Rear Gear change for better mpg - The 1947 - Present Chevrolet & GMC Truck Message Board Network

BigRed63 · 06-21-2017, 08:46 AM

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?

**Palf70Step** · 06-21-2017, 09:38 AM

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.

BigRed63 · 06-21-2017, 10:27 AM

Sorry, should have specified. The PO put the rear end on from an 84 Silverado.

chevy_mike · 06-21-2017, 10:51 AM

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?

BigRed63 · 06-22-2017, 07:09 PM

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.

**mongocanfly** · 06-22-2017, 09:04 PM

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

slorio · 06-22-2017, 10:14 PM

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.
Regards,
Steve
New Orleans

**Captainfab** · 06-23-2017, 12:45 AM

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

Quote:

Originally Posted by BigRed63

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.

chevy_mike · 06-23-2017, 11:01 AM

Quote:

Originally Posted by Captainfab

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.

BigRed63 · 06-23-2017, 03:48 PM

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

06-21-2017, 08:46 AM	#1
BigRed63 Registered User Join Date: Mar 2017 Location: Gladewater, Tx Posts: 35	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

06-21-2017, 09:38 AM	#2
Palf70Step State of Confusion! Join Date: Jul 2002 Location: Gulfport, MS USA Posts: 47,108	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. __________________ Bill 1970 Chevy Custom/10 LWB Fleetside 2010 Toyota Tacoma PreRunner SR5 Double Cab - DD Member of Louisiana Classic Truck Club (LCTC) Bill's Gallery Life isn't tied with a bow, but it's still a gift. Live simply. Love generously. Care deeply. Speak kindly. Leave the rest to God!

06-21-2017, 10:27 AM	#3
BigRed63 Registered User Join Date: Mar 2017 Location: Gladewater, Tx Posts: 35	Re: Rear Gear change for better mpg Sorry, should have specified. The PO put the rear end on from an 84 Silverado.

06-21-2017, 10:51 AM	#4
chevy_mike Never Ending Projects Join Date: Jul 2006 Location: San Diego, CA Posts: 3,836	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? __________________ . 1965 C10 Panel, Tiki Express http://www.67-72chevytrucks.com/vboa...d.php?t=506580 SOLD 1968 Chevy C10, Long, Fleetside, Hot Rod Hauler http://67-72chevytrucks.com/vboard/s...d.php?t=313233 SOLD 1965 Chevy C10, Long, Fleetside, Hot Rod C10 http://67-72chevytrucks.com/vboard/s...d.php?t=415702 SOLD We were given two ears and one mouth for a reason... listen twice as much and speak half as often...

06-22-2017, 07:09 PM	#5
BigRed63 Registered User Join Date: Mar 2017 Location: Gladewater, Tx Posts: 35	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.

06-22-2017, 09:04 PM	#6
mongocanfly Post Whore Join Date: May 2015 Location: Alabama Posts: 14,663	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

06-22-2017, 10:14 PM	#7
slorio Registered User Join Date: Sep 2006 Location: Louisiana Posts: 374	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. Regards, Steve New Orleans

06-23-2017, 03:48 PM	#10
BigRed63 Registered User Join Date: Mar 2017 Location: Gladewater, Tx Posts: 35	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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