Guys, I have a 86 f250 6.9 with a c6 and 4.10 rear end (i'm guessing its a 410 since I run 2800 rpms at 60mph). i have 265/75/16 tires.

I am only getting 11 - 12 mpg with no towing or loads. Is this expected or is it low? what should I be getting and how do I improve with a stock setup? (not interested in turbos, ram airs, etc.)

Thanks.

 

That was normal for mine. I have a 7.3 with 4.10's and 35x12.5 tires. All I could get was 12mpg unloaded. Notice I said "was normal". I swapped in a zf 5 speed(it had the c6 originally), and with everything else the same, I now get 15mpg unloaded.

 

how many miles on the engine?
how many miles on the ip and injectors?

around 2600 rpm probably right?

anywho.nope,not normal.very low.unless your in hill valley and city driven all the time.even then,while empty you should do better empty than a 460 gas job lol.
man,that's really revving.have you considered a 3.55 gear swap?
either that or id consider slowing down to at least 2k rpm,but that's only 46 mph in your case.
3.55's would bring you down to about 2200 rpm @ 60 which be much better.
is this a 2wd truck? do you tow big loads often? if yes 2wd,and no not hauling much,then 3.55 rear end swap is easy and reasonably priced to do yourself.

edit,
yeah,getting an OD trans like Franklin suggests though much more $ and work,would for sure help too.gear her up to 3.55 and OD,and you wont ever look back.
Franklin's MPG gain could have been much better without those 12.50" wide tires too (though the 35" geared his 4:10's up.)-oh nm that one,he had the same tires he started with.so = gain.

Franklin,
where's your garage man.
-oh cool.i found your gallery.

 

Very normal indeed - at 2800 rpms in 3rd gear mine chugs fuel like it's going out of style, you really wanna keep these engines under 2200 rpms or preferably under 2000 rpms to get the good fuel economy they are capable of.

 

Taller tires are going to change your mileage. Are you taking the slower speedometer into account when figuring mileage?

 

265/715-16s are the same diameter as the factory 235/85-16, just an inch wider, so his speedometer is still accurate.

 

Sounds like a carbon copy of my truck when I got it. Yes, those numbers are normal for that gearing and speed unfortunately. Cheapest fix is to swap to 3.08s. Ram air might help slightly but a turbo will not make any difference in MPGs when running around empty.

Your only real choice short of modding the truck is to slow down even more.

 

did you do the soup bowl mod if not that and a more free flowing exuast will add 1 - 2 miles per gallon

 

talkin about rear-ends how can i tell what mine is without pullin the cover off??it is super low though my tach dont work right but bout 70 mph 5th gear it is wide open, the po said it is a ton truck but i dont know if there is such a thing cuz ive squeezed 4 lies already out of him already, i heard someone said they had 4.56 gears could it be?

 

We can calculate if you have an accurate speedometer and you post your tire size. Or (assuming the gears are original) you can look for the axle ratio code on your door sticker and some one here should be able to decode it.

Assuming 5 speed, 4.56 and 235/85/16 tires, I come up with 87 MPH top speed, which is probably too high since you say its running out at 70. I'm guessing your gearing and/or tires are lower.

 

well spedo is about 5 mph off(slow) according to the sheriff dept trailer that tells u how fast u are goin, i topped it ou one day when my tach worked 3200rpms prolly juist passed the 90 mark on the speedo. so id say ur rite i am running 235 85 16's thanks david