Power from Gear change

[Cliff Foster]

I have a quick question. I have a 98 Dodge Dakota ex-cab with a 408ci stroker and 3.55 gears in it. My question is: How much more power if any will my truck put to the rear wheels with a gear change to a 3.91 or a 4.10 gear? Thanks!

[rumblefish360]

You'll feel the difference for sure. As for hard numbers......
Do you have a dyno sheet?

[ehostler]

driveshaft torque X gear ratio = total torque

[Tarrbabe]

Horsepower, none. Torque will be multiplied by the gear ration. You will feel more bottom end but your speed will be pretty much the same in the 1/8 or 1/4 as long as you don't run out of your power range. At least that's my take on it.

Just thinking out loud.

[Cliff Foster]

Well my motor will pretty much rev to the stars (6700rpm). At least that's what they tell me anyway. I should be getting to the dyno here pretty soon. Maybe the next couple of days. The motor has peretty low compression (8.0-8.5:1) because of it being a blower motor. Still needs to be tuned, but I want to get a good baseline from the 408ci stroker. I want to keep the street driveability. That's why I thought that 4.10's would be too much. Especially when I travel. I don't do that very often though.
Why wouldn't the HP numbers go up?

[ehostler]

Since HP is a measurement of torque over time, it would increase at the rear wheels as well.

[pishta]

Horsies will not change, put in a taller rear and your motor will only turn it slower and the mechanical advantage would be greater, or more torque to the pavement at the expense of speed (conservation of energy) assuming the redline remains the same. 4X4 transfer cases can be 50:1 or greater but they literally crawl over the rocks at 5 MPH but have 1000's of Ft.lbs of torque going to the tires.

[creative1]

well, i tend to agree that an engine making 360 hp and 300 ft/lbs of torque will still make exactly those amounts regardless of gearing... but by using the readily available calculators on the 'net, the end results can be quite different. as near as i can tell, on my truck, simply going from a 275/70R16 to a 275/60R16 (about 2 1/2" shorter tire) would decrease 1/4 mile ET's almost ONE FULL SECOND and increase speeds almost 10 mph. now... this is in theory and there is a lot of room for error... but none of the scenarios showed that i wouldnt gain substantially. currently my engine speed at the stripe (1/4 mi) is 3400 rpm... just about 1/2 way through the power band. smaller tires would allow shorter times in 1st and 2nd gear, which means more in 3rd, and my finish line rpm would go up about 1200 rpm... to 4600 rpm.

no, engine power and torque havent changed... but overall torque certainly would change due to the change in overall gear ratios. and while the engine would be the same, the performance characteristics would certainly be different, as would the 'seat-of-the-pants dyno' feel.

[ehostler]

HP and torque at the crank do not change. HP and torque at the real wheels will change with the different gear ratios or tire sizes. Call any dyno shop and ask them, if you don't believe me.

[Cliff Foster]

I was told, form my understanding, that higher number gear ratios increase the amount of power that is put to the ground because of the slightly easeir ability to make the wheels spin. Therefore allowing engine rpms to raise quicker. Moving the power in the powerband of the engine some or would it stay the same.
How much would my gas mileage change? Would I get better because of the ease of turning the wheels. I do have overdrive and a 2800 stall convertor. I did the gear ratio and engine speed comparison and in overdrive my engine rpm from 3.55 to 3.90 would only rise about 300-500 rpm at a a cruise speed of 80-85mph. Or would it get worse because of the engine constantly being at a higher rpm?

[creative1]

numerically higher (meaning lower) gear ratios usually will decrease mileage... but not always. an example of this would be say a low power 4 cyl engine with 2.50:1 (or so) differential and a standard 4 spd trans. the engine is working in high gear that it is in effect lugging all the time. i had an IHC Scout that was like this. it got better gas mileage in 3rd than in 4th, except going down hill.

that 2800 stall converter may hurt mileage if your gears are high (numerically low) enough. that converter will slip significantly until it reaches stall speed. for example: that converter in my truck (3.54:1 diff with 31" tires) wouldnt reach stall speed until the truck reached 75 mph. at the 65 mph speed limit here in kansas, it would be slipping constantly.

on the other hand, 3.90 gears (or so) with a 28" tire would only be doing maybe 55 (i didnt check anything figure wise) at 2800 rpm. see where i am going here?

almost any gear ratio in the 2.XX to 3.30 range is an automatic diff, and it is for economy purposes as long as the engine has the power to pull the car. 3.30 to maybe 3.90 ratios are for performance type applications... muscle car, truck, etc, where power is as important as economy. anything above 3.90 is almost certainly going to be called "extreme performance" and while it will work on the street, it isnt at all economical.

many 1/8 mile cars run gear ratios in the 8.XX:1 area. they want everything at top speed when they cross the stripe. and they will be giong just about as fast at the 1/4 mi line as they were at the 1/8th.

now, as for your question, i dont have any figures. if you have some time slips, you can use some of the java calculators to estimate things. try this link: http://www.wallaceracing.com/Calculators.htm
they should have just about any calculator you need. but if you dont have time slips, they arent much better than just guessing.