Rear end / mpgs

[The Youngen]

Im working on a 71 and it has a 350 with a turbo 350. Currently the rear end is a 3.73. What im concerned with is that when im on the highway, the engine will be sitting at at-least 3000 rpm. Any recommendations for ways to improve this? im not really concerned with towing or drag racing. Id much rather have my engine running low and efficient than screaming. Any suggestions? should i get like a 3.08 or 3.54 rear end? Would like some thoughts.

[heavy70]

3.08's and a 700r-4 tranny!!!
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[Sicklajoie]

What size are your tires?
With 3.73's if you're doing 3000 rpm in 3rd you're doing 75mph.
Slow down!

[dubds10]

I have the same setup and I just cruise the highway around 60-70mph...anything over 70 and you know it lol I don't go over that very often... I get about 200-300miles per tank depending on how I drive it. I'm running 33's and my mileage deff went down when I switched to them lol Personally I'd keep the 3:73, but I'm partial to those gears. I love them compared to other ratios but like others will say an overdrive tranny will do wonders for you if you decide to do some major changes.

[mrhino]

You fuel mileage loss is from getting those 33's up to speed. not down the hwy, if anything your probably are gaining mileage down the hwy with those bigger tires. it takes more engine power to turn a bigger tire, around town and stop light to stop light. if you change gear ratio to a taller gear like 3.08 you probably would loss more fuel mileage the engine the truck would be a dog, and would have to work even harder. however your top end mileage would increase. its a trade off you would have to figure out where you spend most of your time, in town or the hwy, and adjust accordingly. If you were to put in a 700r4 you would take care of both problems. 700's have a short first gear to get the wheels moving easier, and a tall 4th gear to give you low rpm while cruising down the hwy.

keep the 3.73"s and add the 700r4. in both of your cases

[The Youngen]

Originally Posted by mrhino

You fuel mileage loss is from getting those 33's up to speed. not down the hwy, if anything your probably are gaining mileage down the hwy with those bigger tires. it takes more engine power to turn a bigger tire, around town and stop light to stop light. if you change gear ratio to a taller gear like 3.08 you probably would loss more fuel mileage the engine the truck would be a dog, and would have to work even harder. however your top end mileage would increase. its a trade off you would have to figure out where you spend most of your time, in town or the hwy, and adjust accordingly. If you were to put in a 700r4 you would take care of both problems. 700's have a short first gear to get the wheels moving easier, and a tall 4th gear to give you low rpm while cruising down the hwy.

keep the 3.73"s and add the 700r4. in both of your cases

If i had any sort of high budget i would consider a rebuilt 700r4 but from what ive read a good rebuilt 700r4 runs about a grand while i can get a 3.08 for under 200. and im running stock tire size i believe. So 28 1/2", 29" something like that. it will probably be doing on average three speeds. 70, 50 and 30. (got to love small towns with highways.)

[mrhino]

Originally Posted by The Youngen

If i had any sort of high budget i would consider a rebuilt 700r4 but from what ive read a good rebuilt 700r4 runs about a grand while i can get a 3.08 for under 200. and im running stock tire size i believe. So 28 1/2", 29" something like that. it will probably be doing on average three speeds. 70, 50 and 30. (got to love small towns with highways.)

3.08 would put in the ball park of about 2500 rpm, at 70mph. thats a decent cruising rpm.

[The Youngen]

Originally Posted by mrhino

3.08 would put in the ball park of about 2500 rpm, at 70mph. thats a decent cruising rpm.

is 1.7k to low for these engines? with the 3.08 at 30 in second is 1.7. and 35 is 2k.

[mrhino]

Originally Posted by The Youngen

is 1.7k to low for these engines? with the 3.08 at 30 in second is 1.7. and 35 is 2k.

No, I wouldn't think so. lots of the big crusiers cars of the 70's and 80's had 3.08 or taller in them. I had a firbird once with a 350turbo and 2.92's it was a dog off the line, but id bet it would go 150mph though. lol

[The Youngen]

sorry guys. if i had the money i would do the 700r4 but the budget is tight so i guess im going with a 3.08.

[Sicklajoie]

Originally Posted by The Youngen

sorry guys. if i had the money i would do the 700r4 but the budget is tight so i guess im going with a 3.08.

Never heard of people in Westbrook complaining of tight budgets.....
Just kiddin...

[JMac11]

JUst to clear up a myth.....

2.73 700 r4 31x10.50
vs
3.73 700r4 33x12.50

Same Mileage City/Hwy

Hwy because you can actually use OD and Lockup at 65 mph

[N2TRUX]

Originally Posted by The Youngen

sorry guys. if i had the money i would do the 700r4 but the budget is tight so i guess im going with a 3.08.

If it were me, I would go with a 3.42. I feel you will be dissapointed in the 3.08 gears.

[mbgmike]

If you go the 700r route out of a truck make sure its either a real low mileage one or buy a good one. Some of them tend to go out at about 120,000 miles in the 1/2 tons. Mainly the 90's and 91's, gm made a fix for them but if buying used it may not have been. If truck was not used for towing much or at all they lasted longer.

[The Youngen]

Originally Posted by N2TRUX

If it were me, I would go with a 3.42. I feel you will be dissapointed in the 3.08 gears.

How would i be disappointed?

[treveiger]

^^^because your truck would feel "slugish" off the line to some people.

[The Youngen]

Originally Posted by treveiger

^^^because your truck would feel "slugish" off the line to some people.

Im used to be being way under powered. 2009 aveo. Barely has enough power to move itself.

[treveiger]

Then it shouldnt be a problem we drove a car like that in mexico for a rental, with the drop of the clutch itd spin the tires decent. If you have too low of a gear youd be able to jump off the line no problem but at high speeds your motor would be winding way to high thus hurting your mpg and wearing out your motor faster. If you have to high of a rear end your take off will be slugish and your motor will have to work harder to get up to speed so if you stop and take off a lot it will hurt your mpg but highway speeds would be better. You need to find a "sweet medium" for you to get the best mpg and people say the 3.73 and the 3.56 are good mediums but dont know from personal experience.

[Keller]

i have 3.08s in my truck and they suck. get about 8 mpg in nothing but stop and go city driving. part of that is my carb but gonna upgrade to at least a mid 3 this summer.

[treveiger]

Originally Posted by Keller

i have 3.08s in my truck and they suck. get about 8 mpg in nothing but stop and go city driving. part of that is my carb but gonna upgrade to at least a mid 3 this summer.

Do you have a auto?

[The Youngen]

Originally Posted by Keller

i have 3.08s in my truck and they suck. get about 8 mpg in nothing but stop and go city driving. part of that is my carb but gonna upgrade to at least a mid 3 this summer.

i thought the 3.08 was mostly for cruising. Want to trade?

[Keller]

Yes, I have a TH-350.

As far as trades go... whaddya got?! I'm thinkin 3.73 would be perfect for me.

[The Youngen]

a 3.73

[1967stella]

i wish i got as high mpg as some of you guys... I have a 327 with 3.73 gears and a 3 speed auto. im usually getting 140 per tank MAXX with the 20 gallon tank. i am not sure what tranni i have tho. Tires are 255/60r15

[1967stella]

Originally Posted by The Youngen

Im used to be being way under powered. 2009 aveo. Barely has enough power to move itself.

This is funny. My truck was a daily driver untill i bought a 2008 aveo to save the gas.