gas mileage with OD

[blue 74]

Throwing this out to the community to see if this sounds right. I have a 74 with a strong 302. The motor came with the truck and the seller said it had a cam but could not give me any other details. I have been driving it regularly for two years with no power loss (unless I go to high elevations)or engine issues. It has a Holly 570 street dominator carb (not a fan but it works well so why mess with it) Mallory distributor, OD trans, 3.50 gears and BFG 33" tires.

I do a lot of freeway driving and the truck seems to like to be at 70mph. At 70 the tac reads 2100 rpm. My OD gear does not kick in till I am going approximately 65mph and the tac is reading 2600 - 2700 rpm. I am able to keep it in OD when going up most freeway grades, but if I drop below 50-55 it will automatically kick down. It seems like no matter how I drive it I get about 11mpg everywhere. In the city or on the freeway it doesn't seem to matter. Is that normal? It seems like from other threads I should be getting better mileage.

I plan to install a Holley Sniper system in the future (when I can afford it) and my hopes are that it will get a little better gas mileage and perform better at altitude. I was also thinking that I might switch to a 32" tire. Any thoughts?????

 

[bmc69]

11 mpg at 70 is pretty darned good.

 

[blubuckaroo]

If you know the RPM of your maximum torque, and select gear ratio/tire size to put you at that RPM at your expected cruise speed, you'll get your best mileage.

 

[pcf_mark]

I was squeaking 13 around town on a stock suspension, 29" tire, 3:50 gears and EFI. I just switched to 31" tires and I think they soak up a lot of power and I have to use more throttle longer to get up to speed so I suspect I will see a decline.

These trucks are heavy and have zero aerodynamics.

 

[SevenT]

Gas mileage with OD

Blue 74,

I was getting 13 with my 302. EFI, AOD, 3.5 inch lift, 33s and 4.11s. All of this will change with my new ZF I am certain.

v/r,

SevenT

 

[Broncobowsher]

I recently found a sheet where I noted fuel economy over a 1,000 mile round trip to Vegas and back about 15 years ago. 351W warmed over (small cam, aluminum heads, plus typical stuff), 4R70W overdrive, 33s, 4.88 gears, EFI. Give or take a bit, about 13 MPG. And that is doing about 70.

You are in the right ballpark.

 

[bmc69]

These trucks are heavy and have zero aerodynamics.

Yep...and the aerodynamic drag at 70 is twice..2 times...what it is at 50.

 

[Nevadasmith]

You might look at getting 4.11 gears, or 4.56 gears for 32/33 tires like Bluebuckaroo said best mpg is going to be at peak torque...I think even on a stock cam 302 that was 24-2600 rpm, you might try turning off the over drive for a tank of gas and see if you don't get better mpg??? Just as a test....grimjeeper has a great online calculator to help you see what the gear change would do. I have heard tell of efi 5.0l od bronco's getting 18+ mpg when set up right. I regularly get 15-16 mpg occasionally, 18 mpg. But I run a 250 i6 zf5 5.42 gears and 35's tho I never go more than 65, actually 62 if I'm not holding up traffic! Whole different animal but the areodynmics are the same, and a roller 302 should be way more efficient than a log head 250.

 

[Rustytruck]

Your gearing sucks you don't have enough torque to efficiently pull 70 mph at 2100 rpm. peak torque is close to 2600 rpm. I'm not a fan of Holley for a street carb I prefer the efficiency of the Quadrajet.

 

[byson1]

Quote..."I plan to install a Holley Sniper system in the future (when I can afford it) and my hopes are that it will get a little better gas mileage and perform better at altitude. I was also thinking that I might switch to a 32" tire. Any thoughts?????"

Best to decide your main objective. Any upgrade in fuel system will cost some $$ and might get you 2-3 more mpg at best. It will take a lot of driving to recoup any money spent. If improved drive-abilty is the main factor, than most modern EFI systems will be an improvement over your carb.

 

[pcf_mark]

Yep...and the aerodynamic drag at 70 is twice..2 times...what it is at 50.

I've wondered if a lower front air dam would improve economy by directing air around the truck and not under where it can create more drag.

 

[bmc69]

I've wondered if a lower front air dam would improve economy by directing air around the truck and not under where it can create more drag.

If you could measure the difference...I'd be surprised. I think the increased frontal area would cancel any reduction in the Cd.

 

[DirtDonk]

I've wondered too, but even with the trade-off, I bet there is a ton of aero to be gained by lowering the pressure under the vehicle.
But yeah, it wouldn't tip the scales by much I'm thinking.

So blue, can you give us some exact measurements and info on some aspects please?
Such as:
1. Exact tire height (rear tires) with the vehicle on the ground.
2. Speed correction if not taken into account before.
3. Which transmission specifically (different overdrive ratios)

Reason I'm asking is that to get to anywhere near 2100 rpm at an actual 70mph your gear ratio with a typical AOD and 31.5 to 32 inch tires (actual rolling diameter) would need to be 4.11 already.
With 3.50's and 32.5" tires you'd be closer to 1800 rpm at an actual 70.
Hence the questions.

But for the advertised stats, yeah, your 11 mpg is not out of line at all.
And mileage be damned, if it runs better at altitude the cost of EFI is worth it!;D

For comparison, the best I ever got in my '71 with carburetors (five different ones tried) was 17.5mpg at a pretty steady 55mph. This is with 4.11's and 31" (actual) tires.
Even with that setup (a special 450 Holley "Economaster" carb) I would probably have barely gotten 12 at 70 I'm sure.
With the other good carbs (stock and Carter 500 & 625), the best I could get ever was 15.5mpg, again at only 55mph. The Holley 600 ran decently, but never got past the 13mpg mark.

With full modern Explorer EFI complete with all smog items, 4.56's and 29.5" tires (31" BFG's) I get almost 20 (19.8) at a not-so-steady 55-60 mph over hill and dale.
The engine is strong and gearing too low for best economy I think. I'm betting I can get over 20 if I raise the gearing even though it's only running 2000 rpm at 55 now.
The engine is just so strong down low (stock Explorer cam) that it can push a Bronco easily.

No matter what you do to balance the economy factors though, pushing a Bronco through the air at 70mph is never going to net you fuel mileage that you expect, or hope for.

Let us know some more details about the setup though regarding those items I mentioned and maybe we can dial in a gear suggestion better for you.

Paul

 

[centex77]

I used to get 4-5mpg at 70mph on 35’s with 4.10’s and a 3spd manual. Hoping the two extra gears I have now will help that a bit.

 

[Yeller]

I'm with Paul and others speed kills mpg. I get 17-18 at 55-60, I get 9-10 at 75-80. but I have a completely different unmentionable bastard drive train;D that would probably benefit from 4.88 or 5.13 gears instead of the 4.56's pushing 37's.

 

[SavageBurro]

Like they all said, your gearing seems a tad high. I bet, as you are thinking of doing, going to 32s will help.

I used to run 33s with 4.56s and 3 speed, I'd routinely get 15 mpg cruising at 57 mph.

Shoot, right now I have stock 2 bbl carb, heavy 1 tons and 37s w 4.56s and np435 w no overdrive... I average 12 mpg and that includes combo driving in-town, some trail riding, and some 60 mph highway cruising at 2500 rpm.


Talking about the air-dam. Years ago, I had a lifted 83 Toyota on 35s that would get 16-17 mpg. I had a guy tell me to put some sort of an air dam or skid plate of sorts to deflect the wind under the front of the rig. For kicks I half-ass jerry rigged an air dam out of some thin sheet metal and zip ties. My mpg shot up to 20 by just doing that. I took it off due to the fear of my horrible fab job coming off on the freeway but I was impressed with how much that affected the change. Same goes for modern trucks and suv's. I know guys that take off the lower front valance to get a better approach angle / stance but by doing so they lose 1-3 mpg.

 

[DirtDonk]

Always thought an air dam that went up at slow speed and down at high would be a cool thing to figure out on an EB. Maybe not very truck-like, or cool looking like a Bronco should be. But it might be effective.

My first experience with the effect was quite an eye-opener like yours Burro. On a '66 Corvair (that already had killer aero for it's day, and an almost perfectly flat bottom tub) I busted the original valance panel/air dam off on a parking lot tire stop. Didn't think much of it at the time, and didn't want to spend the $19 bucks for a new part that was almost out of sight under the car anyway.
I did notice the gas mileage go down about that time (I check every tank), but did not equate it to more than my driving habits and conditions.
But it finally became too consistent and I though "well, maybe?" so I bought a new one.
Immediately got my 3mpg back! I was shocked, and there really was no general talk about that kind of thing in the magazines in the late sixties and early seventies and we just weren't thinking about it all that much. But we were definitely talking fuel economy starting about that time.

So like yours, mine was good for 3mpg at least at the usual 65-70mph level.
And on Broncos, we could even factor in better cooling too. At least when cooling is borderline at highway speeds anyway.

Paul

 

[Broncobowsher]

I remember looking at an air dam on a BroncoII and thinking that was about the perfect size for an early Bronco. Straight, right width, etc. Then I thought how hideous it would look. Nevermind that the engine cooling is sure to get way better as well. Although to be really effective on an early Bronco it would need to be about 8" tall instead of 3".

Aero is an interesting beast. Little things can make a huge difference. You know you are dealing with really bad aero when you see water drops on the hood pushed forward when driving at highway speeds.

 

[blue 74]

1. Exact tire height (rear tires) with the vehicle on the ground.
2. Speed correction if not taken into account before.
3. Which transmission specifically (different overdrive ratios)

Paul, thanks for the good info. I will check my tire height when I get home and post it tomorrow. Forgive me for sounding lame, but I am not sure what you mean by speed correction. Is that the difference shown on the odometer depending on tire size? As for the transmission, When I purchased the truck it had a blown trans. Since I live in San Diego and most of my driving is on the freeway I opted to get an AOD. I found a shop that had installed many of them in EB's and they had good references. I was told the trans I located was from a 87-89 Mustang (not an AODe). The installer suggested a slightly more expensive torque converter because he said at the speeds I would be driving it would down shift and up shift more frequently with the stock one. He did ask me what gears were in it and I told him 3:50's and he said that those were not the best choice to go with and in the future I should think about a swap. Honestly the only reason I believe there are 3:50's in it is because the original ford tag is still on the rear housing and it says 3:50's. I haven't opened up the front or rear dif since neither are leaking and they are working great. The installer also installed a twin stick with the new trans.

I am thinking of doing a mileage test on the freeway at 60 compared to my normal speed and see what the difference is. But going 60 around here on the freeway is a scary proposition. The altitude issue is because I do drive it to Mammoth Lakes (750 mile round trip) and the altitude there in town is about 7,500'. It runs like a dog there, but it does look cool as hell.

I guess the bottom line is that I don't mind changing the ratio if it means the trans will operate better, but I still want to be able to comfortably drive at 70.

 

[Broncobowsher]

The speed correction is what error is there between the speedometer and the real speed. When you change the tire size you change how far you travel per revolution of the driveshaft. The speedometer doesn't know that, it only reads how fast the driveshaft spins and calculates speed based off the speedometer gears. Changing the tires size (or axle gearing) puts an error in the speedometer. The error will be a percentage error, 10% fast, 5% slower than actual speeds type of thing.