Team Chevelle banner
  • Hey everyone! Enter your ride HERE to be a part May's Ride of the Month Challenge!

Status
Not open for further replies.
1 - 15 of 15 Posts

·
Premium Member
Joined
·
1,090 Posts
Discussion Starter · #1 ·
I am planning to rebuild the trans and rear end for the Chevelle this winter. My question is how do you detirmine the optimal cruise RPM for good mileage. I would assume lower is better, but when do you get to low.

To give you an idea the current plan, I have a 383 with around 325hp. This figure could go up in the next few years. I am building a 700R4 for a transmission and a 12 bolt rear end. The car has 26 inch tall tires. With the current 3.08 rear gear that equals 1625 RPM @ 60 mph.

Would the ideal cruise RPM be affected by weight, Torque, etc.?

Any opinions? Any good reading out on the web?

Thanks,

R.J.
 

·
Registered
Joined
·
279 Posts
That sounds like a good setup to me. The 700-R4 has a steeper first gear so it'll make the 3.08's feel like 3.73's, second and third will be a bit more sluggish... but you should have enough momentum by then.

Weight won't effect the RPM, neither will torque.

------------------
kenny

85 El Camino 355ci
350 turbo - 4.11's
best run so far 12.92
My Page
 

·
Registered
Joined
·
38 Posts
I'd go with a little more gear.I had about the same set-up.Trans did not like going that slow,lugs the engine too much.Are you going with a little bit of a stall in the trans because you may be slipping all the time.Later Clarence.
 

·
Premium Member
Joined
·
3,438 Posts
When Super Chevy magazine was worth reading, Doug Marion did a bunch of econo performer articles with his 71 monte carlo. He was able to get 14 second time slips and over 20mpg from what I remember. He had a tight converter and 2:73 gears, no overdrive. The verdict was to keep the cruising rpm below 2000.

------------------
Dave
70 chevelle ss396 conv
66 chevelle ss396 hdp/conv
72 chevelle
TC member #493
Never forget Earnhardt!
 

·
Registered
Joined
·
659 Posts
I have also heard that internal combustion engines are most efficient below 2000 RPM. I guess you could choose a gear that keeps you below that at whatever speed you cruise.

There might be problems with with lugging the engine too low at that speed, since most all modern factory cars I have driven run around 2200rpm at 70.


------------------
Charles Perrell
[email protected]
65 malibu SS Convertible
283 2v powerglide
 

·
Registered
Joined
·
648 Posts
RJ'
I run a blown duece with a 700r4, 4.11 gears and 31"tires. 1st gear is like having 4.54's and 4th gear with the converter locked up is like running 2.70's. Remember that fun needs to be factored into your formula. I would think that 3.73 gears with the tires you mention would put you in the ballpark of about 2200rpm's at 70 mph. I wouldn't want to lug my motor more then that. With my tires I turn about 2200rpm's at 70mph. Have never thought about gas mileage,just haveing tooo muucch fuuun with it. Fun's gotta be the bottom lie when building.
 

·
Registered
Joined
·
1,451 Posts
you do want to be in your powerband at least, so i'd keep it 1800-2000 at 65-70, if it's low it WILL lug it down. you have to concider weight with these FAT cars. my 307 70' got 16mpg with a 2004r tranny and 3.8 gears tall tires, my current 454 with 331's and a 4spd (no overdrive) gets 14mpg. this shows the rpm band coming into effect and what the weight of the car can do. the 307 was lugging more than it should, if i had a steeper rear gear in it, it would have run higher RPM's but i'd bet id'a got much better MPG.

what you should do (since your planning this out which is VERY smart in my opinion) is go test drive a large SUV, while doing it , check the RPM's at 60-75mph, these new cars are set-up to get the greatest gas milage possible yet not lug the motor down, also check the rpm/speed that the tranny goes into overdrive. this is actualy the most efficient point, if you can get yours to do that, let the other parts (over 35mph or when ever overdrive kicks in) fall where they may, if it's 2000rpm at 70 then so-be-it, just find something close to the same weight/tranny gears
------------------
1970 SS396 AKA 454/4spd
My Chev-Hell Page
GOLD Member #783
Ft.Worth, Tx
(OO=ss=OO)

[This message has been edited by chev-hell (edited 10-24-2001).]

[This message has been edited by chev-hell (edited 10-31-2001).]
 

·
Registered
Joined
·
3,175 Posts
I am running 3.73:1 w/a 26inch tire.
I am at 2900 rpm @60 mph My speedo is off just a little, so that might not be exactly 60mph.
I think if you are worried about gas mileage, you should put a six cyllinder in there with some 2.??:1 or something. Should get around 25 to 28 mpg. But, why do that to your self?
 

·
Registered
Joined
·
203 Posts
Personally, I would go with 3.31. Make sure you use a lockup type convertor for your 700R4. You'll be happy.

BTW, new cars cruise at 1500/1600 rpm because they've been cam'd for low end torque. I bet your 383 won't. Target 2000 rpm.



------------------
-----------------------
Regards,
RLK
70SS LS6 coupe
71SS LS5 elk
72SS LS5 ragtop
02SS LS1 ragtop 35th SA
TC Member #918
ACES member
Metroplex Muscle
 

·
Premium Member
Joined
·
1,090 Posts
Discussion Starter · #10 ·
Thanks for the help.

I do plan on running a fairly tight converter with the lock-up operating. My final goal for the car is to be able to get 20 mpg highway and to run 13 flat with the same setup. I am going to try to make as much torque as possible and I want to keep the car carburated at least for the time being. This car is going to be a long distance cruiser and if I can obtain my goals it will be able to kick the crap out of 90% of what is sold today. (SS_Dave I promise that my next progect car will be able to run in the 10's
)

drptop70ss I remember reading those articles when I was sixteen and I had a '70 Monte. I wish I could get my hands on a copy of them now.

chev-hell that's a very good idea, I have been meaning to go out and beat on (i mean test drive) some new vehicles anyway.

And I did so more calculations and the RPM jump from 3.08 to 3.42 wasn't that great @ 60 MPH. 3.08=1672 3.31=1797 3.42=1857
However the first gear final drive ratio does jump a good amount. 3.08*3.06=9.42 3.31*3.06=10.13 3.42*3.06=10.46 And so without any more checking yet, I think the 3.31 or 3.42 is sounding like the early frontrunners.

Thanks,

R.J.
 

·
Banned
Joined
·
486 Posts
I think that keeping the engine in its powerband and spending alittle time optimizing the carb and ignition will help any car get decent mileage. I drive from Dallas to Houston about once a month with my 3.73 gears, 26.5 tall tires with the engine turning 3600-3700RPM and I average 15.4 MPG, with a best of 16.6 This is all done with a 355ci,2800stall,trick flow heads,286dur-.560 lift comp cam hyd.roller, 750 DP,performer RPM. One of my hot rodding buddies thought I was full of it, until he rode with me there and back. I let him do the math and he was stunned that the mileage could be that good. Dave

[This message has been edited by turbodave (edited 10-29-2001).]
 

·
Registered
Joined
·
1,676 Posts
I second what Doug Marion says about keeping those RPMs down to around 1800-2000 RPMs. Doug Marion is an icon when it comes to ECONO-PERFORMANCE

------------------
69 Malibu 454 10.00 to 1
2400 stall with 3.73
284XE Cam 049 oval heads
2.19 & 1.88 valves 850 dp
1.71 60' 12.42 ET at 107
ACES Member #3635
 

·
Premium Member
Joined
·
8,074 Posts
Good points were made about keeping RPMs below 2000. I would add that it depends on the torque characteristics of your engine. If it is geared too tall and you have to put your foot into it at the slightest hill then you've started to go the other way down the road of diminishing returns. I've heard of one member's long-departed '66 L34 396 (I can't tell you his name but his initials are Keith Tedford) with 3.73s, 4 speed, an L78 cam(!) and no power accessories (p/s or p/b). It ran a Holley 585 cfm and would get 17 mpg (imperial) around town and as high as 20 mpg (imperial) on steady runs on the highway. I've heard of several stock '66 big blocks capable of this in stock form with 3.73s. I've also heard of crappy mileage out of big blocks with tall 2.73s. The engine should be operated in a range where it does not have to strain (either lugging or screaming).
That will be 2 cents...
 

·
Registered
Joined
·
39 Posts
The bone stock '71 Monte I just parted out used to get 20-23 mpg cruising on the highway. You had to stay above 65 and I never did much over 75. It had a 350, th350, 2bbl carb, and 2:73 gears, no overdrive (obviously). I still have the build sheet if anybody wants any particulars. It didn't have a tach so I don't know how fast the engine was going but I do know it sure sounded like it had it easy.
 

·
Registered
Joined
·
777 Posts
airrj,
I would suggest going with some lower (higher numbered) gears like a 3:42 or 3:55. I have a mildly built 350 in my Nova with a 700r4 and 3:08 gears and while cruise is nice, around town it is a dog. I don't know what you run for a cam, but mine is not big and around town, I am less than 1,700 rpm and the motor feels like it is bogged down alot. Once I get it ove 2,000 rpm, it really perks up, but i can't help but feel that the bogging is hurting my milage.

FYI, I run 235/60 tires all around, so that is close to what you will have. Also, cruising on the highway at about 80 mph, it is still under 2,000 rpms! Though the speedo is not quite accurate.

Bill C.


------------------
Bill C.
'71 SS (now with 467BBC)
'70 Nova 350/700r4
ACES #2780
Colo Spgs, CO
 
1 - 15 of 15 Posts
Status
Not open for further replies.
Top