[rad400]

Would there be a good benifit to switch from 3:73 -to a 3:08 rear as far as better gas mileage. how about switching to a low compression engine as well. my current set up in my sig.

[steve]

just take your foot out of it !

[v8drinker]

I thought higher compression was more efficient than lower compression, as long as your octane and tune can support it. Didn't they reduce compression for emissions reasons rather than efficiency reasons? Nox or something?

PS I bet they tell you to run a Qjet and why the 3500 stall for mileage?

[78 GHOST]

Conrad: IMHO you have two big challenges in your combo, the gears and the converter.

I actually run 3.08s with a 3200 RPM stall converter, kinda freaky how it jumps off the line and will run 85-95 MPH all day long.. but then I didn't want a tall geared car I couldn't drive anywhere.

In your shoes, you might see a better result if you look at the rear gear and as mentioned, a well set up Q-Jet is hard to beat in terms of fuel efficiency.

You didn't mention the type of driving you do which helps determine the best route to choose. Around town versus highway.

Driving habits are the single biggest determining factor in fuel mileage. I drive a 4400 LB Ford F-150 Lariat 2WD Supercrew that usually averages around 17 MPG around town and 20 MPG on the highway. I ordinarily don't rev the engine over 2000 RPMs at all. On the last tank, I focused on maintaining my momentum, such as slowing long before arriving at a red light, and only using 1800 RPMs to get going, the result was 18.4 MPG. That is about 7.7% increase in fuel efficiency just by taking my foot off the pedal a little.

I consider mechanical changes in light of the financial aspect. Changing your engine, gears, and converter might cost you $5000 which is going to take a LONG time to amortize. Personally, if I had a stock converter on the shelf, a set of gears I could install myself, and a nice Q-Jet I could have built by Cliff, I might consider doing so.

However, there is only so far you can go with any combination. I once had a '78 T/A with a low compression 400 and 2.56 gears running a Q-Jet that I had dialed in to run a best of 22.5 MPG on the highway. That car was tuned to the razors edge and probably would not do any better than that unless I had done some aero mods to the body.

While the Gear Vendors overdrive is very expensive (around $2700) it's good for a lotta HP and gives you about a 30% reduction in cruise RPM, in addition to being able to split each gear to keep the engine in it's happy RPM band.

Overall, you might find a better result keeping what you have and bolting on a GV unit. Cruising RPMs are a big factor.

[Z Code 400]

Low compression reduces Nox emissions and is more octane friendly. High compression is more efficient. As V8Drinker pointed out, you have a lot of parts that are not exactly geared towards fuel economy.

If you brought the car to our shop and asked our opinion, I would suggest a Crower (only because you are already using one) #60240 camshaft, a Q-Jet and a TH200-4R transmission...Robert

[Slick 50]

The 3.08's will help your MPG on the freeway but you will lose accelleration on the bottom end some. Pontiacs have great torque, for the steet go with the 3.08's, they are a great compromise for the street.

I believe the lower compression engines were due to fuel and EPA. The fuel additives to make high octane fuel cause bad emmisions(lead). It always blew my mind that the EPA engines of the 70's and 80's had less power and burned more fuel for the sake of the enviornment. Oil industry-$$$$$$$$$$$$ ????

[rad400]

thanks guy for your input, would 2400 stall be better for the crower 60210 229/239@.50 .480/.501 lift. and 3:73 rear.

[Stuckinda60s]

Long gears will help quite a bit in a flat state like Delaware (or around Houston and Dickinson, right guys?). In a hilly area, they may actually reduce mileage due to converter slippage and the greater throttle opening required for climbing hills. The tighter stall converter [b]might[b/] help, but once again, because you live in a flat area, possibly not as much as you might think. It depends on how much slippage you have at highway speed and, as I said, that increases when you climb hills. A tighter Continental type converter would probably help more than simply going to a 2400 stall by another manufacturer. I echo the recommendation for the 200-4R, using a Continental converter, and the QJ. They would be the biggest improvements and would actually help acceleration. Changing to a 3.55 and maybe even a 3.23, coupled with the trans and carb changes would probably give you more scoot and definitely improve your mileage.

I've always felt the longer gears, 3.42's as opposed to standard 3.73's, in my 2000 GMC have actually reduced my mileage around home because it's mountainous. I'm running the 4.8 and believe the 5.3 would actually give me better mileage around home with these gears.

[ericwy]

I am getting about 15mpg city driving with the combo listed in my sig and I use
the secondaries. I was getting 11-12 before putting in the OD 700R4.
I might be able to squeeze another mpg by carefully tuning the carb, but
the spark plugs already look good w/ slight tan/grey deposits & very clean.
Its currently tuned for better performance not mileage and its a daily driver.
Planning a short road trip end of this month so I will see what the highway mileage is.