Ok, I was yakking with the brother in law (who is an expert in everything it seems), and he was chiming off about gear ratios. Tis the season to visit with relatives, and get into friendly spats...
Anyway, to make a long story short..he was arguing that it was better to have gears such as 4.11's or 3.90 for highway driving. I kept telling him that it made more sense to have gears close to 2.56, 2.78, etc.. as the rear wheels will turn more often on those then the 4.11's.
Who is correct?
Am I wrong, or does not 4.11 gears mean that the driveshaft turns 4 times for every 1 turn of the rear wheels?
In my thinking, I would think that gears such as 2.56 or 2.78 would be better because it would mean lower rpms at the highway speeds...thus making it cruise better.
Just so everyone knows...we're talking about a '69 firebird vert, 400-4v, automatic 350th trans..
Your thoughts? What would be the best gears for Sunday cruising on the highway..or simple around town driving... Not the race track...!
I was always taught that the high gears such as 4.11, 3.90 meant racing....while the low numbers like 2.56, 2.78 meant simple cruising...
The engine back in the day 350 400 455 etc. HAd plenty of torque with the short duration cam they produced GOBS or low end torque.
The best all around ratio for driving were the 2.56-2.78 gears as the engines produced plenty of torque to get out of the hole easy enough and the ratio gave the best gas mileage STOCK pontiac set came with.
NOW when Pontiac made a racer / performance engine the duration of the cam shaft profile raises creating more valve overlap making the low end on a automatic feel slushy (yes even the big cube motors. TO GET to those BIG horsepower numbers everyone likes we need to move it up the rpm range... ALWAYS A trade off .
SO NOW we need to get back some low end that is where the gear ratios came in 3.55 3.90.
IF you were to stick 2.78 gears in a race motor set-up you would have a gas hog too because the carb would not supply the correct throttle angle for optimum air flow mixture and the car below 3000 RPMs will be a dog. WHICH is where most of driving on the street really happens. ONLY spikes above 4000-5000 happen when the power is being applied for freeway merging etc.
Anyway what I an trying to get at is the right gear needs top be matched with the correct engine build combo for optimum gas mileage and good street torquey feel.
So 2.78 may work for one motor and 3.55 for another for the best mileage (yes even with a 3 speed tranny)
I can certainly say that 3.90-4.33 would be for HO application and GAS MILEAGE AND CRUISING IS NOT A CONCERN...lol.
In the overdrive application YES 3.73 or 3.90 may give the better mileage. BUT we are usually in the fuel injected era and that is a TOTALLY different beast than a carburetor installed car.
BOY, I hope some one got something out of all that rambling...lol. Jim
I used to run my 400 with the RA IV cam, auto and 2.56 gears and got 18 mpg at 70 mph. The same engine was installed into a four-speed '77 TA with 3.23 gears and got about 15 mpg.
If you want to keep the same acceleration off the line after changing to a much bigger cam, usually the difference needed is going to be at the most a 20-25% increase in ratio if the engine is tuned properly. That means that a 2.56 should be changed to a 3.08 - 3.23 for example.
