gas mileage

[LanceB]

I know gas mileage and corvettes don't exactly go hand in hand, but, anyone know a decent checklist I can do to make sure I am getting the best mpg? It is a 77 L-82, changed gears from 2:73 (not real sure why these were in there) to 3:50 something for a better takeoff (if necessary), headers, true duals, and it maybe gets 7 miles a gallon. I change the filters, fluids regular, and just had carb rebuilt. Not real sure if there is a good way to improve mpg. Ideally I would like to be in the teens somewhere. Seems like it burns gas no matter what and I don't have a too heavy of a foot.

 

[JohnZ]

Was the mileage any different before you had the carb rebuilt? Is the choke operating and adjusted correctly? 7 mpg is certainly not typical for a properly-tuned L-82.

 

[TWINRAY]

What kind of driving gives you 7 mpg? I'd do a trip at 55-60 mpg to determine your best potential mpg. With headers, it will take longer for the motor to warm up than if you had a standard manifolds with a heat riser. I would WAG that the best you could hope for is trip mileage of 15 mpg tops. As a comparison, I get trip mileage with my '69 of 19 point something at a 60-70 mph average but it's an auto, 3.08 rear, with the 350/300 & a quadrajet.

 

[Bob Chadwick]

with 3:08's and an automatic I get 11 - 12 in town and 16 at 65 - 70 on the highway.

 

[TWINRAY]

with 3:08's and an automatic I get 11 - 12 in town and 16 at 65 - 70 on the highway.

Hi Bob,
Maybe could be that the hi-compression motors (pre '71) were that much more efficient. Plus I have no polution devices other than a PCV valve but I do have PS & PB. I don't know what I get around town - probably 14 at best.

 

[Bob Chadwick]

Weren't the earlier cars a little lighter with the chrome bumpers versus the rubber bumpers and some other crash stuff.

 

[TWINRAY]

Weren't the earlier cars a little lighter with the chrome bumpers versus the rubber bumpers and some other crash stuff.

'69 weighs 3354 (has AC)
'67 small block weighs 3021 (no power anything 'cept the 327/350)

 

[Willis76]

How accurate is that 20+ year old odometer? Just a thought, stranger things have happened to me, buy I'm not going there ..........

 

[Vettehead Mikey]

How accurate is that 20+ year old odometer? Just a thought, stranger things have happened to me, buy I'm not going there ..........

Odometers do not change calibration with time. They a simple gear reduction mechanism from the transmission output shaft vis the speedo cable.

BTW, my '73 regularly gets 18-19 mpg on the highway. Don't know what you guys do to your cars to get them to run so badly.

 

[Culprit]

Odometers do not change calibration assuming that the car still has the same diameter tires and rear gears that it was shipped from the factory with. Larger diameter tires would 'worsen' your gas mileage computations because the car is going farther than the odometer thinks it is. Don't know by how much though, and I don't have the time to do the calculations right now.

 

[81 Corvette]

i get about 11, combined city/highway. didn't seem to change much with the gear swap, but i've only run a few tanks thru her

81 w/ rpm air gap, road demon, headers, 3.54 gears, no ac, air pump or clutch fan

 

[6 Shooter]

about 29 mpg . . . ya got to love a 6 speed . . .

Later . . . . . .
6 Shooter

 

[The73vetteman]

I started a thread on this earlier this year - do a search on my name to find it. I was concerned that I was only getting 10 mpg at best out of my L48. The concensus view from people in the CAC was that for city driving, which is 90% of what I do, 10 was typical.

As someone said, the Corvette was never advertised as an economy model.

John

 

[Ol Blue]

LanceB

Did you change the speedometer drive gear in the transmission to match the new rear end gears? If not, your odometer will be off. Gears are only around $4 or $5 at Chevy, but they come in a pack of two.
Ol Blue

 

[brusso]

LanceB

Did you change the speedometer drive gear in the transmission to match the new rear end gears? If not, your odometer will be off. Gears are only around $4 or $5 at Chevy, but they come in a pack of two.
Ol Blue

You know what the sad part is? If he had not changed the speedo gear, after rear end gears, the actual mileage is worse than 7 mpg. With lower gears, the drive shaft is spinning faster than before per tire revolution. The difference between the two sets is about 22%. So if you were to travel 10 measured miles, the odometer would read over 12 miles driven.

3.50x10=35 (ratio x tire revs = drive shft revs)
2.73x10=27.3

27.3/35= .78 or 78% right?

So if used 1 gallon for this trip, using the odometer, you would think that you 12 mpg. When you actually only drove 10 miles.

 

[GaryS]

Unless you were driving a really tricked out BB, 7 mpg is way off base. My 72 LT1 with 4:11 gears got 15 mpg on the highway at a steady cruising speed. I wouldn't expect anything less than that in your car. Did you do an actual fuel mileage measurement? By that I mean: fill the tank full, drive the car some distance, like 100 miles or so. Fill the tank again, noting the gas. Divide miles driven by gallons.

I have read guys estimating their fuel milege by what their fuel gauge shows. THere is only one way to get true mpg and that is measurement of fuel and calculating miles driven.
Gary

 

[photovette]

I replace my old Quadrajet (which I had rebuilt previously) with a new Edelbrock Quadrajet. My gas mileage (city) went from 10mpg to 17mpg. Real crappy gas milage is in the carb--it can be no where else.

I have a 355/350hp motor (1yo), automatic transmission, 3.55 rear, and a/c.

Around town it gets 17mpg, on the Interstate at sustained 70mph it gets 17mpg. At sustained 55mph, it gets 23mpg. OD transmission would be nice, but the expense of changing it would never cover the fuel savings. When my TH-350 finally dies, I'll make the switch. This car is an every day driver.

Then, of course, by keeping my C3 instead of buying a new C5 or C6, I'll save enough money to fill a medium sized swimming pool with gasoline. I'll keep the so-so gas mileage, you C5-6'ers keep your car payments. No disrespect intended.

 

[81 Corvette]

brusso is correct with the gearing. I went from 2.87 to 3.54 and thats a 23.34% change. I enter all my gas info into an excel spreadsheet to track that info by fill up, avg week, avg month, and year. I havn't swapped the speedo gear yet, so i have to adjust my mileage down to account for the gear swap. First couple of tanks on the new gear was giving about the same result as the old gear. I still think its too early too say definitavly what the change is. If anyone would like a copy of my spreadsheet, drop me a pm.

 

[LanceB]

gears were changed last month. I will change the speedo to match (of course my shady mechanic didn't tell me that was necessary, among other things, and I know that he knew that, but that is another story). Not sure why the gas is so bad. To be honest, I have not done an exact calculation, but I did on my other 81 and estimated it by comparison. The 77 has a smaller tank, and I estimated for that also. I have to fill up the 77 almost everytime I take it to run errands even on pretty short trips. I thought the rebuilt carb would take care of some of it. I need to have an exact before I post next time, but I know it is very bad.

 

[GaryS]

To be honest, I have not done an exact calculation, but I did on my other 81 and estimated it by comparison. The 77 has a smaller tank, and I estimated for that also. I have to fill up the 77 almost everytime I take it to run errands even on pretty short trips. I thought the rebuilt carb would take care of some of it. I need to have an exact before I post next time, but I know it is very bad.

WHile your mileage may be bad, you really don't have an accurate idea of how bad especially since our Corvette tank gauges are notoriously inaccurate.
Gary