Higher gear for differential

[eyebuzz]

Hello. Looking to improve MPG a bit - currently getting 7city/12hwy (would be happy with 10/16+). And beyond MPG it would be nicer to "cruise" a bit more on the highway - currently "force" is a better word to describe the ride on an expressway - even at 55. '72 c20 Suburban with 350/350 - so it probably has a 4.11 - 4.56 in the differential. I don't plan to tow (and probably never tow anything over 1,000lbs - but will, on occasion, haul 1000+ lbs in the truck over hills and such)...on to the questions:

1) To me, it seems easier to change the gearing to something higher within the differential than to look for/install a whole new rear end - but can this been done? (from everything I see here, yes - but a mechanic told me it can't be done...so I'm here asking the question directly)

Assuming, yes you can change the gears in the dif:

2) Can I go as high as a 3.07 with no issues (being a c20 was my transmission set up differently than a c10 - that kind of stuff)?

3) I recognize this needs to be a professional job - any idea of cost ranges (at what point will my picture and the word "gouge" be synonymous)?

Thanks,
TT

[raycow]

If you have the H052 rear (removable center section and round rear cover) there is no taller gearing available than 4.11. Your only practical fix is either a complete rear end swap or an overdrive transmission. If you have a Dana rear (made all in one piece with an irregular-shaped cover) you can get taller gears for it, at least down to 3.54.

Do you have leaf or coil springs?

Ray

[68longhorn]

have you considered overdrive?

[eyebuzz]

Thanks Ray and Longhorn - The differential seems to be the H052 (cover is round as a full moon). My hesitancy to switch the rear end comes more from locating and shipping a rear end (looking at the set up it doesn't look that hard to do). I'll think about it and compare to overdrive - see what's out there...so this leads to a few questions --- BTW - coil springs.

Overdrive:
1) Overdrive would just help out on the highway - correct? (solve two of my three concerns - hwy MPG and cruising...not city MPG)
2) From what I read it looks better, for the automatic, to replace the transmission - switching in one with a 4th gear vs. an external overdrive. Opinions?

Rear End Swap:
1) Any difference for a 72 pickup to a 72 Suburban (or even Blazer/Jimmy) in terms for the rear end - can I use one from any 3/4 ton?
2) For 3/4 tons what was the proportion of Dana or H052 differentials (rough estimate)?...very many Danas with taller than 4.11?

Thanks,
TT

[68longhorn]

An overdrive trans will most likely only help on the highway, If you go with a 700R4 the steep 1st gear may help mileage in the city slightly because the engine will work less to get her moving. (Thats theory based, I have no proof of this occuring). Another thing you could try would be locking up your converter in 2nd or 3rd gear, It makes a solid link from the motor to trans so you don't get slippage. It is used in drag racing and increasesthe mph of a run, but never seen it done like that on the street. The hang on units like gear vendors seem neat, and I know people who say they work great. Not 100% sure,but I believe you can overdrive each gear, so your 350 could go 1st, 1st-OD, 2nd, 2nd-OD, 3rd, 3rd-OD. That could help in the city.

I'm not familiar with the rear options for your truck, but I've been told steeper gears help fuel economy in stop and go because it doesn't work the motor as hard on starts. Hopefully someone with a little more experience can chime in with what they have done.

[Redcap]

As far as Dana 60 rear ends, 3.54 is a common gear set.

[raycow]

Do you have leaf or coil springs? This determines the selection of rears you can use, unless you want to weld on the required mounting brackets.

I believe an overdrive transmission with a lockup converter would help your city mileage if you wire it to lock in 2nd and 3rd (don't worry, this was the stock setup on a lot of factory installations). This would not be true for an external O/D like the Gear Vendors or US Gear, because then you would still be driving through your original converter. Also, a version of the TH350 is available with a lockup converter. By itself, it should help your city mileage, but you would still need a rear end swap to address your highway issues.

Ray

[eyebuzz]

Thanks Ray...Coil springs.

I'm guessing the hassle of finding and shipping a rear end is going to out-way the "ease" of a new transmission - so for now I'm leaning towards the new transmission. I see at Bowtie Overdrives they talk a good game about their product. I used to live in the Bay Area and loved looking around "Pick-N-Pull" (I think that was the name...it was long time ago) - but here in NY I don't have anything like that so to find a rear end it would probably be something sight-unseen.

Between my three issues - city MPG, hwy MPG, and cruising at highway speeds - I'd be most interested in solving the last two...so a 4spd transmission should do that. (It is kind of a shame as the current Turbo 350 works pretty good).

Best,
TT

[Captainfab]

I'm not sure if you're aware of this, but you could also swap in a 14 bolt rear end from a '73-'87 C20, or K20. or maybe even newer truck. Of course this would require cutting and welding to install. Just mentioning that in case those are easier to find in your area.

[eyebuzz]

Thanks Captainfab - I probably won't - but that's what's great about this forum, it might help someone (and at least I know). Cool stuff.

TT

[chris mc bride]

IMO the ideal gear for non perfomance vehicle would be 3.42 or 3.73. for towing I would go with 3.73 light loads to 4.10 for heavy loads. If not eager to change rearends and limited to how tall a gear you can run then I would say a overdrive tranny is in order. If you go 700r4 or equal you might look at lockup converter also. Persoanly I like 3.73 with 700r4s lower 1st gear and then overdrive 4th. Great off the line and pretty good milage on highway.