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Old 06-21-2017, 08:46 AM   #1
BigRed63
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Rear Gear change for better mpg

I have a 63 C10 with a 250 inline 6 put in a 700r4. When I had the tranny swapped I was told I had a 2.73 rear end. I am currently getting 11mpg, and I don't ever expect to get great gas millage out of this truck, but I was told going to a lower gear would probably help. I'm mostly city driving with some hwy miles. Does anyone else have this kind of setup and any thoughts on a good rear end gear setup?

Last edited by BigRed63; 06-21-2017 at 09:20 AM. Reason: change title
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Old 06-21-2017, 09:38 AM   #2
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Re: Rear Gear change for better mpg

Overdrive with a 2.73 isn't a great combo, when using the OD portion of tranny. Keep it in none OD for in town driving. Personally, I have never heard of a 2.73 in the pre 72 years, mostly just 3.08 or 3.73. My 82 had a 2.73 with a T350. I average 11-12 in town and 15 ish on the highway (65-75MPH)

a 3.73 with 700R4 is a good combo for Hiway. Your intown will remain about the same though, least in my experience. As you know, the 250 is not a gas mileage engine.
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Old 06-21-2017, 10:27 AM   #3
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Re: Rear Gear change for better mpg

Sorry, should have specified. The PO put the rear end on from an 84 Silverado.
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Old 06-21-2017, 10:51 AM   #4
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Re: Rear Gear change for better mpg

If you do have a 2.73 gear, you didn't need a 700R4. In OD you are totally lugging the engine, which will actually get worse mileage and is not good for the engine in general. With the .64 OD ratio in the trans, you should be at a 3.50 or 3.73 ratio.

Most older engines seem to like a cruise RPM in the 2200-2400 range, as they are most efficient there. What's your OD cruise RPM?
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Old 06-22-2017, 07:09 PM   #5
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Re: Rear Gear change for better mpg

Sorry for late response, was wanting to get an accurate reading before posting. OD cruise is only about 1700 and 2000 in D, this is why I'm wanting to change out my rear gears. Hopefully soon I'll get some estimates on how much it will cost.
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Old 06-22-2017, 09:04 PM   #6
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Re: Rear Gear change for better mpg

Agreed with above..2.73s and 700r4 is not the best choice..here's a rpm calculator tool that might help..http://www.csgnetwork.com/multirpmcalc.html
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Old 06-22-2017, 10:14 PM   #7
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Re: Rear Gear change for better mpg

Big Red:
One question. With transmission and rear end both changed, are you sure your speedo and odometer are accurate? There are free apps for a smart phone or a GPS where you can compare your speedo to see if it is accurate. I would do that first before you conclude on your current mileage.
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Old 06-23-2017, 12:45 AM   #8
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Re: Rear Gear change for better mpg

This is confusing....is that in D or OD?

Quote:
Originally Posted by BigRed63 View Post
Sorry for late response, was wanting to get an accurate reading before posting. OD cruise is only about 1700 and 2000 in D, this is why I'm wanting to change out my rear gears. Hopefully soon I'll get some estimates on how much it will cost.
I agree on a gear change to either 3.42's or 3.73's depending on tire size.
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Old 06-23-2017, 11:01 AM   #9
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Re: Rear Gear change for better mpg

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Originally Posted by Captainfab View Post
This is confusing....is that in D or OD?



I agree on a gear change to either 3.42's or 3.73's depending on tire size.
How it reads is, in OD he is at 1700 RPM's and in D he is at 2000 RPM's.

3.73's would be a good choice, if it was my money to spend. Actually at 2000 RPM's in drive, I would have left the transmission alone.
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Old 06-23-2017, 03:48 PM   #10
BigRed63
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Re: Rear Gear change for better mpg

This is my first project truck, still have a lot to learn, but the PO told me he thought it had a 3.73 rear end, this is why I chose the 700r4. I didn't realize it was any different until I swap the tranny and the transmission guy told me it was a 2.73
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Old 06-23-2017, 04:33 PM   #11
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Re: Rear Gear change for better mpg

Quote:
Originally Posted by BigRed63 View Post
This is my first project truck, still have a lot to learn, but the PO told me he thought it had a 3.73 rear end, this is why I chose the 700r4. I didn't realize it was any different until I swap the tranny and the transmission guy told me it was a 2.73
Welcome to the truck world and your first lesson. Never trust what a previous owner tells you. Always double check yourself.
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