You have posted a list of ratios. What it means is the engine turns this many times for the tires one time. Since all non over/under drive trannys turn 1 to 1 with the engine, a 2.75 ratio for example the engine is turning 2 and 3/4s times per the tires one rotation.

As long as the engine can handle the power, the lower the numbers like 2.75 the wheel is turning faster at a lower engine speed. With 3.55 or 4.11 the engine is turning a lot faster to acheive the same tire speed.

The I6 in your truck needs to build more speed to make it's horsepower and the 2.75 ratio is not allowing that to happen. A 3.50 or 3.75 ratio will allow the engine more rpms and it will make it happier, and better gas milage.

hope this helps

John

 

jowilker: Thanks for the info. It was never explained to me that way before. But it seams to me that if an engine is turning faster it would use more GAS to make it turn faster, is that correct? If so how can it have better MPG, turning more RPM's

 

You need to know the torque, not horsepower band for your engine. Usually, engines are most efficent near the torque peak. The 300ci six and the deisels tend to have peak torque at a lower rpm than the gas V-8's. Lets say that your engine torque peaks at 2000 rpm and you like to cruise at 65mph. Find the gearing, with your current tire size, that will give you that rpm/speed combination in high gear.

For example, my old Ranger 2.9 V-6 had the Mazda 5-speed OD, 3.45 gears and 14 inch tires. It all worked out to a 68mph speed at peak torque, and it got the full EPA estimated highway mpg at that speed. My current F-150 4x4/5.0/E4OD/3.55/29in tires gets way less than the EPA estimated 18mpg, more like 15 or less. I have to drive at 75mph to get the engine up into the torque band and at that speed the wind resistance eats up too much gas. At lower speeds, it labors and wont stay in OD, again eating up gas.

Jim

 

i thought the peak efficiency had to do w/the vacuum. i read somewhere that a vacuum guage can tell where your engine is operating most efficiently. most gassers creat their peak torque at 3-5k rpm's. try cruising at that range and look at the gas bill.

 

Jessfactor, that's peak horsepower you're thinking of. Most gas V8's make thier peak torque below 3500 rpms. My 351 makes it's peak torque around 2800, which is actually a little under my top freeway cruising rpm, since I have low gears in the truck.

 

What you have to remember is the amount of gas used is controlled by how much you open the throttle, not how fast the engine is spinning. If your engine is performing in its optimal RPM range it will function with just a little throttle and keep you moving down the road. If you lug it down you're going to open the throttle more to keep your speed up. This doesn't necessarily mean you're going faster, just that you might have your foot too the floor and be dumping a lot of gas in the carb to keep it going. Think of it as driving up a steep hill in 3rd gear vs. driving on flat ground. If you go up the hill and stay in 3rd gear you can put the pedal to the floor but the RPM's will stay fairly low because you're not in your power range. You're still dumping fuel into your carb full speed but not going any faster. Now if you drive that same truck in 3rd on flat ground you can hold the throttle at half and travel twice as fast as you did on the hill because the engine isn't overloaded and can operate in the power range. This is the same thing happening with poor gearing, you can't get to that power range but you can still dump more gas in the carb trying.

Hope that made sense, I've had a bit to drink and I'm a little incoherant.

 

Interesting, thanks Ivan.

 

Modern passenger car engines sometimes have torque peak way up there but most, not all, truck engines are near 2000-2500. A vaccum gauge would make a lot of sense, especially when combined with a tachometer. Most small airplanes have both. You set a fixed rpm with the propeller control (similar to a transmission gear selection) and vary the power with the throttle, as indicated by the manifold pressure gauge (a vacuum gauge, but calibrated differently). Low rpm and high pressure (low vacuum) is bad for the engine.

Jim

 

Doc, No I dont really know how to make the change. If I got a rear-end (just the gears) from a ford Bronco, could I just take my gears out and replace them with the ones from the Bronco?
John

 

Basically if you get the entire drop out from another 9in, and the axle spline count is the same then yes you just pull the axles, drop the entire center section out clean up the sealing surface use some silicon on it (I don't use the gasket just blue RTV) install the new center section reinstall the axles, and fill with gear oil. pretty basic simplized description but the general idea. if you are just needing to change the ring, and pinion it's a little more complicated but still not difficult

 

Thanks for all the info. I am still thinking about it.