MPG Difference between manual and auto?

[EB70]

I am preparing to build the "super cool, really bad" off road Bronco. However, in between I need something to wheel. I have a 70 with a lift and locker. I'm going to gently mod a few things and then wheel it with some large tires for snow stuff and a trip to the beach.

Assuming that things are geared properly for tire size. What is the difference in MPG between an auto and a stick in general? Stock or modded?

FWIW- I will likely be running 4.56's, 37's and my tranny options are 3-speed or c4. I have a couple of c4/dana 20 combos that I would like to get some use out of. Also, my project Bronco has all the driveshafts etc.

I know the MPG will not be great either way. I also know the auto will likely take more, just curious as to what you have seen?

Thanks as always.

 

[jckkys]

The EPA mileage ratings aren't vary accurate when compared to real world results on a particular vehicle. However they offer a way to isolate a specific difference between two vehicles that are otherwise identical. You would want to find a comparison including a C4. OD with a lock up converter or C6 transmission won't tell you what you want.

 

[Viperwolf1]

If properly equipped with the right gears I doubt you'll see a difference.

 

[jw0747]

If properly equipped with the right gears I doubt you'll see a difference.

x2 and mpg always depends on how you drive it. Don't count on much difference.

 

[Broncobowsher]

Auto/Stick will be about the last variable in economy for that rig. Tires, gears, wind drag are all going to push you under 10. The driving style will be a bigger difference than transmission type at this point.

With the tires and gears, and the desire to actually wheel it, go C4. Anything built as much as you have with a 3-speed will just be an exercise in clutch abuse or very boring dirt roads in an over-built rig.

 

[EB70]

It will be used almost exclusively for snow and sand, not a rock monster. I will say that if I do everything perfect, I can pull 15 out of my EB with 33's and 3.54's. Going to lose that ability, but should gain the ability to munch through 3' of snow!

Sounds like it won't make much difference as far as the tranny goes.

 

[jckkys]

To your original question, you can build the same Bronco with both an auto and manual transmission but no other change. The next best method would be to compare 2 Broncos alike in every way except the transmission. EPA did that. Parasitic loss is inevitable with a none locking torque converter. I'd guess that to be about 5 percent. Going to 37" tires will more than erase any fuel economy gains made by going with a manual transmission.

 

[EB70]

I figured there was a loss of some sort. I know the MPG is going to be bad, but the performance will be good in other ways!

 

[broncnaz]

The older transmissions had more parasitic loss than todays trannys. Old autos 20-25% loss old manuals 10-15% loss. It wasnt uncommon to see manual trans vehicles get 3mpg or better than a auto vehicle. Although many times the numbers were closer as a lot of the manual vehicles had lower axle gearing
Of course as was said your pretty much not going to see any real differance with that setup as the 3sp pretty much sucks gearing wise and the C4 wont be any better due to slippage. Autos tend to need 100-300 more RPMs to maintain the same speed than a manual tranny would. Lots of things going on in a auto torque converter slippage, have to turn the pump, clutch disc slippage ect. As things wear slippage can go up.

Overall if your looking for any mileage gains from a transmission you should step up to a newer tranny. The have a lot less parasitic losses than the old ones. plus gearing tend to be a lot better as well less RPM drop between shifts keeps the engine in its powerband.

 

[EB70]

I've been playing around with RPM calculators. It looks like 4.56's will be fine with either 35's or 37's. I currently have it set up with 3.50's and 33's. It isn't too doggy, especially after I have driven Toyotas for 20 years! But I would say that it isn't snappy either. I'd like to get the cruise RPM closer to 2500 RPM and I think the 4.56's will get me there.

If life was ideal, I would do the 3550 swap and drop the gears to 4.88's etc. and get efficiency and some crawl. However, I own two C4 transmissions and both are good so I would like to use them.

I have never deep snow 4x4'd with an automatic, but I think it would be killer. I have spent 20 years wheeling a stick and I'd like to try an auto.