Rear gear ratio for mileage

[Enginerd]

Hi guys,

I have a 1978 C-10 with a 350 SBC, SM465 tranny, and 3.73 geared 12-bolt.  Would swapping to a 3.08 rear gear (or numerically lower) help my mileage much?

Thanks,
Gary

[zieg85]

If you do mostly highway driving yes, but otherwise probably not.  What is your current usage and mpg?

[Stewart G Griffin]

i think so---for highway cruising.  And it would likely improve more with 2.73.

Might be cheaper/easier to go with taller tires.

[Enginerd]

If you do mostly highway driving yes, but otherwise probably not.  What is your current usage and mpg?

I live in a rural area where most of the drive time is used to get to town, and then just a short distance through a small town.  I'm not sure what the current mpg is since my odometer is not working

I think so---for highway cruising.  And it would likely improve more with 2.73.  Might be cheaper/easier to go with taller tires.

I'd really like a set of 2.73 gears, but I cant buy 2-series gears new.  I'd also be happy with a set of 3.08 gears, which I can buy new, but I think I need a different carrier to use them, wouldn't I?  As for tires, I just bought all new stock size tires for the factory rims, so I don't want to get new rears again. 

[zieg85]

I would trade you in a heartbeat if you were anywhere close to NW Indiana.  3.73 is a perfect gear using a OD.  IMO your best bet would be to find a whole rear end and swap it out.

[travisr1988]

Wouldn't taller tires just throw off your odometer and speedometer? which really wouldn't help your mileage, it'd just make you think it is, a narrower tire will help a little, but I don't think a taller tire would help. Anyone care to explain the taller tire theory?

[Tx_Phil]

Taller tires equate to a numerically lower final drive ratio. The taller the tire the fewer times it needs to turn to cover the same distance as a short tire. This results in fewer RPMs and less fuel consumed.

The trade off is that you now have a longer lever arm thus taking more energy to get the tire turning. In this case taller tires cost you more gas.

If you drive 50/50 hwy/city taller tires will typically burn more gas. If you drive 75% hwy then they can save you a little gas.

[travisr1988]

That makes sense. Thank you for the explaination Tx_Phil

[Tx_Phil]

Glad I could pitch in.

I'm a computer geek more or less so my wrenching skill aren't really good enough to help out around here. When I do stumble across something I know the answer to I try to help.

[Blazin]

Based on what you explained as your driving habits in my opinion it would take you a long time to pay yourself back with the next to nothing increase in MPGs you will see of the cost of swapping gears, or even the whole axle.

[Boone83K10]

I have the same type of driving you have, except mine is in the mountains of NC. I have 3.08 auto hub 10 bolts with 700r4... I am only getting 7 mpg (average). I don't think the swap would be worth it.