If a truck has a gear ratio of 3.90 and has a 425 Cat does this mean my truck will suck in fuel mileage?? like under 6 mpg??

 

i'd say it also depends on what trans and tire size it has.

my dad has a t800 3406b 9 speed 4.11's with 24.5's.. he gets around 5-5.5

 

transmission is a 15 speed od and the tires are 24.5's

 

it will probably get around 5.5 at 65 mph.

 

It depends on a lot of different factors.

What year is the engine?

Is the engine electronically injected or not?

Does the engine have EGR?

What is your normal cruising speed and what is the engine RPM at that speed?

Are you driving mountains or flat ground?

Are you pulling van/reefer/container, flatbed, tank?

Are you in town or OTR?

How much idling do you do?

A 425 Cat with 3:90's and an OD Trans and 24.5 tires with electronic injection and no EGR could get near 7MPG OTR if properly driven. That would be a cruising speed of about 60-65MPH with engine RPM of about 1350-1450.

 

Assuming your tires spin 475 times per mile, your tranny has a 0.73 ratio in the top gear, then with the 3.90 rears, your RPMs at 60 MPH should be:

3.90 x 475 x 0.73 = 1,352 or
1397 at 62 MPH, or
1,465 RPM at 65 MPH.

No reason not to get at least 6.5 MPG (with a heavy load) if you stay between 60 and 62 MPH (but closer to 60). Note: your tachometer will probably show 20 to 50 RPM less than what the engine really spins at. I have a digital tachometer and I found I get the best fuel mileage with minimum RPMs (CAT says, "stay between 1300 and 1350!).

Mine is a CAT C15 with 3.42 rears, 0.73 tranny, and low profile tires (512 reves per mile).

3.42 x 512 x 0.73 = 1278 RPM at 60 MPH or 1320 RPM at 62.

 

I have a 3406b, 3.90 rears, 18 speed and get 6 mpg. That's 65 or under. If you go over 65 and especially 70 or over............ look out.......... for a fuel stop that is.

I made the mistake ONCE of coming out of NC at about 75 mph, when I got home I fueled up and did the math, 4.5 mpg. Last time I did that.

 

Driver performance is about 30% of your MPG, the other is aerodynamics and then specs...... I run a flat-top FLD w/470 Detroit and was pulling a van (9ft of trailer dragging in the wind) and I was able to manage 7mpg as long as I skip-shift, and didnt stomp the pedal. If not, then she would get 6.0-6.7mpg.

Now that I'm pulling a light tank down and empty back i'm getting between 8.0-8.3mpg.

Its a 470 S-60, straight 10spd and 3.90rears. I only go 63mpg as well and make plenty of time.

 

There is not a lot of difference in time saved between 60 mph and 70 mph. It's almost impossible to maintain a constant speed......any speed.....unless your running across the endless hwys of the southwest.

But the difference in fuel economy between 60 and 70 is considerable. You will pay for that extra 10 mph. In 10 hours of driving, you might gain 50 miles.

Aerodynamics plays a big part in the fuel economy game. Weight doesn't matter that much. But a 20 to 30 mph headwind will cost you 2 to 3 mpg! OUCH!

Also, that extra 10 mph faster speed increases the frontal pressure area of your rig greatly! The faster you go, the more air resistance. The more air resistance, the more fuel consumed.

That's why a lot of drivers like driving at night. Usually there is no wind. The air is cool. No need for the A/C. Cool air is good for the truck and tires. And for fuel economy.

 

Theoretically, considering you don't stop for any reason at all, 10 mph over 10 hours will cost you 100 miles. If you factor that, your speed, and time during travel, you are looking at losing roughly an hour and a half of driving time by dropping ten mph. If you can afford to drop that much time, go for it, speed in combination with wind resistance makes a huge difference in fuel economy. However, if you are pushed for time and fuel economy doesn't matter at the moment, keep in mind, in a day of driving you will lose and hour and a half worth of distance by dropping those 10 mph.