885speed

iTrader: (1)

Hey guys, I've been cruising down the highway lately(not a typical thing for me) and I'm just curious on what is the best RPM to cruise with that is economy friendly.

After a little trial and error I noticed that doing ~50MPH in 4th at about 3500-4000RPM is more conservative on gas compared to around 2500RPM in 5th, which it tends to use ALOT more fuel.

Or... is this question kind of pointless?
I base that question on how I've learned that every car has its own "personality" especially the manual ones.

Atilla the Fun

There's something wrong with your engine set-up if you're getting better mileage at higher rpm.
My first thought is too big of a cam, as bigger cams do like higher rpm at cruise, especially if the tuning wasn't compensated.
For a basically stock camaro V8, except the L69, you should get over 25 mpg at a steady 65 mph at 1800 rpm.
The L69 I had never saw interstate use, so I can't say what it wants.
If your T5 is original, it should have the 0.725:1 fifth. If your car came with the LG4, you should have a 3.23:1 axle, and if your car came with the L69, it should be 3.73:1.
So, if your car is basically stock, work on fine-tuning the engine rather than playing with the gearing. GM chose gearing to exceed 20 mpg at a steady 55 mph, back in those days.
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If youre running the 700R-4, the LG4's standard 2.73:1 gearing sucks, and you'll actually get better mileage by going to a 3.08:1 gear.

885speed

iTrader: (1)

Well, I'm honestly suspicious if a previous owner put a cam in it, because my timing tab on the front case cover has been hacked off to obviously change the timing set, but no idea if the cam has been messed with, it seems to have a little rougher idle then my brothers 92 rs (same engine)

I do know for a fact though that my SES light comes on at about 55-60, brother says its the EGR valve which I believe.

Well, I honestly have neither of those engines, I have an LO3, T5 and stock 3.08 rear

Could my EGR be lowering my MPG?

If so I would like to figure out how to recalibrate it if its still good(yes yes I know about the tech articles)

EDIT: I REALLY! want to learn tuning, but it seems too expensive for me
(making minimum wage at sweetgay)
Is there a more cost-effective way to tune the engine myself?

EDIT EDIT: wow, after doing some basic math, its telling me I get barely over 5MPG?!?!
I seriously need a cable to hook up to my car/laptop to do some diagnostics!

this cant be right at all, I do between 50-60 miles in a week(roughly) and I go through about ten gallons of premium(seems to do a good deal better on MPG than on regular)
guys, I really need help with this one. I think a small question just turned into a small dilemma...

Atilla the Fun

Okay, L03s came with a 3.08:1 axle and a 0.625:1 fifth. This sucks. You'll really benefit from a 3.42:1 gear, if you need good mpg, or a 3.73:1 gear if you want stoplight fun.
First, my experiences with more than a dozen of my own camaros is that EGR helps mpg, when it is functioning correctly. At part throttle.
First, check your initial timing with the connector unplugged.
Second, reconnect it and check total advance at 1500 rpm, at part throttle. It should be 35-40 degrees or so. My LO3 is gone, so I can't verify that.
hird, check if your injectors are leaking or stuck, and if the fuel pressure regulator, built into the TB, is leaking.
Fourth, replace the O2 sensors.
A stock new LO3 car, way back when, would get gas mileage in the mid twenties, easy. Mine had the 700R-4, and got 23 mpg with both the original 2.73:1, in both third and OD, and with a 3.42:1 axle in OD.
So 23 mpg is your target.
Another thing to do is examine every inch of the high-pressure fuel line, from the tank to the TB, to verify it's not leaking.

80schild

iTrader: (5)

2500 RPM in 5th @ 50mph sounds high to me, especially with 3.08 gears. Is it possible someone has changed the gears? 50mph in 5th should be under 2k RPM easily.

885speed

iTrader: (1)

I'm already looking into buying a new rear end setup, just wasnt sure about it till now.
However though, would a 3.43 posi make any MPG difference then an open style one?

Also looking into EGR work to check it out.
as well as a fuel system check(sometimes on cold start-up the car will start, lose RPMs and die)

Not throwing any codes at me, so I'm guessing O2 sensor is fine, but will check it out too.

I honestly have no clue, all I know is that its open differential, and my 145MPH speedo reads almost exactly 10MPH slower than normal, but thats what I'm expecting because its a junkyard find, originally had a 120 speedo.

doc221978

This is actually a simple question that is getting over-analysed..... easier to keep it simple. It has been proven by GM, Jacobs, and a few others, that an engine makes peak mileage at peak volumetric efficiency. When I had my s10 it got 18 mpg at 60 mph and 26 mph at 70. 70 was at 2,250 rpm's which was about where the 4.3 in it made peak efficiency.

It's a matter of experimenting to find the sweet spot in the torque curve.

Atilla the Fun

Sorry doc, wrong again. Take any new '98-'02 F-car with the LS1/T56 combo, which are capable of 31 mpg highway, and replace the stock 3.42:1 rear gears with 4.10s, and you'll never see 30 mpg again. The LS1 makes peak volumetric efficiency up around 4000 rpm. Gear it to cruise up there, and you'll never top 25 mpg.
You want your engine just above lugging. 100 rpm too much is better than 50 rpm not enough.
And back to the original poster, posi won't affect your highway mileage.

Atilla the Fun

Man, what Doc posted is really buggin' me. I had this '78 Camaro. 350/TH350/3.08:1. Rated 270 TQ @ 2400 rpm, which was 55 mph.
I installed a 700R-4, but made NO other changes. Turning just 1800 rpm @ 65 mph got me 25 mpg. Up from previously being stuck in the teens for mileage.
Going 75 mph dropped that car downa couple mpg, even though it got closer to the torque peak. That car was well tuned before the trans swap, but had NO engine mods.
Do not try to cruise at peak torque with a mild cam. It is NOT THE answer. It IS a waste of gas. And engine life.