comparing two cars

[analogkid455]

I had a 1987 Pontiac Firebird that got 27 -28 mpg on the highway cruising at 75 mph. It had a carbed chevy 305 v-8, .7 O.D., 2.73 gears and turned 2000 rpm at 75 mph.

Now I have a 1987 442 with a carbed Olds 307 v-8 with .67 O.D., 3.73 gears and it gets 10-13 mpg at 60 mph on the highway. Everything has been checked and gone through and the engine runs perfect. I know the 3.73 gears will hurt mileage, but that much? Cd will effect it also. The 442 is about .4 and the Firebird is around .3 . The only other thing I can think of is, the 307 only had 8:1 CR while the 305 had 9.4:1. Why would I be getting less than half the mileage? My 1974 Olds Omega with a 500hp 468 Big Block Olds with no O.D. and 3.73 gears gets 10-12 on the highway. I might as well drive that!

[cmittle]

My first thought would be fuel. Maybe the first one was lean this one is rich. It may run fine, but until you get a Wideband O2 or something similar to check the a/f ratio you wan't know for sure. Of course theres a lot of differences between engines and how efficient they are at converting combustion into useable power, including head flow, spark plug gap, exhaust flow characteristics, etc....

That being said don't compare numbers between other cars as a goal. If you're confident that the car is tuned up and running correctly you have to learn what driving method can get you the best mileage.

[some_other_dave]

I bet the gearing is a pretty big contributor... You've also got worse aero, and more engine (so more friction losses and so forth). It all adds up.

See if you can get a "taller" rear end and possibly a taller overdrive top gear. That will at least get you a decent amount of it back.

-soD

[cfg83]

analogkid455 -

I made a spreadsheet where I assumed that the cars have the same Cd :



I ignored the O.D. and engine size because they were within or below 5% of being the same. I used the ratio from the final gears to *extrapolate* RPM and MPG at 75 MPH. Before the aero penalty, your 442 should be getting in the neighborhood of 20 or more MPG at 60 MPH.

Hrmmmm, the 442 weighs 3203 lbs :

1987 Oldsmobile 442 Specifications

The Firebird is over 3200 lbs and seems to have a .333 Cd :

Camaro & Firebird Technical Data - ThirdGen.org
1987 Pontiac Firebird & '87 Pontiac Trans Am - ThirdGen.org

I need to factor in the O.D. because that 4% difference must be a negative multiplier.

CarloSW2

[Who]

Cd is kind of meaningless unless both vehicles have the exact same frontal area. You need to compare the CdA (Cd x frontal area).

There is a huge difference between 3.72 and 2.73 gearing. What are the OD gears in each? Are they the same?

[analogkid455]

cfg83, that is exactly what I calculated, 20 mpg. And thanks for the spread sheet. I know v-8's can get good gas mileage. My buddy had a 97 Z-28 with a v-8, 3.42 gears and .50 OD. We got 33 mpg out of a highway trip from Frederick, MD to Baltimore, MD. It ran 14.00 in the 1/4 mile. It was bone stock.

This is for the people that didn't read my first post very well.

I understand that the two cars don't compare very well but, I just don't understand why there would be such a HUGE difference. The 3.73 gears compared to the 2.73 gears is a big factor. The tire height is the same, although the 442 should have less rolling resistance because the tires have a much smaller contact patch (215/65/15 for the 442 and 235/60/15 for the Firebird) and also, the 442 has slightly more OD. .67 to .70.

I thought it was running rich as well but, it doesn't smell like it is. My Omega was running rich (burns your eyes out!) as a you know what and still getting 10-12 mpg without OD, 3.73 gears, 468 cubes with a non-computer carb, weighing 3620 lbs and a Cd of who knows, maybe .50?

I must have a fuel leak! (on the 442)

[sszewczuk]

hey saw your post about the 442 mpg. Did you ever get the problem identified? Just curious

[darcane]

Originally Posted by cfg83

analogkid455 -

I made a spreadsheet where I assumed that the cars have the same Cd :

Which is taking out a BIG factor in the equation....

3rd gen firebirds are used regularly in land speed racing for a few reasons:
RWD car with room for a big V8 and seats four
Very low Cd for a car of this type
Very low frontal area for a car of this type.

3rd gen firebird has a CdA of 6.43 sqft (nearly identical to the same year Corvette)

I can't find specifics on the Olds, the closest I can find is an '88 Caprice with a CdA of 10.11 sqft which is close in size and Cd, mostly just longer.

This will make a big difference in freeway cruising. That combined with the higher revs will account for most of the difference.