of the torque converter, but at cruise, there’s plenty of ground to play with re: gear selection. I struggle with the idea that a mathematically equivalent cruise rpm is too high an rpm in an automatic but acceptable in a manual.
 

DaveInDenver

Middle Income Semi-Redneck
Speedo correction would seem to only be important to give you correct information? I mean, if your calculations say you should be going 70 MPH at some RPM then you are regardless if the speedometer says you're going 65 or 75. Perhaps with an automatic the computer needs to accurately know, but I have no experience with that.

You may be interested in this spreadsheet I built up trying to decide on gears back when. I started stock with 225/75R15 (I used 29" for calculations) + W56C (0.85 O/D) + 4.10. I say speed limited because with those 5.29 gears at 65 MPH my engine was doing 3k RPM, so it's really a matter of holding >3k for long periods was terrible for MPG. I never did (and still don't) worry about holding my truck engine at sustained RPM while climbing passes. I wouldn't blink at even 4k for a burst in the old truck. But on flat ground 5.29 + 33" was a bit buzzy for hours. Let me tell ya, it got real tiring crossing CO/UT/NV getting to Rubicon trying to keep up with supercharged 80s who wanted to do 75 and I was sitting at 3,500 RPM.

Anyway, in this spreadsheet you'll see the 22R-E HP peaks at 4800 RPM and torque peaks at 2800. I compared ratios by assuming I wanted to be at 65% or 75% of each of the HP and torque peaks to settle on cruising RPM biased for power or MPG. I then selected a top speed to come up with various ratios. Which is really cruising speed w.r.t. this discussion, top speed is really the redline, but you understand what I mean.

What you have to remember is power is constant, being the product of torque x angular velocity. Lower gearing multiplies torque but reduces proportionally the shaft speed, so you really aren't changing the power you have at the rear wheels. You will change the acceleration profile since you're multiplying or dividing torque. The apparent power (to call it something) may seem to change because you change how your truck moves through the HP peak.

Anyway, what you see is comparing stock to modified is a few things. First, if you assuming a cruising speed of 75 MPH it shows 4.129 as the ideal ratio to bias HP for MPG, which is 65% of HP peak (3,120 RPM). That made sense then why Toyota selected 4.10 in my mind. Then with a 32.6" actual tire seems to say 4.642 would give you the same 65% peak. That's why I think people used to suggest 4.88, you're close to the same ideal point on the curve with some help with inertia of the extra weight.
 

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bkg

Explorer
What’s interesting about your last paragraph- Toyota chose different ratios for automatic and manual...
 

DaveInDenver

Middle Income Semi-Redneck
What’s interesting about your last paragraph- Toyota chose different ratios for automatic and manual...
Don't they still? I honestly can't keep up anymore.

In 1991 the standard shift, 22R-E with 225/75R15 tires got 4.10. With an automatic it would have 4.30 I think. The trucks that came from the factory with 31" tires got 4.56 and 4.88 respectively. Those factory 4.88 thirds were the thing to get.

My 2008 V6/6MT has 3.73 ratios and the 4 cylinders still got 4.10 in 2008. I don't know if automatics got different ratios, though.
 

Dipodomys

Observer
Good post. I put 33s on my 2008 DC with the auto trans and am really feeling the difference with the stock gears (3.73, I believe). The guy at the local gear shop said the general rule of thumb is to go up one "gear size" for each change in tire size. So if I went up two tire sizes then that means I'd probably want to go from 3.73 to 4.56, assuming that 4.11 is the "middle" gear ratio that I'd be jumping over. I'm not super thrilled about taking such a crude approach when considering such a large investment, not to mention the frustration that will result if it doesn't turn out the way I had hoped. The truck is definitely heavier than stock, so maybe an additional "step" is warranted.

On another note, does anyone know how to definitively determine the factory gear ratio installed in a stock Tacoma? Yes, I know about the axle and transmission code on the driver's side door jam, but I have yet to find a site that interprets those codes for my year/model. I have a 2008 DC with the auto trans. I THINK that the factory gearing is 3.73, but I get conflicting results on the internets. Advice appreciated.
 

thezentree

pretend redneck
My understanding is that you can jack the rear of the truck up and spin the wheel. With 3.73 gearing, the wheel should perform 3.73 rotations for every full rotation of the driveshaft. (Someone correct me if I'm wrong!)

You got it backwards - one revolution of the wheel for every 3.73 revs of the driveshaft.
 

TernOverland

Supporting Sponsor Ternoverland.com
A reference point for fuel economy and gearing: My stock 2007 AC manual transmission truck, with a heavy duty Leer shell got 18mpg on the highway. Now my truck with 4:56 gears and 33's, gets 15-16 on the highway. It is 1,000lbs heavier and the rotating mass of the tires has gone up 78 pounds. I'm also 5" taller, considering the lift and the tires. That gives it a bigger profile to the wind. I attribute most of the loss to the factors mentioned, other than actual gearing. All in all, I'm surprised that the loss isn't more.

Another point to remember is that gearing does not make horsepower. If you keep going down in gearing, and up in tires size, you will compensate the RPMs, but you will feel the power loss. I feel it most on steep highway grades, and when towing.
 

knoxswift

Active member
Why do you say that? I’d like to understand your reasoning.
It's the RPM range changes.
I guess if you don't plan highway driving more than 4.30...might be ok.
Also my comments are based on manual transmission.
The website I linked does an excellent job of showing rpms in each gear.
For Toyota targets is around 2200-2300rpm for decent gas mileage.
Choose your driving style tire size and average mph in most desired gear.
For highway and 33s 4.30s are almost perfect...4.10s make Toyota equivalent to stock (but who wants that...lol)
 

knoxswift

Active member
A reference point for fuel economy and gearing: My stock 2007 AC manual transmission truck, with a heavy duty Leer shell got 18mpg on the highway. Now my truck with 4:56 gears and 33's, gets 15-16 on the highway. It is 1,000lbs heavier and the rotating mass of the tires has gone up 78 pounds. I'm also 5" taller, considering the lift and the tires. That gives it a bigger profile to the wind. I attribute most of the loss to the factors mentioned, other than actual gearing. All in all, I'm surprised that the loss isn't more.

Another point to remember is that gearing does not make horsepower. If you keep going down in gearing, and up in tires size, you will compensate the RPMs, but you will feel the power loss. I feel it most on steep highway grades, and when towing.

I had similar results. With 4.10s you'll see 18mpg. With 4.30s I've had as good as 22mpg.
It really is also one must target their driving habits and style.
I'm in a lot of stop and go where torque is desirable to move the extra mass you mentioned.
However when not in stop and go I drive like I stole it...so I couldn't go to tall or would lose the high end.
The gear calculator will help find that balance if one is honest about how they actually drive.
Gears can compensate for the loss by bringing the engine rpms into the desired curve sweet spot on intended use...without major investment we cannot change the design and engine curve so one needs to keep this in mind when changing tires and gesrs.
 

bkg

Explorer
It's the RPM range changes.
I guess if you don't plan highway driving more than 4.30...might be ok.
Also my comments are based on manual transmission.
The website I linked does an excellent job of showing rpms in each gear.
For Toyota targets is around 2200-2300rpm for decent gas mileage.
Choose your driving style tire size and average mph in most desired gear.
For highway and 33s 4.30s are almost perfect...4.10s make Toyota equivalent to stock (but who wants that...lol)


I don't think you are at all correct in any of these statements, unless you're maybe starting with a 3.73 gear ratio. Just take a look at the charts I posted above... And you're ignoring weight additions, height, rolling resistance, etc...
 

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