Interesting Observation
There is consistency to the madness at Toyota in gearing the 5 sp transmission. Here is a table. See a pattern?
Gearing Summary |
|
|
Gear |
mph/1000rpm |
|
1st |
5.2 mph/1000rpm |
|
2nd |
9.6 mph/1000rpm |
|
3rd |
14.8 mph/1000rpm |
|
4th |
20.5 mph/1000rpm |
|
5th |
24.1 mph/1000rpm |
You can see there is a pattern to to the ratio. First gear is just about 5mph/1000rpm, second gear is about 10mph/1000rpm, third is 15mph/1000rpm and so on. This is with stock tires by the way.
There is a reason why the shift point on the Toyota truck is the way it is.
First some rational in transmission gear choice. For a small engine to generate a large amounts of horsepower, it need to be reved to a high rpm assuming it still generates the same torque at high rpm. Horsepower is simply, torque times rpm times a constant. The problem with a low torque high reving engine is that it lacks flexibility. For example, a big engine that gives plenty of performance rotating at 1500 rpm at 30mph, if we double the rpm to 3000, the vehicle would be going at 60mph, and still get good performance. That vehicle would be easy to drive since it does not requires much shifting, and the gear box can be a wide ratio gear box. My Ford F150 with the 4.9L straight 6 has only 3 forward gears in its manual transmission. Asmall engine on the other hand that has the same performance at 4000 rpm at 30mph; in order to double the speed to 60mph would require the engine to also double, and spin at 8000 rpm. The rpm gets too high at speed when is already high to begin with. That is just the way the math works out. So in gereral, a high reving engine need a close ratio gear box to have performance at all speeds.
There is always the question of whether torque or horsepower is better. For any given engine speed, higher torque directly translate to higher acceleration. However, if the engine has half the torque and twice the horsepower, I can always put a 2:1 gear box to double the torque and half the engine speed to arrive at the same torque at the same speed. The down side is as mentioned above, the engine is already operating at a high speed, it will not have the flexibility to vary the final output speed by much. The trade is that a smaller engine with lighter weight generate the same power as the larger engine.
In the case of the Toyota truck, the 4cyl engine is relatively small compared to american trucks. It does need a close ration gear box to take advantage of it high reving capabilities. However, a gearbox with too close ratio will limit the top speed of the truck, or that the truck would have to rev to a high rpm on the freeway, which is tiring on a long drive. Therefore, as a compromise, the gear ration is wide at lower gears since there is not much wind resistance at lower gears. As the vehicle speed picks up, and there are more wind resistance, the gear ratios are closer together so that the engine can spin at a higher rpm to fight the aerodynamic resistance. At high vehicle speeds, a too large of a drop in rpm when shifting to the next gear could create a gap where the engine is suddenly too low in the rpm range to overcome the wind resistance to accelerate; therefore limiting the top speed.
As seen in the chart above, in first gear, the rpm drops by 1600 rpm when shifting from first to second at 3500rpm shift point. In second gear, the rpm drops by 1230rpm when shifting from second to third at the same 3500 rpm shift point. Third gear drop is 970rpm. Forth to fifth gear is only a 520 rpm drop from 3500 to 3000 when shifting at 75 mph. At 2800 rpm, the engine still have enough torque to accelerate the truck to perhaps 90mph at near peak torque of 3730 rpm to overcome air resistance at 90mph. That is quite impressive for a 4 cyl. engine in a 2ton truck. This is possible due to efficient gearing.
There is however, a tradeoff to the high speed oriented gearing on the truck. In order to get the truck to high speed, the lower gearing on the truck is quite wide. The first to second shift drops the engine rpm from 3500 to 2400 rpm. On a flat road, 2400 rpm at 23mph is more than adequte to accelerate the truck. However, on a steep off-road hill, 2400rpm will lug the engine. The second to third shift is also quite wide. Many times, on a fireroad, I find that I am stuck in second gear because on third gear is too steep a reach.
On the Toyota truck that problem of the long reach is taken care of with the 4low gearing. See below. The only remaining problem is that once you are in low range, your speed is limited to about 40mph without having all the gears screaming. You have to come to a complete stop before you can shif back to high. But in high, you would not be able to shift out of 2nd to 3rd gear due to the wide ration. This issue usually only happens in running in mud where you need the high torque and speed.
My Opinion on Marlin Crawler
It seems like everyone who owns a Toyota wants a Marlin Crawler and a ARB. When I got my truck, I wanted an ARB and the Marlin Crawler 4.7:1 gear before I even knew how the locking hub worked. After driving it for a year and 18k later with much driving on dirt roads, I am beginning to see the wisdom of the Toyota engineers in the gearing department. The stock transfer case has a low range of 2.28:1. The 5speed transmission has a total reduction factor of 4.6 (1st gear divided by 5th gear). This means that there is an overlap of gears. Third gear in hi range is about same as first gear in low range. Logically, the Marlin Crawler made perfect sense. It had a reduction of 4.7:1. When you compare this to the reduction factor of 4.6 for the transmission, there would be no overlap of gears at all. Given that the Toyota has a small engine, a close ratio gearbox is really needed in general off-road driving. On a dirt road where there is a moderate incline is where the stock low gearing really shines. I find that many times, 1st gear is too slow to make much progress while 2 gear would lug the engine on a bumpy terrain. Putting the truck in low range, I effective have a close ratio gearbox. The 2.28:1 low range divides the ratio spread in half. Now, 2rd gear is like first while 4th gear is like second gear. When I need a gear between first and second, I go to low range third. Had I got the Marlin Crawler, I would be stuck with a very low gear on the trail. Therefore, if I wanted the crawler gear, I would get the double transfer case with stock gear in one and the low gear in the other one. Then I would have the best of both worlds.
My Opinion on Locking Differential Locker
For now, I do not have any locking differential. For one thing, I cannot afford it at this point. The other reason is that a locking differential will probably get me into trouble that is beyond my experience. Currently, with open differential on my truck, I will climb a hill until one wheel in the front axel and one wheel in the rear axel slips, then I stop and back down. With only one wheel on each axel slipping, I am always in control, and can back down straight. Had I had a locking differential, I would no doubt head up the steepest hill I can find and gas it until all four wheels are slipping. At that point, I would have lost all traction as well as steering. I will probably slide sideways down the hill. That can be dangerous without a lot of experience backing down a difficult spot. It is best to learn on an open differential rig first before adding locker. Having an open differential also allow you to understand terrain and traction much better. I find that with practice, and picking just the right line, I could go up things that would seem impossible with open diff. I have a good challenge without endangering myself or destroying my truck. Even with open diff, I already almost rolled my truck going down a steep hill that I climbed just by picking the wrong line. I will get the ARB when the time comes where my skills and pocket book allows.