axle swap gas mileage

[John black]

I have an 85 k5 with a tired 305 holly 600 and intake stock manifolds dual exhaust 10 bolts with 3.08s and a th350 on 33x12.5 mastercraft mts I have a newish 350 with an rv cam and an opportunity for 14 bff and d44 with 4.10s was wondering if these might help my mileage because I'm only getting about 8-10 right now

 

[JoshHefnerX]

Hard question to answer, going up in cu in, up in rpm. Will be loading the engine down a lot less. I think 10mpg isn't really that far from avg though. I had a 77 for a couple of years and no matter what I did w/ it I got 10mpg...

 

[paratrooper307]

Probably not going to help. While you may gain a little bit in certain situations because the engine isn't as stressed, you're going to lose mileage from more cubes, more RPM, and more weight from bigger axles.

 

[John black]

Hopefully it will at least balance it out some highway I get close to 15 towns is closer to 6ish because my for has to stay into it for it to move decently

 

[John black]

"Foot"

 

[6872xtc]

That is why the 700R4 is somewhat better. Your truck would have had one from the factory, but the early ones weren't great.

Since it has overdrive, you can have more of both worlds, decent acceleration with lower gears and lower rpms on the highway. It also has the lower 1st gear than the TH 350.

But that would be a complete drive train swap! And do you know what transfer case that you have? There were a lot of swaps back in the day that kept the 208 and replaced the 700R4 with a TH350, so maybe you still have the 208.

 

[John black]

Yes it's a 208

 

[6.2Blazer]

Mainly depends on what type of driving you do. I would guess the highway MPG would drop a good bit because of the higher RPM. With the 3.08 gears you running very similar RPM's at highway speeds as compared to a 700r4 equipped rig with 4.56 axle gears. If you see much 70 mph freeway time the engine will be singing pretty good. Around town it probably will help. The other downside is the rear axle is much heavier and you will lose some mileage just because of that.

 

[John black]

Most of my driving is 55/60 some town driving very rarely does it see interstate it will probably see some light wheeling on local roads maybe a trip to try out an orv park not to far from here hopefully it would help driveability because it has not acceleration

 

[6872xtc]

In that case , I would do it! The rewards of how it would feel, would be worth it to me. I would guess that the mileage won't change much, maybe go down a tiny bit, but I doubt it. ( in my opinion )
But I also value the driving experience more than people who drive economy cars.

 

[John black]

Yea if I wanted a 80hp car I would get one

 

[Blue85]

Be careful seeking MPG improvements, as it's easy to spend more than you'll ever get back in fuel savings. The truck is never going to work very well with 3.08 gears and 33" tires, so that's a better reason to make some changes. Yes, a 700 would be the best thing for your MPG, but even overdrive is not as powerful a weapon as driving slow is. So figure out what makes sense based on how much you will drive it and how you will enjoy it.