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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: Enginerd on March 01, 2013, 04:34:46 pm

Title: Rear gear ratio for mileage
Post by: Enginerd on March 01, 2013, 04:34:46 pm

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

Title: Re: Rear gear ratio for mileage
Post by: zieg85 on March 01, 2013, 05:18:43 pm

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

Title: Re: Rear gear ratio for mileage
Post by: Stewart G Griffin on March 02, 2013, 12:50:43 am

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

Might be cheaper/easier to go with taller tires.

Title: Re: Rear gear ratio for mileage
Post by: Enginerd on March 02, 2013, 01:11:58 pm

Quote from: zieg85 on March 01, 2013, 05:18:43 pm

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

Quote from: Stewart G Griffin on March 02, 2013, 12:50:43 am

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. 

Title: Re: Rear gear ratio for mileage
Post by: zieg85 on March 02, 2013, 01:36:21 pm

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.

Title: Re: Rear gear ratio for mileage
Post by: travisr1988 on March 02, 2013, 09:10:14 pm

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?

Title: Rear gear ratio for mileage
Post by: Tx_Phil on March 02, 2013, 11:14:25 pm

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.

Title: Re: Rear gear ratio for mileage
Post by: travisr1988 on March 03, 2013, 02:50:33 pm

That makes sense. Thank you for the explaination Tx_Phil

Title: Rear gear ratio for mileage
Post by: Tx_Phil on March 06, 2013, 07:01:07 pm

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.

Title: Re: Rear gear ratio for mileage
Post by: Blazin on March 07, 2013, 07:31:31 am

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.

Title: Re: Rear gear ratio for mileage
Post by: Boone83K10 on March 07, 2013, 10:30:21 am

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.