Overdrive+Gear Ratio=Gas Millage

[Daddy Brim]

I have a 67 C20 with 350 engine, turbo 400 trans. It also has a Gear Vendor overdrive, it is mounted were the carrier bearing would be. I had the cover off the rear end ( it is a Dana 60 ) and counted the teeth on the gears, 41/16 so it has 2.56 gears. I'm thinking this is probable a good set up for gas millage. The PO. used the truck to pull a 5th wheel trailer, so I guess it has good pulling power. Is this correct or wrong. This is one area I have no knowledge of.

[Billett]

Are you asking whether we think it will get good mileage? Is there some reason you can't drive it on the road? I think that would probably be the best gauge. Most c10 mileage threads on here usually have a consensus of 10-12 normally for sbc's 8-10 for bbc's and 16+ on ls engines. From what I've sifted through it seems that although the trans and gearing does obviously help, the size and lack of aerodynamics on these is what really prevents any really good mileage.

[BB72CHEVKT]

If it truly has 2.56 gears I do not think the Gear vendor overdrive is increasing mileage.

[cdowns]

i doubt u counted the right gear teeth// rotate that rear and look on the big gear to see numbers stamped in it

[Daddy Brim]

Originally Posted by Billett

Are you asking whether we think it will get good mileage? Is there some reason you can't drive it on the road? I think that would probably be the best gauge. Most c10 mileage threads on here usually have a consensus of 10-12 normally for sbc's 8-10 for bbc's and 16+ on ls engines. From what I've sifted through it seems that although the trans and gearing does obviously help, the size and lack of aerodynamics on these is what really prevents any really good mileage.

I do drive the truck almost every day, and my gas millage seems to be what everybody is getting. I think my question is, With the price of gas, Is there anything I can do with this truck to squeeze a few more mpg. I'm trying to educate myself on different rear end gears, and if I should change the gears or just leave it alone.

[67swb72klb]

that 2.56 gear does not seem right in a 3/4 ton truck

[Sideways]

Originally Posted by Daddy Brim

I have a 67 C20 with 350 engine, turbo 400 trans. It also has a Gear Vendor overdrive, it is mounted were the carrier bearing would be. I had the cover off the rear end ( it is a Dana 60 ) and counted the teeth on the gears, 41/16 so it has 2.56 gears. I'm thinking this is probable a good set up for gas millage. The PO. used the truck to pull a 5th wheel trailer, so I guess it has good pulling power. Is this correct or wrong. This is one area I have no knowledge of.

Is there a set of numbers stamped on the ring gear?

[davischevy]

Originally Posted by BB72CHEVKT

If it truly has 2.56 gears I do not think the Gear vendor overdrive is increasing mileage.

I agree. The only way to tell is going to be with a fuel mileage run from point A to point B and back, with the OD engaged and without.

Sidenote; My stock 1/2 ton has 3.08 gears, and I don't believe it would pull itself with 2.56 gears, let alone with overdrive.

[Daddy Brim]

Originally Posted by cdowns

i doubt u counted the right gear teeth// rotate that rear and look on the big gear to see numbers stamped in it

I counted the teeth on the big gear 41 then the gear parallel on the right 16. is this correct. I can pull the cover back off and look for a number, but probably not until next weekend.

[Billett]

I agree with these guys, 2.56 doesn't seem right. Also, I'd say the only way to get more mpgs out of these is an ls swap. The reason I say that is because I considered swapping to a manual trans with more than my current setup of 3 gears to one with 5 or 6. In the end it would cost me about 6300 miles worth of gas just to do the swap and I'd only end up with maybe 2 more mpg. With an ls you get a great engine, great trans, 10 more mpg, and it only costs about twice as much if you stick with stock parts.

[Sideways]

Not the side gear,were the four gears are housed.You want the pinion behind those that the drive shaft yoke is hooked to .

[SS Tim]

My guess is that you are counting the ring gear and the right spider grear from your description. The pinion gear is meshed directly to the ring gear. Probably have 4.57 gears that came in leaf 327/TH applications but there was a 4.10 option as well. Can you post a SPID so we can see what you started with? Either way these gears would be a good choice with an G/V overdrive, giving you a 3.56 or 3.20 final drive ratio. Is your O/D working? Just can't see 2.56 gears working in your truck much less overdriven.

[Daddy Brim]

Originally Posted by Sideways

Not the side gear,were the four gears are housed.You want the pinion behind those that the drive shaft yoke is hooked to .

I see I'm wrong with my statement of 2.56 gears. Transmissions and Rearends are one area that I know nothing about, I'm trying to educate my self. I'm going to pull the cover back off were will I find the number that is stamped on there ?

[Daddy Brim]

Originally Posted by SS Tim

My guess is that you are counting the ring gear and the right spider grear from your description. The pinion gear is meshed directly to the ring gear. Probably have 4.57 gears that came in leaf 327/TH applications but there was a 4.10 option as well. Can you post a SPID so we can see what you started with? Either way these gears would be a good choice with an G/V overdrive, giving you a 3.56 or 3.20 final drive ratio. Is your O/D working? Just can't see 2.56 gears working in your truck much less overdriven.

Spid pic.

[Sideways]

Originally Posted by Daddy Brim

I see I'm wrong with my statement of 2.56 gears. Transmissions and Rearends are one area that I know nothing about, I'm trying to educate my self. I'm going to pull the cover back off were will I find the number that is stamped on there ?

If you look on the large ring gear you will see the numbers,divide the small number into the large number. eg:41/10 =4:10

[Daddy Brim]

Originally Posted by Sideways

If you look on the large ring gear you will see the numbers,divide the small number into the large number. eg:41/10 =4:10

Thanks I will pull the cover back off, but it will haft to wait until next weekend.

[SS Tim]

Fun, your truck seems to have fleet order numbers for the rear axle, springs and suspension. Originally a four speed truck as well. If you find any codes on the axle especially the tubes I would like to see what you have.

[Daddy Brim]

This is what I have found so far. I will look for more.

[1972RedNeck]

Lets say this. It is is physically impossible to fit 2.56 gears in a Dana 60. Period.

[Vintage Windmills]

Number should be where the yellow circle is here:

[factorystock]

Originally Posted by SS Tim

Fun, your truck seems to have fleet order numbers for the rear axle, springs and suspension. Originally a four speed truck as well. If you find any codes on the axle especially the tubes I would like to see what you have.

This truck is a 1967 C 20, coil spring rear was the standard set up with HO52. Leafs and Dana's were not a regular options on '67 2WD C 20s. This truck has a special order rear suspension, in other words a GMC rear suspension.

[Daddy Brim]

[QUOTE=SS Tim;5936989]Fun, your truck seems to have fleet order numbers for the rear axle, springs and suspension. Originally a four speed truck as well. If you find any codes on the axle especially the tubes I would like to see what you have.[/QUOT

Where can I find the codes from my spid for the rear axle, springs and suspension.

[jocko]

if there is no code and callout RPO on the SPID (and there is not one in your case), that means it should have the standard rear end ratio - a 4.10 for C20s I believe. However, with factorystock's comment about this being a gmc rear - hard to say for sure.

[SS Tim]

Originally Posted by Daddy Brim

Where can I find the codes from my spid for the rear axle, springs and suspension.

With fleet codes it would be very hard and beyond the documentation I have at the moment. My earlier ratio posts were based on a 68 model and I didn't catch my error until factorystock's post.
As this is not an RPO code option rear end the actual ratio could be anything, but a 4.10 or 4.56 seem the most likely.

[piecesparts]

Your original statement had that the truck had a TH-400 tranny in it, with a Gear Vendors OD and a 350 engine. I would have to say that there has been some serious changes made to the truck from what your placard in teh glove box leads one to believe. The truck had a 327 Chevy engine with a 4 SPD tranny. Now go to the picture of the OD unit and look at the crossmember and it shows some elongation work from a torch to make the room for the drive components.

My guess is that since you have the TH-400 and a TH-350, the truck has been modified. The TH-400 is a heavy-duty tranny and it uses more fuel than it's counterpart the TH-350 to do work. That is just a fact of life. The OD unit is either there to help the fuel mileage or to help in pulling heavy loads down the road. Is it a unit that gives you gearing up for economy or gearing down for pulling loads. What are your RPMs at different speeds and is your speedometer accurate? What tire size are you running. With that info, we can figure out what you are doing with the OD unit.