Real world results for Overdrive in an FE

[Racer Z]

I signed up for the purpose of hearing the answers. I may want to do something like this. Keeping the rev's down at cruising speeds and hopefully climb up to 11 MPG.

[mlheppl]

No real life experience, but I've been looking into this some myself for my highboy with a 435. Gear vendors boasts that it will increase your mpg's by 22% which at first sounds pretty impressive until you do the math. That will jump your 9 mpg's up to 11 mpg. I would expect that 22 to 25% would be the average or norm for whichever type of overdrive system you went with including an AOD or other overdrive tranny. The problem is I don't think that you'd realize much of that increase potential. I seriously doubt that the increase in fuel economy would be realized during in town driving. Rather I believe the potential is when driving on the highway or freeway.

IIRC, you're wanting to use your bump to pull the jeep up to 4 wheelin country. During towing, the OD would be pretty much useless most of the time so you won't see much gain there. So then your potential fuel savings would be when driving on the highway and not towing anything.

No facts, just opinions, but that was my
Mike

[Racer Z]

IIRC, you're wanting to use your bump to pull the jeep up to 4 wheelin country. During towing, the OD would be pretty much useless most of the time so you won't see much gain there. So then your potential fuel savings would be when driving on the highway and not towing anything.

No facts, just opinions, but that was my
Mike

Towing our race car (eventually on a trailer) over the Grapevine and other hills puts me in the same position. But most of the time I'm running light. I currently have a C6 (automatic 3-speed) and was thinking that a 5-speed manual might be good. My concept is that first should be a Granny Gear while fifth should be over 1:1
But, that kind of thinking wasn't popular forty years ago. Not sure what my choices are that will bolt up to my FE block (390). I know there were 3-speeds and 4-speed manuals as well as the C6.

[GSequoia]

Same story Z. I like my C6 and will like it towing but I won't always have a Jeep on my back. Also getting something with a really low first would be nice but that will just be an added perk to going overdrive (If I go OD it will likely be in a manual), if the numbers don't make sense I'll just do a rebuild on the C6 and call it a day (since the C6 is one auto I'll gladly trust for tow duty).

[Racer Z]

The C6 has proven itself over the years. No question there. My 69 F250 came with a shift kit installed. Had a nice firm shift. If I was on the gas it would kick pretty hard. But my 70 is a slush box in comparison. It has such a lazy shift that it's very hard to tell when it actually shifts.

[GSequoia]

The C6 has proven itself over the years. No question there. My 69 F250 came with a shift kit installed. Had a nice firm shift. If I was on the gas it would kick pretty hard. But my 70 is a slush box in comparison. It has such a lazy shift that it's very hard to tell when it actually shifts.

Hey, I just noticed you've got L.A. on your location tag... Whereabouts?

[Racer Z]

Hey, I just noticed you've got L.A. on your location tag... Whereabouts?

Venice, and I got a buddy near you in Lawndale.

[My427stang]

Love OD in my FE powered Mustang

However, you need to do some math to pick the right gear and box combo.

If running a manual transmission, you want to look at 1st gear and OD multiplied by your final drive.

Any lower than 12.25:1 (1st gear x rear axle) makes 1st gear very short and can be annoying.

Anything taller than 2.90:1 in OD (OD x rear axle) would make 5th only usable in real flat situations in a truck.

If running an AOD, focus on high gear more, 1st isn't as fussy, because it'll shift when it wants.

Here's a real world example of my Mustang, keep in mind, its 489 cid and makes a lot of torque, so my numbers allow a little taller rear axle gear than a truck would want.

ORIGINAL 5 SPEED BUILD
2.87 1st gear x 3.70 rear = 10.61:1 1st gear compound
.64 OD x 3.70 rear axle = 2.37 final drive compound in 5th

This setup was OK in town, but needed a little more low end at stoplights, 5th gear I found myself driving way too fast, and really didn't need to use it unless I was rocking along. In 5th it was too low of RPM at 70 mph to be efficient.

RECENT GEAR CHANGE
2.87 1st gear x 4.11 rear = 11.80:1 1st gear compound
.64 OD x 4.11 rear axle = 2.64 final drive compound in 5th

This setup is fantastic. 1st gear in town is nice, the motor doesn't have to work as hard and it doesn't feel like granny low. On the highway it raised my rpm by only 350 rpm at 75 mph, but I get better mileage.

If I went with 4.33 in the axle, 5th would still work well, but 1st starts getting short. Keep in mind this is a car, that 4.33 combo would probably work well in a heavier truck. Bottom line, be sure to do the math before you pick an OD/rear gear combo.

[JG F100]

I have a 4 speed overdrive in my truck with a 360FE, not sure where it came from but last time I did research on it was an RTS something or another. First and second gear are pretty close and short followed by 1:1 3rd gear then OD. love it at highway speeds. I wouldnt expect great MPGs in these trucks, maybe if you had fuel injection and a good tune but its hard to overcome the weight.

[mlheppl]

Love OD in my FE powered Mustang

However, you need to do some math to pick the right gear and box combo.

If running a manual transmission, you want to look at 1st gear and OD multiplied by your final drive.

Any lower than 12.25:1 (1st gear x rear axle) makes 1st gear very short and can be annoying.

Anything taller than 2.90:1 in OD (OD x rear axle) would make 5th only usable in real flat situations in a truck.

If running an AOD, focus on high gear more, 1st isn't as fussy, because it'll shift when it wants.

Here's a real world example of my Mustang, keep in mind, its 489 cid and makes a lot of torque, so my numbers allow a little taller rear axle gear than a truck would want.

ORIGINAL 5 SPEED BUILD
2.87 1st gear x 3.70 rear = 10.61:1 1st gear compound
.64 OD x 3.70 rear axle = 2.37 final drive compound in 5th

This setup was OK in town, but needed a little more low end at stoplights, 5th gear I found myself driving way too fast, and really didn't need to use it unless I was rocking along. In 5th it was too low of RPM at 70 mph to be efficient.

RECENT GEAR CHANGE
2.87 1st gear x 4.11 rear = 11.80:1 1st gear compound
.64 OD x 4.11 rear axle = 2.64 final drive compound in 5th

This setup is fantastic. 1st gear in town is nice, the motor doesn't have to work as hard and it doesn't feel like granny low. On the highway it raised my rpm by only 350 rpm at 75 mph, but I get better mileage.

If I went with 4.33 in the axle, 5th would still work well, but 1st starts getting short. Keep in mind this is a car, that 4.33 combo would probably work well in a heavier truck. Bottom line, be sure to do the math before you pick an OD/rear gear combo.

You also need to take tire size into account. Let's say you're running 245/60/15's on your mustang, and GSequoia is running 235/85/16's on his F250. The 235/85/16's have a 16" larger circumference. That is going to lower his rpm's considerably.

[no_treble]

Good points. I didn't do much of a complex calculation, but more of a 1-to-1 comparison and got as close as I could. Hopefully what I did will hold up in the real world... I may have gone slightly too tall based on my own differences in tire size and the rear-end, but I'll have to do a seat-of-the-pants test once it's all back together to be sure.

After asking some forum members questions to get a better understanding of the tranny options and ratios, I compared the curb weight, engine, transmission, rear-end and tire diameter of my truck to the 1989 Ford F-150 I was getting the 5-speed out of. The 89 was running a 302 (albeit EFI) and a M5R2 with 3.08 in the rear, a stock tire size of 235/75R15, and it had an advertised curb weight of slightly over 4k lbs. My truck is also running a 302, has a slightly larger (by 0.2") tire size of 255/70R15, and an advertised curb weight of slightly less than 4k (and has trimmed some with more aluminum on the engine and a polyethylene gas tank). Since I will be using the truck on the interstate a fair amount, I picked the rear gear for my 9" that was closest to the 3.08, which is a 3.00. I will try it and see, and if it feels too tall I may move down to a 3.25:1.

We'll see how it goes in a few months when I get it back on the road.

[Racer Z]

This is an online gear ratio calculator.

http://webspace.webring.com/people/cz/z_design_studio/

We use it for our 240z Race car (hence my name Racer Z). It has the common 240z tranny ratios pre-installed, but allows you to key in your own gear ratios, tire sizes, and final drive ratio.

Keep in mind that real trucks need a low first gear. Lets say you got a full load (bricks, rocks...) and need to start on a hill....

I don't know what my C6 has for gear ratios, 3rd is 1:1 and I'm pretty sure my Dana 60 is 4.11. The stock tires were 9.50 x 16.5 (I think), currently I have 11.50 x 16.5. Tires without an aspect ratio are usually an 80 series tire.
The calculator now tell me that at 6,000 RPM I'm doing 75 mph. I don't have a tach, but this seems right. At 7,000 rpm I would be doing 87 mph. I did take the truck up to 90 once, just for a few long seconds, and the motor sounded like it was ready to explode.

[mlheppl]

Here's another calculator.

http://www.grimmjeeper.com/gears.html

[Racer Z]

Haha. It says I'm doing 39 mph at 5,000 rpm with my 11.50 tires.

Maybe if I had a 4x4 with mud whompers it would work for me.

[mlheppl]

Yeah the calculator is set up for 4X4's. You'd probably have to manually enter 1.0 for high and low range in the transfer case section for a 2wd vehicle.