N0DIH

04-25-05, 02:48 PM

I would like to start a thread to discuss the theories behind fuel economy. All are welcome to post what you have learned over the years, and what works, what doesn't and what worked and theories on why.

I have looked at EPA estimates on fuel economy as a general "gauge" to a known fixed set of conditions for a known combination.

Aka, a 1976 Olds 98 Regency is EPA rated at 12 city, 18 highway. We know that the car was a 200 hp smogger 455 with a mild cam and a QJet, THM400 w/2.41 or 2.73 gears. This car weighed in around 5000-5200 lbs. It is a "C" body, basically a long B or a short D body in those years. Having the 12 in front brakes and 11in rear drums very kin to our D bodies.

Ok, 18 highway for a brick wall front end BB Olds 455 with out OD. Not bad. I got 16 highway for my 1980 Turbo Trans AM with a 1970 Pontiac 455 with 10:1 compression, small valves with good flow and puny 2in duals. 3.08 gears and a THM350 and a 4000 lb car with moderate drag. That is real world economy at 75 mph, 3000 rpm cruise rpm without cruise control. Not very related to an EPA measurement.

So, my questions are:

1. If I take a 307 Olds V8 with a 2.56 gear and a .67 OD in a 89 Brougham, and baseline my mileage to say 24 mpg (guessing here) and I then remove that engine and drop in a 455 Olds with same everything, only keep a cam that has torque peak at same rpm, or less, what will my mileage be?

2. Now, if I take that same car and go to a 2.14 gear and reduce engine rpm further, with the massive torque of the 455, will it help or hurt mileage?

Look at a diesel semi, they run 1000 rpm with 80000 lbs and get 6-7 mpg. My dad's RV with a 8.1L BBC gets 6-9 mpg @ 2250-2600 rpm with 30K lbs and also a brick wall for aerodynamics. Why doesn't my 455 get MUCH better as it is having a much lighter load?

I have looked at EPA estimates on fuel economy as a general "gauge" to a known fixed set of conditions for a known combination.

Aka, a 1976 Olds 98 Regency is EPA rated at 12 city, 18 highway. We know that the car was a 200 hp smogger 455 with a mild cam and a QJet, THM400 w/2.41 or 2.73 gears. This car weighed in around 5000-5200 lbs. It is a "C" body, basically a long B or a short D body in those years. Having the 12 in front brakes and 11in rear drums very kin to our D bodies.

Ok, 18 highway for a brick wall front end BB Olds 455 with out OD. Not bad. I got 16 highway for my 1980 Turbo Trans AM with a 1970 Pontiac 455 with 10:1 compression, small valves with good flow and puny 2in duals. 3.08 gears and a THM350 and a 4000 lb car with moderate drag. That is real world economy at 75 mph, 3000 rpm cruise rpm without cruise control. Not very related to an EPA measurement.

So, my questions are:

1. If I take a 307 Olds V8 with a 2.56 gear and a .67 OD in a 89 Brougham, and baseline my mileage to say 24 mpg (guessing here) and I then remove that engine and drop in a 455 Olds with same everything, only keep a cam that has torque peak at same rpm, or less, what will my mileage be?

2. Now, if I take that same car and go to a 2.14 gear and reduce engine rpm further, with the massive torque of the 455, will it help or hurt mileage?

Look at a diesel semi, they run 1000 rpm with 80000 lbs and get 6-7 mpg. My dad's RV with a 8.1L BBC gets 6-9 mpg @ 2250-2600 rpm with 30K lbs and also a brick wall for aerodynamics. Why doesn't my 455 get MUCH better as it is having a much lighter load?