I have a 1989 Ford F-350 non-turbo IDI diesel, 5 speed 4WD pickup single cab long bed. I am planning to put 285/75r16 tires on it. Does anyone have any idea what kind of gas mileage I will get?

 

12-20, depends on how you drive and the state of tune.

 

I have an 89 7.3 idi with the C6 (no overdrive), 4.10 gears, 2 inch lift and 285/75/16 mud tires. I get 12 mpg all day every day. I bet that a 5 speed will get a little better than that if you keep your foot out of it. When I changed my tire size from 265/70 to 285/75 the fuel economy didn't change enough to notice.

 

that about sums it up. my IDI is in a van and has a c6 behind it, and i get about 14-15 driving fast and agressively while loaded to over 8000#.
your 5 speed will be an improvement over mine, but your axle gears also make a big difference. if you have 4.10 gears, which you probably do, the bigger tires should help quite a bit with economy as well as increasing your top speed.

 

Depends alot on what gears you have, but regardless on the hwy if you cruise between 1600 and 1800 i would expect 20 if its tuned up.

 

I wish I could run 1600-1800 rpm's down the road. I have a 1987 F-250 with a 6.9 and C-6 tranny. With 4:10's for gearing I run 2500 to 2700 Rpm on the freeway and get 12-13 mpg. Wish i had a gear vendor's overdrive!!

 

I have a 1988 Econoline with the 7.3 IDI and the 3-speed auto, and it gets just under 15mpg on the highway (no faster than 65mph). In the city it's substantially less.

 

You asked the question...Does anyone have any idea what kind of gas mileage I will get? In my world...a diesel engine will not get any gas mileage!

Did you mean "fuel mileage"?...If you're going to use this diesel forum, you better start using the correct nomenclature or we will think you don't really dig the diesel scene man!

Ha ha, just messing with ya...A good diesel man can take a joke!

Look at my signature...we have a similar set-up...my mileageis in there.

 

If mileage is a concern, why put on wider/taller tires? I mean, if it's to be able to handle more weight when hauling/towing I get it, but otherwise you're not helping yourself with tires that large.

I don't bother tracking mileage in town because I just don't put enough miles on her to care, but my truck flirts with low 20s on long highway runs if I keep it under 75. Slightly different setup than yours, though, and I'm running the OE size 235/85/16 on mine. Might go 265/75/16 on the next set just for a better stance, but that'll probably be a few years off and more due to rot than mileage.

Mike

 

The second you put different than stock tires on it, kiss your fuel economy goodbye. I had 265/75/R16s on it when I bought it and it was a TURD.

I'm running stock 235/85/R16s on my POS and it gets between 15 and 18 mpg depending on how it's used. The 235s are far superior in the snow than the other tires.

 

21 on the highway with my truck below. 16 city.

 

I owned one way back when. A 1990 F-250 extended cab, E4OD, 3.7 gears and the best I could ever get was 19 +. I should have had the tranny re-"built" and kept the truck. I really don't remember what tires it had, what ever it came with new.

 

Towing my toyota on a flat bad trailer from Rimrock WA to Prosser WA, mostly down hill but still dealing with traffic slowing ect, sitting on 35's i got 14mpg towing.
If i could keep my foot out of it and drive the truck a little nicer when not towing i bet it would be around that 14mpg or better especially on the highway.
Rob

 

i average about 15

 

all city my truck gets about 16. all back roads i can hit 20.