Mileage vs rpm vs gear ratio???

[PackRat239]

My 93 F-250 is set up with a ZF5, 7.3 NA, and 4.11 gears and 33 x 12.5 x 16.5 tires. I am turning over 2000 rpm cruising on flat ground at 55 mph in 5th, and I am turning 1500 rpm in 5th at 40 mph, and I can almost watch the fuel gauge drop at highway speeds as I drive. Not too bad around town as I try to keep it around 1500, but that is not possible anything over 40 - 45. With the cost of fuel these days, I am looking for ways to get better mileage. The truck runs great, with great power for an IDI, no smoke, pulls hard. I can climb hills in 5th at 40 mph and accelerate up them. I drive around town in 5th, and get better mileage. I like to drive the truck so I need to burn less fuel. How much will it improve if I go to 3.55 gears. Is it worth the work and money? Remember, this is a four wheel drive, so I have to do it twice. I have a rear Dana 60 with 3.55s already. How about going to 35" tires? Any ideas or suggestions?

Thanks in advance.

 

[Hyde]

Have you calculated mileage? With my t19 I don't have overdrive and with 3.55s I imagine our highway rpm might be similar, I don't have a tach however. At 65 I get between 16 and 17mpg, doesn't change considerably between my 235/85s or my 35x12.5s. I have a gps odometer so don't have to calculate for change with tire size.

 

[Brad S.]

Good question PackRat, I'm kinda curious about this also.
There are a lot of factors on this kind of question. I think going to a taller but bigger tire might not get you where want to go.
I went from a 235/85/16 to a 265/75/16 and didn't really see any better mpg. The rpm's did go down maybe 150 rpm, then went back to the 235/85/16.
I think if you went to a "narrower" tire and the 3.55's it might help.
The amount of lugging your engine might do with 3.55's & 35" tires, might cancel the rpm decrease.
I think the 255/85/16 tire is a good size for what your looking for.
Riotwarrior, (Al) uses this size maybe he'll chime in.
If your after cheaper fuel cruise over to the wmo/section and do some reading.
(Wouldn't be a true wmo user if I didn't plug that section)

 

[jwalterus]

in the flood victim on different fuels I get the following:
#2 w/PS: 14.7town, 22.3hwy
#2 /Howes: 14.5town, 21.7hwy
#2 w/o PS: 14.5town, 20.1hwy
#1 w/PS: 12.1town, 17.3hwy
P-40 diesel: 14.5town, 20.7hwy
WMO: haven't burned yet, but I have about 300 gallons waiting to be filtered this summer
I put between 50-100 miles on my truck daily, and am meticulous about recording fuel mileage info

 

[justinray]

Go dyno your truck, request to do the dynotest in 5th.. This will throw off the measurements.. But these are the ones you want.. Find peak hp and torque in 5th, start calculating which gearset will keep you between those at the speed you want to cruise at. You may want to do a pull in each gear to see how it's not just a direct relationship, since the turbo produces boost relative to the load on the engine. If you do that, find an rpm that is between the hp and torque peak in all gears (if one exists, if not find the most repeated) and use that rpm to figure out the gearset you want. This is the most efficient way to do it that I know of.. At most, you pay for 15 pulls (Robert's 15 speed) and then your gears, and you know where to plant your tach.

 

[Sycostang67]

I fell your pain, having almost the same setup(I gots a turbo). I normally take back roads to work and average 16mpg. My normal route has been closed for the last few months forcing me onto the freeway and down to only 13mpg.

You will get a more substantial drop in rpm from the gear change, but it is costly. I spent about $1500 to change gears in a 4x4 F-250 I used to have. I would only do one or the other, swapping gears and upsizing tires would most likely bring your mpg down.

 

[RLDSL]

You have to step back and ask yourself what you are going to use your truck for. If you pan on doing any *serious* towing, then 3.55 is going to be a bit much
I find when towing heavy that the range in 4th Od is perfect for highway running, for steady pulling, so a 3.73 would likely put you in that ballpark in 4th ( since you have 4x4 you have a low range ) but if just running around town and not pulling anything, then toss the tallest gears you can find in it your engine has oodles of torque, and like I said, if you go offroad, thats what low range is for.
* with the caveat, remember not to lug the blamed thing in the name of fuel mileage, you have a shifter and a brain... use them

 

[tanman_2006]

The dyno in OD to set your rpm's isnt really a terrible Idea.

I'm a detroit guy (hence all the 6.5's) and I dont lug engines BUT w/ 3.73's in OD at 50mph I can knock down some serious MPG's.

Towing I like my 4.10's and am actually waiting to get my 4.56's in, I want to install a ranger OD infront of my NV4500 to get me back into 4.10 cruising rpm's. Tall/wide/heavy tires play a big roll. I lost 3mpg going from michellin 32x10.5's to a Micky T ATZ 33x12.5 on my dmax. I can't get max mpg back to 19, stuck at 16 (other issues being explored)...

 

[asmith]

* with the caveat, remember not to lug the blamed thing in the name of fuel mileage, you have a shifter and a brain... use them

just curious what do you consider lugging? what rpms would you shift at to get the best mpg, power and engine life while running mostly empty (couple hundred pounds in cab and bed)?

 

[PackRat239]

I am lucky if I get 10 mpg on the road as she sits. Since torque peak is at 1800 rpm, I don't think keeping her at 1500 - 1800 is lugging her down any. Yes, she has plenty of torque to pull 3:55s, but I need to know it is worth the cash to do the swap. I also have a D60 front with 4:10 gears, which is what I originally intended to swap in. I am not sure where to ga at this point. Keep the ideas and info coming. Thanks Guys.

 

[TronDD]

What mileage are you actually getting? Have you calculated it over several tanks? How many miles do you put on the truck each year?

You have to do the math. If it costs you $2,000 to reduce the mileage but would take 20 years to make it back in fuel savings, it'd be silly.

Tim.

 

[PackRat239]

You are right about that. It is difficult to do an accurate test as I have not installed the fuel fills in the bed yet after the tank swap, and it would be kinda hard to fill up with a funnel and hose.. What I need is a 10 speed Road Ranger in place of the ZF5, or maybe a 2 speed rear? ... Shift+R improves the quality of this image. Shift+A improves the quality of all images on this page. I wonder how much a turbo would help mileage?

 

[GOOSE]

Gearvendors or a brownie.......

 

[RLDSL]

just curious what do you consider lugging? what rpms would you shift at to get the best mpg, power and engine life while running mostly empty (couple hundred pounds in cab and bed)?

Well, when I was turning wrenches at a bus co, the factory rep dropped by and explained it as thus: You can lug these things at ANY speed regardless of RPM or load. If you cannot accelerate to full RPM at 2/3 throttle, then you are lugging the thing. And that doesnt mean standing on it at 2/3 throttle till it slowly creaps up 5 rpm at a time, you have to use your head, it should accelerate at a *normal* rate, in other words, you could be spinning at 2500 and be lugging the thing

Now, all of this schmaltz aside, peak torque is around 1750, which is also around shift point.... under load , you can climb hard in that range even towing heavy , just so long as your not floorboarding it ( and it will pull REAL nice ( Ive found mine pulls best in between 1750 and 2200 ) as far as going down running empty, you just have to be mindful of your foot. I have a double OD and my RPMS can get down there pretty good, around 1200, but I keep a VERY light foot on itand if I hit a hill I pay very close attention as to how much fuel is going to it, at those rpms if at any point its to where I couldnt accelerate easily , I simply drop a gear and play it safe, and get the engine in the correct operating range for pulling there is no cast in stone number. The goal is simply to prevent undue loading .
When Im on back 2 lane roads, I cant even use 5th gear, its simply too high, I just keep my brownie in OD and use 4th and that works great ( 5 over it too high, the split at 4 over is perfect and saves having to shift to 5 direct and fight the curves and 4 over gets the engine in a better mileage range when going up and down and pulling out of curves )

Which brings to mind, lower rpms does not always equal better mileage. If the road you are on has a lot of ups and downs to where you are having to stuff your foot into it a lot in 5th gear to pull the hills, you may actually find that you will get BETTER fuel mileage on that stretch by letting the engine wind up a bit in 4th and not have to give it so much throttle to get over teh hills.