Gears, gears and more Gears!! The truck was origanly a 5.4 but im doing a 99 v10 swap and want to change my gears. Curently has 3.73s, would really like to go with 4.30s but hurd good things about 4.56s. Will 4.56s kill my mpg goen 70 to 75mph compared to 4.30s? I do have a very heavy foot at times racing lil ricers and cummings so i dont want 90 mph to be my top speed.

 

My truck:

34" tires, 4.30 gears, 2,000 RPM @ 55 MPH = 14.5 MPG
65 MPH = 13.5 MPG. I don't recall the RPM at 65 MPH, but you can see that the MPG at 55 is pretty sweet for what it is. I think you'd lose some MPG with lower gears (4.56) but with 35" tires you might still be at 2,000 RPM at 60 MPH.

 

Im confused here just upgraded my tires to 34'' running 3:73 and v10 automatic tranny...now for maximum fuel efficency isnt it wise to run 40-45mph on freeway at 1200 rpm...why does everyone keep there engines at 2000 rpm are you not burning more fuel at 2000 than 1200rpm please clarify im confused thanx.

 

4:56's or dont bother..

you might even wanna consider 4:88's for teh extra jump...

thats what i'd do...

 

The engine is overloaded at that low (1200) rpm. 2000-2100 seems to be where this engine is the most efficient. Just because the rpm is low doesn't mean the engine isn't working. When I had my 3.73s in my truck it was extremely doggy, got horrible mileage even at highway speeds and sounded like it was constantly "working" to maintain. I made the move the 4.88s (with my 35" tires) and couldn't be happier. @ 65 mph and turning 2100 rpm, the engine is barely heard and holds OD up hill, even while towing in most cases.

This isn't unheard of to get an mpg increase with an rpm increase.

 

More fuel at 2000 than 1200? No, not necessarily. At least not very much.

The amount of fuel burned is the amount injected. The amount injected depends on the amount of power output you request through your right foot.

Highway mpg is largely dependent on aerodynamic drag and frictional losses. Aerodynamic drag is most dramatic, as it is CUBED function of speed. In other words, to go twice as fast, you need 8 times as much power.

So the increase in mpg you see at 40 mph and 1200 rpm is not so much the engine speed reduction but the dramatic reduction in aerodynamic drag.

 

wow...interesting point...my objective is to optimize maximum fuel efficency when I drive at 2000 or 1500 rpm im burning a lot of fuel... seems like when Im driving at 1200 rpm im only sniffing gas...the difference is huge as the needle barely moves lol... the environment is flat stretches of highway im running the truck on cruise control and any slight incline the transmission shifts and rpms adjust...with gas prices on the rise im willing to slow down and reduce cosumption.

 

Your truck is barely moving too.

It's a balancing act when it comes to efficiency. Heck, never start it and your mileage will be magnificent.

 

Is that with a auto? Happen to know your top speed?

 

That is the cheapest and easiest way to improve mpg.... especially with the HUGE frontal area and very poor aerodynamics of these trucks.

Lowering the truck and putting smaller and narrower tires would help as well.

But bottom line, fuel economy was about #18 on the list of design priorities for the Super Duty!

 

2356 rpm with 4.56s, 35s, and a 5 speed sound right?

 

Yes automatic but the your 5th is nearly the same as my OD ratio.

Top speed is really inconclusive in a truck but would be over 100mph since I have close to 3000rpm left to rev from 65mph.

 

If you plan on doing a lot of driving at 70-75mph and do not tow much or at all you may prefer the 4.30s. The 5 speed manual has an OD ratio of .76 then to add to that the NPI V10 does not like to rev as much as the PI and tends to fall off pretty quick at 4k+ rpm. If your rpm is up too high in OD you can forget about a downshift to 1:1 unless you slow down from 70-75 mph.