pardon the ignorance/stupidity but,
can anyone explain the differences in the axles offered and the limited slip differential ect.

thanks alot,
norm

 

With regards to what, the numbers, like 3.08, 3.31, 3.55, 3.73? Gear ratio primer. These numbefs above are all to 1, so 3.55:1. For every 3.55 revolutions of the driveshaft, the rear axle will turn once. In summary, the higher the number, the more wheel torque, faster acceleration.

With regards to open (standard), limited slip, and electronic locking? Differential primer. Open is bad, limited slip and locking are good. Locking is better off road, LSD better on road.

 

The standard differential is what usually called an open differential. A limited slip differential is an option.

An open differential will allow all the power to go to one wheel causing it to spin free if it has less traction than the other. Such circumstances as a muddy road or wet boat ramp come to mind.

Under those same circumstances, a limited slip differential will help apply more power evenly to both wheels but there are limitations.

The FX4 model has an electronic locking rear differential which actually physically locks the axles so both wheel get the same power regardless of traction when activated.

Then there are the rear ratios.

The 3.73 ratio will allow more engine rpm per each mph. its best suited for towing applications and off-road. better torque at the rear wheels but slightly less gas mileage.

The 3.15 ratio at the other end of the spectrum will provide less engine rpm per each mph and will provide the best high gas mileage.

3.31 and 3.55 are compromises of each.

 

I must note that there is no direct correlation of axle ratio to fuel economy, but there is of axle ratio to wheel torque. For example, 3.73 compared to 3.15 is 18% shorter, but it does not mean 18% more fuel used for the same distance.

 

You don't believe there is any correlation or do you mean it isn't the same proportional difference as calculated torque?

 

There is no correlation. For instance, some people put heavier and larger tires on their trucks, then change the axles from 3.73 to 4.56 and increase fuel economy, despite running 22% higher engine speed for the same vehicle speed in each gear. If the gearing means the transmission is always downshifting and the driver pushes on the accelerator pedal more, he will use more fuel. Everyone's driving style is unique, as their truck powertrain/tire/load varies too. Lower RPM does not mean less fuel consumed.

There is a correlation of wheel torque to acceleration, but is is not the same percentage as gearing.

 

Well then its a matter of context. Given all other variables being the same, there is a correlation between axle ratio and fuel economy.

 

Actually, no there is not. Most people think that shorter gears (numerically higher) means more fuel consumed, which has yet to be proven since 1997 F-150s. Stock 4x4 Screws get the same fuel economy with any axle ratio and either engine, for example. 2WD/4WD vary, as do cabs (weight), but not so much engine/gear. My opinion is that one might as well have the better acceleration/torque multiplier of shorter gears.

 

I spoke to someone about this on sat night who does off roading and he has had this same debate about axle ratios/fuel economy. He basicly said that you cant get something from nothing. He said it as simple as if you want more TQ then you will have to be willing to lose Fuel economy. He said it was all relative and that it's simple math. A truck with a 3.55 diff doing a 200 mile trip will use up less fuel than a truck with a 4.55 rear end. When you look at it as simple as that then there's def a correlation between Axle ratio and MPG. It would be like saying your truck wont use anymore fuel when at 70mph than 65mph because i can tell you now that it takes a big hit when doing that 5mph more. It would be nice to see a table showing what speed a 3.55 would be at at 2000rpm compared to a 3.73 at 2000 RPM. The 3.73 must be lower so to get that extra MPH you will need extra RPM which means Extra fuel.

Maybe someone can in put the data into the find MPH and find RPM calculator for trucks with same setup and different axle ratios. Im not sure how to fill the thing in. http://www.4lo.com/4LoCalc.htm

i done 2000RPM 32" tire dia and 1.0trans ratio and 1.0 transcase ratio and got 53.65mph for a 3.55 and 51.06mph for 3.73 so there def a diffence when both cars would be at the same speed.

I done the same but for MPH and when both trucks are going 60mph the 3.55 turns at 2236.5rpm and the 3.73 turns at 2349.9 so there def more fuel consumption at any given speed.

 

You guys can say it makes no difference in fuel economy if you want to. I can not comprehend half of what some of you said. Seems to be going in circles. I do know that in my 65 Mustang when I swapped the gears from 3.00 to 3.50s, I do not go but about 3/4 of the miles on a tank of gas. That has happened every time I changed gears. Would be difficult to convince me that I do not burn more gas with lower gears.

 

I too have swapped the rear-end gears in more than a few vehicles. There is always a difference in gas mileage.

If you take two F-150s completely identical except one has the 3.15 gears and the other the 3.73 gears, run em both the exact same distance with the exact same load at the exact same speed, the 3.15 geared truck will get a few more miles from the same amount of fuel that the 3.73 geared truck.

To read that someone believes there is no difference at least gave me a good chuckle.

 

The flaw on that is the load is not the same. By load, I mean how hard the engine is working. I don't mean different vehicle weight. One can change the operating speed of the engine such that the lower load from shorter gears compensates for the higher engine speed. In short, 2500rpm @ 15% throttle can use less fuel than 2000rpm @ 30% throttle.

People on here with 35" tires have gone from stock 3.55 or 3.73 to 4.56 gears and improved fuel economy. 4WD Platinum trucks with 3.73 gears are getting the same fuel economy as the 3.31 4WD trucks. Research Brake Specific Fuel Consumption (BSFC) and engine load for reasons why. As I said, it comes down to lugging the engine, or more downshifts under certain driving conditions.

 

We it must make a difference because if it dont then why not just sell 3.73 axle? It just dont make any sense that getting less rotation from the same power gets you the same MPG. Thats like me rideing about in 1st gear on my mountain bike and doing 20 miles and saying that it would be the same as me doing 20 miles in any of the other gears. The difference from 1st to second is very small but over the 20 miles it would make a difference in how hard i have had to work.

 

What you are doing is changing the gears as well as tire height to come back to the same or similar net ratio change. If that is the case, then of course changing gears will not change fuel consumption much. It will change from any difference in resistance or weight, but not from gearing. You are changing two factors in order to stay at the same ratio. I did not get that out of your earlier statement. All I could see was a statement that changing gears did not affect fuel consumtion.

 

I said people that already had 35" tires and stock gears changing to shorter gears improved fuel economy. Maybe they lost it when they went from stock to 35" tires, but 9% larger tires with 21% shorter axle ratio is not the same net gearing.

The human body is not the same as the internal combustion engine.

Has anyone followed my suggestion?