2500 RPM @ 55 MPH Help!

[gunz]

unknown as my tach is still on the fritz, but its not running hard.

 

[Hydro-idi]

yeah i just bought this 87 f250 a few weeks ago and have noticed my tach only works when engine is warm for some reason. but you must not have the old c6 in your pickup if you can cruise 70 mph with fairly low rpm. im cranking roughly 2400 rpm at 55 mph with 3.55 gears and c6. seems high to me but surprisingly i got 21 mpg going home a few weekends ago going down 99. better fuel mileage than my previous gmc sonoma with 4 banger.

 

[ocnorb]

I have a ZF5 and 235/75R16's and I run 2100 rpm at 70 mph.

Never seen anything close to 21 MPG!

 

[Hydro-idi]

my mileage is calculated going 55mph steady down highway. im a college student who dreads filling up 2 20 gallon tanks so i try and conserve fuel keeping her right at 55. i still dont understand why my mileage is so much better than other idis out there. im running original injectors and new fuel pump with it turned up one click as well......says previous owner. what trans do you have to run that low rpm at 70 mph?? wish that was my case!

 

[ocnorb]

what trans do you have to run that low rpm at 70 mph?? wish that was my case!

Sorry- a ZF5 is the 5-speed transmission Ford used in these.

 

[Hydro-idi]

maybe ill stop asking so many questions haha. i am new to the idi world and this web blog site deal too. it is very nice to actually have answers to some of the questions that i have because not very many mechanics in my area know much about idi fords...even diesel mechanics . but thanks again guys i appreciate it.

 

[papastruck]

Look around and you can score them much cheaper than that.

I've seen complete set-ups go for $800-$1000. At $4 a gallon they don't take too many miles to pay themselves off.

I ran a quick payback calc on it. If you spend $1500 on an overdrive that net you 3 MPG increase you will break even at 22,500 miles. Any miles after that is pure gravy.

Next time you see one please let me know. I'm willing to pay that.

There's a bit of luck involved to get that low. I bought a rebuilt unit from a reputable dealer, but by the time I'd gotten finished swapping out the incorrect adapter (even though the correct part number was stamped on it - turns out they mill down and reuse that particular adapter without changing the number on it) and other odds and ends that needed to be replaced and bought, I was out another grand because, not wanting to spend another year scouring ebay, I went directly to GV.

However, they do pop up on craigslist out west, often attached to old Fords for cheap. Those you can verify before buying.

 

[papastruck]

maybe ill stop asking so many questions haha. i am new to the idi world and this web blog site deal too. it is very nice to actually have answers to some of the questions that i have because not very many mechanics in my area know much about idi fords...even diesel mechanics . but thanks again guys i appreciate it.

In case you haven't stumbled on it, head up to the top of this sub-forum and have a look at the tech articles and FAQs. I highly recommend printing them out and stashing them in the John for some lengthy morning reading material.

 

[Agnem]

Proper engine timing has one hell of an effect on MPG as well.

 

[Hydro-idi]

when i bought truck, previous owner said he had diesel mechanic properly time fuel pump when it was replaced and said the injectors were all working properly even though they are originals. maybe that is why my mileage is good??? dont really know. also realized my rpms seem to be a little lower at idle since i made a road draft tube hooked up to exhaust. not too sure why that is the case