Howdy neighbor if she runs good she don't need fix in..if she smokes idling hot replace injectors, my van is running 50% oil blend, no smoke at all,,injectors are new and it is timed correctly. when injectors are bad they dribble,before popping and cause incomplete combustion,,(smoke) clean your crankcase breather, can cause smoking

 

Well shoot. Turns out my truck does have a 4.10 rear. The axle code on the door tag is 35, which is a 4.10 open 6250 cap axle.

My conundrum is this. I was originally shopping for a 3.55 axle truck with a zf5 and diesel for a few reasons, mainly because I was wanting something that I could DD without breaking the bank but still be able to haul with when I need to, which is happening more and more as the weather gets warmer. I have been getting about 16-17 average MPG driving 700 miles a month, mostly short trips under 20 miles a day to and from school, but a once weekly 65 mile drive for college.

Now as I already said, my truck is kind of a drag going up hills with a load. I have a potential gear swap, straight across trade a 3.55 gear set for my 4.10. By going with higher gears, what would be my mpg gain vs towing capacity loss?

700 miles / 16 mpg = 44 gallons * $3.99 avg/gal = about $175 a month on fuel
700 miles/18.5 mpg = 38 gallons * $3.99 avg/gal = about $150 a month on fuel

Adjusted to yearly totals
$175 * 12 = $2100
$150 * 12 = $1800

Total savings yearly: $300.

The above math is assuming I gain 2.5 mpg from the gear swap. What do you guys think? If I'm only going to save about $25 per month, I probably won't bother with the re-gearing. Too much hassle for too little monies saved.


-Jameson

 

If you are getting 17 out of a 4:10 and you tow with it leave it alone.

Also, even if changing your gearing does make a mileage improvement it's not necessarily going to equal a mathematical figure. If it was that simple you'd go with 2.75s, (I know, not available), and push 30mpg. In reality you could even lose economy especially when towing as the truck is working harder to move.

FWIW I replaced the 4:10s in my '69 F250 with 3:73s. I saw a 200RPM drop at freeway speeds, yet did not gain an iota of fuel economy.

 

Mathematically, yes. Realistic? Nope. That would be a sight to see... A diesel trying to run on 2.75 gears...

That's what I'm thinking though. It doesn't break the bank to drive every day, and it would almost be more hassle than what it's worth to swap axles. Besides, $25 a month isn't crazy savings..

-Jameson

 

Keep the 4.10's. Make sure the fuel system is timed and up to snuff and start saving for a turbo.

 

It really just comes down to final ratios at the top and bottom, engine rpm and average use/speed. I want your 4.10 axle but hard to say if it would really do you any good. The question is what RPM are you running at highway speeds? Do you tend to speed?

Some quick math with a guess on actual tire size(235/85/16=31.7" diameter) gives me this, sound about right?
4.10 4th 2175RPM @ 50MPH
4.10 5th 1820RPM @ 55MPH
4.10 5th 2000RPM @ 60MPH
4.10 5th 2200RPM @ 65MPH


3.55 4th 1880RPM @ 50MPH
3.55 5th 1575RPM @ 55MPH
3.55 5th 1720RPM @ 60MPH
3.55 5th 1860RPM @ 65MPH

I'm no IDI expert and roll a C6(for now) so I never cruise below 2000 but my understanding from reading this forum is that 1800 is a good cruising RPM for the IDI. Also just for consideration 4th is a more efficient gear as it is direct drive. So which is better for you mileage wise comes down I would figure to how fast you drive. But my best guess would be 3.55s. They would let you cruise along in 4th when in the 40-50MPH area on back roads, and be good when you have to bomb down the highway at 65.

What that is worth mileage wise, I have no idea. If I had to guess 1 MPG.

 

My two main cruising speeds are 55 and 60. That's where I spend the most time.

60mph rpms are just over 2200 rpm, 55 mph is just under 2200.


-Jameson

 

Actually, 60 is about 2250, and 55 is about 2050 rpm.


-Jameson

 

I'm putting a 6.9 in my '89 F150 that has 2.73 gears.

But it's purely for fun and mpg. Truck won't be pulling anything.