3.07-3.73 swap:big mileage difference? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

bigcarguy · 08-07-2002, 11:06 AM

How big of a gas mileage difference would I see if I replaced my 3.07 rearend with a 3.73 posi unit? I have the TH-350 auto with a 307 engine.

RON WOODGEARD · 08-07-2002, 11:15 AM

Without knowing the tire size and doing a numbers thing it's a guesstemation,, I would suggest filling the tank up and driving your rig around in second gear for a few miles then check your mileage, It's gotta be somewhat close.

You may be surprised, you may see an increase. With 307 gears in town you are probably lugging the engine. with shorter gears you could bring the vacuum up into the proper working range. it all depends on where you do the most driving and the RPM of your engine, but this would be an easy way to get somewhat of an idea.

RON

Bill72BB · 08-07-2002, 11:22 AM

I'll give you a ballpark figure your milage will drop approx 17% depending on how you drive ,I find that with 3.73 or 411 I tend to have a lead foot stop light to stop light.
The question is do you want to be faster off the line or a freeway cruiser ? Larger Diameter tires will help the milage also.

Good luck Bill

bigcarguy · 08-07-2002, 11:34 AM

So taller tires will improve mileage on higher gears?

palallin · 08-07-2002, 11:35 AM

I got over 20 mpg on the highway when I put a 3.07 rearend in my old '67 swb fleetside.

bigcarguy · 08-07-2002, 11:43 AM

I'm really interested in the posi out of it. I reckon I could try it out and if I don't like it then I can just swap back. I was looking into buying a 3.73 posi rearend out of a 70, I reckon I could put it on, try it out, and decide what to do with it then. It'll mainly be a city driver, but I'm taking it across country once or so, but it'll primarily be a city driver.

JHickson6 · 08-07-2002, 01:01 PM

Try this page out www.ring-pinion.com/calculators.shtml

Click on "Engine RPM"

Input your two different gear ratios and you'll see the difference!!

JHickson6

JHickson6 · 08-07-2002, 01:22 PM

Oh I just realized that you can't swap a 3.07 for a 3.73 beacuse they use a different series carrier. I can't find the other post about swaping differentials. Something like if you have 3.07's than you can only swap in gears up to 3.73's, but not 3.73's you can do 3.54's with no problem though. If you have 3.73's you can only go higher like 4.10's. I'm not dead on about that but there is something about swaping those gears.

jku72 · 08-07-2002, 02:23 PM

I did the oposite swap. Went from 3.73 to 3.07.
Mileage with 3.73: 13mpg
" " 3.07 18mpg
With the 3.07 I can cruise 65 at ease, with the 3.73, the engine was racing. I run a near stock 350.
I did have to swap carriers, but I used a junkyard set of gears, and took the carrier with the ring gear, shims and all, for 75 bucks. I have even thought of going with 2.67, because I have a sm465 with granny low, so I would always have good takeoff power. I am not sure if I will do this, since the truck has gone into semi retirement, only makes local trips now.

67_SS396 · 08-07-2002, 03:47 PM

I am running a pretty stout little BigBlock, 'glide and 3.73's (soon to be 4.10 or 4.56). I get roughly 10-12 on the hwy at 3600 rpms and have 26" tall tires.

My old 65 had a 307, muncie M-22 and 3.31's out of a 'velle with 26" tall tires and I could get 15-16 on the hwy. With the 396 I could pull 16-18. That's with A/C in the 65. No A/C in the 67 as of yet.

bigcarguy · 08-07-2002, 03:55 PM

what about just around town? I'll only take it on a few long distance trips, but for the most part it'll just stay in the city. I was thinking of swapping the whole rearend, not just the gears. The rearend with the 3.73's has a posi unit, which is why I'm most interested in it. How much does a new posi cost?

jku72 · 08-07-2002, 04:38 PM

I would think that if you kept it under 40 all the time, you may actually do better with a 3.73. Less strain that way. I live in a rural area, so 3.07 makes more sense, but in town a 3.73 may be better. You decide.

Bill72BB · 08-07-2002, 05:46 PM

[QUOTE]Originally posted by bigcarguy
[B]what about just around town? I'll only take it on a few long distance trips, but for the most part it'll just stay in the city.

If that is the case I'would go with the 3.73 posi .Much more fun around town and won't lug the engine.

Bill

johnymac · 08-07-2002, 07:23 PM

I too have 3.73 gears. I lovem, wouldn't change for anything.
My truck sounds like a winston cup car of the highway.
My solution is to next upgrade the tranny to a 200R or a 700R.

You wouild have the best of both worlds.

A costly solution, but just a thought.

**MikeB** · 08-07-2002, 10:14 PM

You CAN put 3.73 gears in your 3.07 carrier if you get a "thick" 3.73 gearset designed for the 3.07 carrier. Randy's Ring and Pinion has these, as does Summit Racing and others.

weasel29gm · 08-08-2002, 12:02 AM

My 69 has 3:73's had 3 differnant motors , 307 ,305, 350 60 over the last was a killer. The first 2 got 14-15 in town and a little better on a trip. I like the 3:73 its a good all around gear.The posi makes it a blast to drive, the 60 over with a Muncie it was to scary that's why its goin'down sized!!!!

08-07-2002, 11:06 AM	#1
bigcarguy Finally a Bulldawg Join Date: Apr 2001 Location: Athens, Georgia Posts: 220	3.07-3.73 swap:big mileage difference? How big of a gas mileage difference would I see if I replaced my 3.07 rearend with a 3.73 posi unit? I have the TH-350 auto with a 307 engine. __________________ Ah yes, the college life. School all morning, work all night, study when ya can. Uh...

08-07-2002, 11:22 AM	#3
Bill72BB Thats the Hillbilly way Join Date: Jul 2001 Location: Torrance,Ca Posts: 5,980	I'll give you a ballpark figure your milage will drop approx 17% depending on how you drive ,I find that with 3.73 or 411 I tend to have a lead foot stop light to stop light. The question is do you want to be faster off the line or a freeway cruiser ? Larger Diameter tires will help the milage also. Good luck Bill __________________ BILL Torrance Calif 90504 Member DUF Club Member Old Farts Club 72 3/4ton Custom Camper BB402/400 trans Dynomax Cerama-coat headers , Hei distributor ,Edelbrock 1411 air,ps,pb,leafsprings ,98 chevy 60/40 seat,S10 shoulder belts tilt,3.54 Dana 60 rear. 16" Eagle wheels with LT265/75 16 tires,Gaylord Lid,H4 headlights with 100W relay conversion. " If I knew I was going to live this long I would have taken better care of myself " Marriage changes passion...suddenly you're in bed with a relative. A GUY'S PRAYER.... Lord, I pray for a nympho with huge boobs who owns a liquor store and likes to fish and hunt and has a Harley .Amen ,

08-07-2002, 11:34 AM	#4
bigcarguy Finally a Bulldawg Join Date: Apr 2001 Location: Athens, Georgia Posts: 220	So taller tires will improve mileage on higher gears? __________________ Ah yes, the college life. School all morning, work all night, study when ya can. Uh...

08-07-2002, 11:35 AM	#5
palallin Registered User Join Date: Jul 2002 Location: St. James, MO Posts: 1,239	I got over 20 mpg on the highway when I put a 3.07 rearend in my old '67 swb fleetside. __________________ '69 Longstep K-10: 327/SM465/T-221/Closed Knuckle Dana44/12-bolt.

08-07-2002, 11:43 AM	#6
bigcarguy Finally a Bulldawg Join Date: Apr 2001 Location: Athens, Georgia Posts: 220	I'm really interested in the posi out of it. I reckon I could try it out and if I don't like it then I can just swap back. I was looking into buying a 3.73 posi rearend out of a 70, I reckon I could put it on, try it out, and decide what to do with it then. It'll mainly be a city driver, but I'm taking it across country once or so, but it'll primarily be a city driver. __________________ Ah yes, the college life. School all morning, work all night, study when ya can. Uh...

08-07-2002, 11:15 AM	#2
RON WOODGEARD Resident Young Old Dude ! Join Date: Jul 2001 Location: Nevada Posts: 1,948	Without knowing the tire size and doing a numbers thing it's a guesstemation,, I would suggest filling the tank up and driving your rig around in second gear for a few miles then check your mileage, It's gotta be somewhat close. You may be surprised, you may see an increase. With 307 gears in town you are probably lugging the engine. with shorter gears you could bring the vacuum up into the proper working range. it all depends on where you do the most driving and the RPM of your engine, but this would be an easy way to get somewhat of an idea. RON

08-07-2002, 01:01 PM	#7
JHickson6 I Love This Stuff! Join Date: May 2002 Location: Winnipeg, Manitoba, Canada Posts: 739	Try this page out www.ring-pinion.com/calculators.shtml Click on "Engine RPM" Input your two different gear ratios and you'll see the difference!! JHickson6 __________________ My Rides 1967 C10 In Pieces for now 1985 Ford B700 SourPuss Bus - BIG BLUE 2003 Silverado CITY WORKER YELLOW!!

08-07-2002, 01:22 PM	#8
JHickson6 I Love This Stuff! Join Date: May 2002 Location: Winnipeg, Manitoba, Canada Posts: 739	Oh I just realized that you can't swap a 3.07 for a 3.73 beacuse they use a different series carrier. I can't find the other post about swaping differentials. Something like if you have 3.07's than you can only swap in gears up to 3.73's, but not 3.73's you can do 3.54's with no problem though. If you have 3.73's you can only go higher like 4.10's. I'm not dead on about that but there is something about swaping those gears. __________________ My Rides 1967 C10 In Pieces for now 1985 Ford B700 SourPuss Bus - BIG BLUE 2003 Silverado CITY WORKER YELLOW!!

08-07-2002, 02:23 PM	#9
jku72 Registered User Join Date: Jan 2002 Location: cornville, maine Posts: 659	I did the oposite swap. Went from 3.73 to 3.07. Mileage with 3.73: 13mpg " " 3.07 18mpg With the 3.07 I can cruise 65 at ease, with the 3.73, the engine was racing. I run a near stock 350. I did have to swap carriers, but I used a junkyard set of gears, and took the carrier with the ring gear, shims and all, for 75 bucks. I have even thought of going with 2.67, because I have a sm465 with granny low, so I would always have good takeoff power. I am not sure if I will do this, since the truck has gone into semi retirement, only makes local trips now. __________________ jku Cornville, Maine 68 C10 burb 42 Willys MB 46 Dodge WF-32 1.5 ton 53 & 56 Olds' 60 Cadillac 22 Dodge Brothers

08-07-2002, 03:47 PM	#10
67_SS396 Registered User Join Date: Mar 2000 Location: Blue Ridge, Tx Posts: 1,172	I am running a pretty stout little BigBlock, 'glide and 3.73's (soon to be 4.10 or 4.56). I get roughly 10-12 on the hwy at 3600 rpms and have 26" tall tires. My old 65 had a 307, muncie M-22 and 3.31's out of a 'velle with 26" tall tires and I could get 15-16 on the hwy. With the 396 I could pull 16-18. That's with A/C in the 65. No A/C in the 67 as of yet. __________________ Fawker Fab three seater rock crawler buggy 498 BBC/SM465/203/205 doubler, propane, 5.13's, 42" PB Rockers '66 C10 - The new hot rod '77 3+3 Dually - 12 valve 4wd conversion tow rig project '28 Ford 2 door sedan street rod RevLimit Offroad "Hang on beer, we can do this."

08-07-2002, 03:55 PM	#11
bigcarguy Finally a Bulldawg Join Date: Apr 2001 Location: Athens, Georgia Posts: 220	what about just around town? I'll only take it on a few long distance trips, but for the most part it'll just stay in the city. I was thinking of swapping the whole rearend, not just the gears. The rearend with the 3.73's has a posi unit, which is why I'm most interested in it. How much does a new posi cost? __________________ Ah yes, the college life. School all morning, work all night, study when ya can. Uh...

08-07-2002, 04:38 PM	#12
jku72 Registered User Join Date: Jan 2002 Location: cornville, maine Posts: 659	I would think that if you kept it under 40 all the time, you may actually do better with a 3.73. Less strain that way. I live in a rural area, so 3.07 makes more sense, but in town a 3.73 may be better. You decide. __________________ jku Cornville, Maine 68 C10 burb 42 Willys MB 46 Dodge WF-32 1.5 ton 53 & 56 Olds' 60 Cadillac 22 Dodge Brothers

08-07-2002, 05:46 PM	#13
Bill72BB Thats the Hillbilly way Join Date: Jul 2001 Location: Torrance,Ca Posts: 5,980	[QUOTE]Originally posted by bigcarguy [B]what about just around town? I'll only take it on a few long distance trips, but for the most part it'll just stay in the city. If that is the case I'would go with the 3.73 posi .Much more fun around town and won't lug the engine. Bill __________________ BILL Torrance Calif 90504 Member DUF Club Member Old Farts Club 72 3/4ton Custom Camper BB402/400 trans Dynomax Cerama-coat headers , Hei distributor ,Edelbrock 1411 air,ps,pb,leafsprings ,98 chevy 60/40 seat,S10 shoulder belts tilt,3.54 Dana 60 rear. 16" Eagle wheels with LT265/75 16 tires,Gaylord Lid,H4 headlights with 100W relay conversion. " If I knew I was going to live this long I would have taken better care of myself " Marriage changes passion...suddenly you're in bed with a relative. A GUY'S PRAYER.... Lord, I pray for a nympho with huge boobs who owns a liquor store and likes to fish and hunt and has a Harley .Amen ,

08-07-2002, 07:23 PM	#14
johnymac Registered User Join Date: Mar 2001 Location: Dover, NH USA Posts: 194	How about using an overdrive tranny I too have 3.73 gears. I lovem, wouldn't change for anything. My truck sounds like a winston cup car of the highway. My solution is to next upgrade the tranny to a 200R or a 700R. You wouild have the best of both worlds. A costly solution, but just a thought.

08-07-2002, 10:14 PM	#15
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,470	You CAN put 3.73 gears in your 3.07 carrier if you get a "thick" 3.73 gearset designed for the 3.07 carrier. Randy's Ring and Pinion has these, as does Summit Racing and others. __________________ Mike 1969 C10 LWB -- owned for 35 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- sold 1981 C10 Silverado LWB -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 24 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then!



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08-08-2002, 12:02 AM	#16
weasel29gm SLOW BUT SURE,BUT CLOSER Join Date: Feb 2002 Location: eleanor wv usa Posts: 3,091	My 69 has 3:73's had 3 differnant motors , 307 ,305, 350 60 over the last was a killer. The first 2 got 14-15 in town and a little better on a trip. I like the 3:73 its a good all around gear.The posi makes it a blast to drive, the 60 over with a Muncie it was to scary that's why its goin'down sized!!!! __________________ 69 C-10, OWNED 35YRS... 350 over 30, 350 Turbo, 3:73 Posi P/S, P/B Black with red and black int. ''LOVE THESE OLD ''TRUCKS.......

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