wow, sorry I created such a controversy. I'm still going to drop the 6 speed in my truck and I'll let you all know how it works out. How often do you see a '67 with a 6 speed, I think it'll be cool, so I'm going to do it. Of course it'll take about six months to save up the cash, oh well, you only live once

 

I'm not trying to boast about my car so much as point out that more gears will help with mileage and still allow you to have a high performance car.

20 MPG and 30 MPG are quite different in my book. I'm not exagerating either. I measured my mileage on a highway trip where I drove 75-80 MPH, but never let the engine get above 2000 RPM. I got over 30 MPG.

This cars does have some tech advances, but you'd be surprised how similar it is to our old engines. This thing is still a pushrod, single cam motor. It does have fuel injection, but a well tuned carbureted engine is not losing 1/3 gas mileage to EFI.

It does not take more power to maintain the same speed through a lower overall gear ratio. To move the same vehicle the same speed, requires the same power (provided all other factors are contstant). It does require that the power come at the lower RPM.

Take the same 60's Mustang. Keep the first 4 gear ratios and the same rear gear ratio, add 1 or 2 OD gears. The mileage will probably improve, the wear on the motor will probably be less. What's to debate here?

I'd like to apologize for having a bit too much attitude in my post last night. I'm dealing with a dying parent and I had 2 glasses of wine! I'm on the edge of a nervous breakdown!

 

BTW, at 2.50 gears, you are effectively running overdrive anyway. Run this car with 3.73 gears, improve the quarter mile times and add on overdirve to keep the same mileage.

 

Having driven a '72 Ford my dad owned awhile back(the only 4 speed I've driven, as I'm still young), then now having driven a friends Corvette with a 6 speed T56, I would be under the impression that an FE engine, if built well and all, would have plenty of power in the low RPM range that is needed for an OD tranny. I mean, seriously...how much torque is a 390 putting out compared to the SB Chev 350?

 

Gtex, I can simpathize with what you're going thru, I lost my Mom last January. Now that, that's said,, take any manual trans and turn the input by hand in direct ( high gear in non OD transmissions) notice the effort in turning it. Then, turn the output shaft by hand in third gear , notice how much harder it is to turn. This in effect the same thing thats happening in your trans when it's in OD. This also illustrates how much extra effort is involved in turning an OD gear. My Stang with a 2.50 geared rear isn't the same as having an OD trans and a lower gear ratio rear. The overall rpm reduction is the same, but the effort involved is different. And as Andysutt said about the technology differences adding to your mileage also holds true, the aerodynamics, and EFI make a huge difference over a carb in efficiency.

 

You're arguing with a guy with a degree in physics.

Given the same weight and same drag coefficient, the motor is seeing the same load regardless of what gear or rear gear ratio is used. Unless there are substantial design differences between the gears, which is unlikely in a given transmission. Why would they design 6th different than 5th, except for the size of the gear.

The only thig that changes is the RPM at which the engine sees the load needed to maintain a given speed. Short of pinging from overload, I can see no disadvantage to running at a lower rpm. In the case of my car, it actually has enough torque at the lower RPMs to overcome the air resistance at highway speed and even accelerate in 6th gear.

 

While I don't have a degree in physics, I do have 30 years of practical real life experience. Try what I posted and you'll see that your theory about it taking no more power to turn an OD gear than a direct or under driven gear is wrong. There is nothing different in the basic design of the gears, it just takes more force to turn an OD ratio than a direct ratio. And some engines, do not run as efficiently at lower rpms than others. Most newer engines are designed to operate more efficiently at lower rpms that OD ratios offer. The engine in my Ranger is built to operate at a higher rpm than what Ford last built them to run. I didn't believe what someone else told me about my mileage going up instead of down when I swapped to 3.73's from 3.08's but now after the deed has been done, he was right. Sure I'd like to cruise down the interstate with my motor turning less rpms, but I also like the acceleration it has now with the 3.73's and the fact that, no matter what abuse I dish out to the Toploader 4 speed that I paid $150 for, it'll take it without a whimper.

 

Hi, mind if I jump in the off-topic conversation?

Don't gear ratios give you torque multiplication? For example, if you started in 2nd gear it doesn't accelerate as fast as when you start in 1st. Seems like the engine is seeing less load in 1st. I'm trying to visualize what's happening to the load, am getting a headache.

Barry

 

Who's on first?

 

I'm trying to figure out what gtex mean't by "the motor seeing the same load regardless of what gear or rear gear ratio is used."

Still got a headache

 

I think he was referring to the final drive ratio, at the point where the tires touch the ground. But that still would be different because in OD you're trying to turn some components faster than they need to be then reducing the rpms down to get the same result. As much as I hate to think that I'd agree with what the europeans do as far as auto tech, they have it right when they avoid OD ratios to get the same effect. They geared the lower trans gears down to improve takeoffs and went with a higher rear gear ratio to acheive higher speeds out on the Autobahn. They know that OD geared components will not take sustained operation at high speeds or rpms.

 

As long as we are moving the same vehicle the same speed! (Many other factors have to be constant as well). But the gear ratio (and total gear ratio) is irrelevant. What's magical about a transmision gear ratio of 1:1 , when we change the overall ratio by using different rear gear ratios and different tire sizes. At the same speed all components after the transmission spin the same speed (assuming we dont' change the the rear gear ratio or tire size). In OD, the engine and many of the transmission internals are turning at slower speeds as well. As aI said above, your motor has to be making enough power at that RPM to over come your load (load being, what power is required to keep this vehicle moving through the air).

If 1:1 is "ideal" ,then what rear gear and what tire size is "ideal" :-)

 

So you are saying it takes the same amount of torque on the chain of a bike to get it rolling 2MPH in 18th gear as it does in 6th?

 

Yes and no, I'm not saying anything about acceleration (getting a bike moving from 0-2MPH is acceleration). To mainatin 2MPH on a bike on a flat road, your resistance is from the air you move through and there is some friction with your bike components (a product of the normal forces, normal forces being primarily your fat **** on a bike:-) . In a car this friction is even more neglible compared to the air resistance at speed. Look at your car as a black box, your engine pushes against the air resistance. Does this force change when you change gears? No!

Now, going from a stand still or moving uphill and things change. I'm mostly looking at the "static" condition present when OD is used.

 

You kinda confused me there Greg. When I am maintaining 50MPH down the road in second gear and change to third my engine is appplying more torque to the input shaft of the tranny to maintain the same speed. The engine is however doing less work (horsepower) and requires less fuel to maintain the speed. The engine is producing more torque at the flywheel to keep the same amount of torque at the wheels. The wheels require the same amount of torque to go 50MPH no matter what gear you are in.

Why would you downshift to go up a steep hill with a big load?(very big for your truck) The engine has applied its maximum torque to the input shaft of the tranny and has failed to produce the required amount of torque to the rear axles in the higher gear to maintain your speed.