Right now I get 16 mpg w/2:73 gears, if I go to 3:73 gears how badly will that hurt my mpg?

------------------
'86 IROC
T-TOPS, TINTED WINDOWS, BRAKE LIGHT BLACKOUTS
GM GOODWRENCH 350
EDELBROCK TES HEADERS
FLOWMASTER
EDELBROCK 600CFM CARB.
KN AIRFILTER
ACCEL HEI DISTRIBUTOR
2:73 GEARS
Rebuilt 700R4, with manual valve body
B&M Megashifter
Soon to be installed
and Exhaust Cut-Out
"There is nothing more addicting than speed"

 

Something is not right, you should be getting way more than 16 mpgs with the mods you are showing.

I would work on getting your mileage more where it should be..in the mid 20s at least.

FYI, I get US 30 mpgs on the highway with my L98.

 

But Glenn, if this is the Glenn I think it is, don't you have "highway mode" invoked?
I agree, 16mpg is terrible with your mods and 2:73s. I used to pull 30mpg highway with my 305TBI 2:73 equipped convertible. What like 75mph at 2000rpm?
But yes, your mpg will decrease with the 3:73 rear end. How much I can't really say.

------------------
Rob P
89RSconvtZZ4TPI
Edelbrock Hi-Flow Intake
SLP Dual Cold Air Intake
1 5/8" Headers
Semi-Siamesed Runners
MSD6AL/AdjFPR/IROC frnt@rear swaybar/wonderbar/steeringbox/alum drvshaft/ Alston SFC/3:23posi disc rear/MAC LCA/H.Adams Panhard Rod/KYB struts/ shocks/
92Z28convt5spd
71Impala convt 402BB
BETTER DRIVING THRU SUPERIOR HORSEPOWER!

 

You are right Pete, but even before invoking Highway Mode, I still got 25 US MPGs on the highway with my L98 and 3.23s.

Even with a carb, those 2.73s should get him darn close to 23-25 when its tuned right with a 600 CFM carb. And the Edelbrock can be "leaned" a little; which is what Highway Mode does.

 

How do I lean out the carb?

------------------
'86 IROC
T-TOPS, TINTED WINDOWS, BRAKE LIGHT BLACKOUTS
GM GOODWRENCH 350
EDELBROCK TES HEADERS
FLOWMASTER
EDELBROCK 600CFM CARB.
KN AIRFILTER
ACCEL HEI DISTRIBUTOR
2:73 GEARS
Rebuilt 700R4, with manual valve body
B&M Megashifter
Soon to be installed
Exhaust Cut-Out
"There is nothing more addicting than speed"

 

You can buy a calibration kit, part number "1487" and go leaner on the rods and jets.

How is your driving style? Your siq says you have a full manual valvebody your shfit points could be decreasing mpg.
you cold also get a carb that has vac secondaries like a holley 600cfm. Edlebrocks like to start dumping some fuel from the secondaires at 2500 rpm, even though you do not feel the 4bbl power.
SSC

 

I don't drive the car all that hard, it's not a full manual valve body, I can leave it in OD and it shifts. Could playing w/ the A/F ratio help, and if so which way do I turn the screws.
I recently went thru emissions and failed big time, one reading was 7x the limit and another 3x, could this be why I am getting horrible mpg, plus I don't have a converter.

------------------
'86 IROC
T-TOPS, TINTED WINDOWS, BRAKE LIGHT BLACKOUTS
GM GOODWRENCH 350
EDELBROCK TES HEADERS
FLOWMASTER
EDELBROCK 600CFM CARB.
KN AIRFILTER
ACCEL HEI DISTRIBUTOR
2:73 GEARS
Rebuilt 700R4, with manual valve body
B&M Megashifter
Soon to be installed
Exhaust Cut-Out
"There is nothing more addicting than speed"

 

If your numbers were 7X the limits, I'd guess and say your are running WAY rich. And yes, this will kill your gas mileage.

------------------
Rob P
89RSconvtZZ4TPI
Edelbrock Hi-Flow Intake
SLP Dual Cold Air Intake
1 5/8" Headers
Semi-Siamesed Runners
MSD6AL/AdjFPR/IROC frnt@rear swaybar/wonderbar/steeringbox/alum drvshaft/ Alston SFC/3:23posi disc rear/MAC LCA/H.Adams Panhard Rod/KYB struts/ shocks/
92Z28convt5spd
71Impala convt 402BB
BETTER DRIVING THRU SUPERIOR HORSEPOWER!