2nd Gen Non-Engine/Transmission Help fuel mileage gone!!

[catoiler]

help! Changed from 245/16 to 285/16 tires and soon after the mileage in city dropped to 8 mpg. always got 15 mpg before (big foot)air filter is ok,fuel filter is new,exhaust not plugged. any suggestions? thanks mike

 

[mikel]

First thought is did you fix your speedo? If not then you're covering more ground than shows up on the odometer which is throwing your mpg calculations (you are using a calculator and the odometer not the overhead "computer", right?) off by a fair amount.



The second thing is I would bet that your truck *does* burn more fuel pulling those 285's around than it did with the 245's, but not as much as your numbers show.



Combine an inaccurate odometer w/ larger tires and I can see how it may look like you are getting such low mileage.



Hope this helps,

Mike

 

[Shovelhead]

Have you tested "on the road" yet?

I'd think the larger tires would increase your MPG on the highway.

The bigger tires take more effort/fuel to get moving each time you stop and start.

 

[JGK]

Catoiler, I'm hoping that's not a typical effect of 285's. I'm having a set put on today. However I am only changing from 265's up to these, so the leap isn't as big.



I found a conversion formula on some web site a few days ago. It seems to me I calculated the odometer difference going from 245s to 265s, based on tire diameter, to be about 5%. Going up to 285s from 245s calculated an 8% difference. That would apply direclty to miles traveled and MPH. But not fuel consumption in the same way. I'll see if I can find that page again and double check.



-Jay

 

[JIM S.]

I have a friend that runs 285's and he just got his timing set to 16*... his report was 23 MPG on the hiway(97 4x4 5 speed) there must be some other reason for the drop in milage.

Jim

 

[Blakers]

Originally posted by JGK

:

I found a conversion formula on some web site a few days ago.

Go to 4 Wheel & Off Road.com



Go to "Truck Tech" on the left hand side and then click "calculators". There are several good ones to choose from.



Blake

 

[cmckinney]

Here's another link to figure out the size difference between the tires.



http://www.4lo.com/4LoCalc.htm



-CM

 

[catoiler]

on road no good

over highway mileage is down also. last trip to Las Vegas took a tank to get there and another to get back. before could go there and still have enough to get back on one tank. :{ played around with some of the calculators and it still don't jive. maybe it's the design of the tire?maybe i'll go to 265 instead of 285. thanks

 

[HC]

I can't help much; 24V engine here. But let's say you lost 20% due to bigger tires with more aggressive tread, and another 10% if you haven't corrected for the speedo. (both numbers are higher than your losses should be). You'd STILL be getting over 10 mpg. So I agree, something happened besides the tire change. You might tap a few more specific suggestions over in the 12V engine and drivetrain forum. The only thing I can tell you is to do an oil analysis and see if an injector has gone bad and started dribbling fuel continuously or something like that. Good Luck!

 

[Blakers]

Re: on road no good

Originally posted by catoiler

trip to Las Vegas took a tank to get there and another to get back. before could go there and still have enough to get back on one tank

Man, that is pretty good! Wish I could get to Vegas on one tank!



Seriously, your original post of 8 mpg - is that towing? If not, your original 15 mpg is pretty bad for that truck.



I wonder if something else happened to it. I run 33" tires in the summer that are about the same diameter as your 285s and I get 18 mpg empty.



Blake

 

[catoiler]

Blake,that is empty,not towing. this truck has never gotten the mileage that others say they have gotten. right now i have 352 miles on the trip meter and the tank is almost empty!figure in about 10 % correction and it still stinks mike

 

[Blakers]

Originally posted by catoiler

Blake,that is empty,not towing. this truck has never gotten the mileage that others say they have gotten. right now i have 352 miles on the trip meter and the tank is almost empty!figure in about 10 % correction and it still stinks mike

And mine's a 4 wheel drive. Tires size and moderate load changes does not seem to effect mileage that much. That's also a combination of 72 mph and rural roads at 45-60 mph.



I use my trip meter and generally get about 325 miles on 1/2 tank.



What are your driving conditions and what speed do you drive? Anything unusual? Does it feel like your truck is rolling along freely or is it always working to keep up to speed?



Maybe you can through some more facts out for us to digest in this discussion.



Blake

 

[Steve Campbell]

Is there any way...

that the injection timing could have "changed"?

 

[Oil Ape]

Any time you go to a larger tire you are making the engine work harder to do the same amount of work. Granted with the low end torque of these trucks a minimal increase in size may work the engine a bit more, while at the same time increasing the distance traveled giving better fuel economy.



But there is a size barrier that once you get too big, your economy begins to drop. I had the same problem with my truck. I bought it used with 285's on it. Looked great but didn't get very good fuel economy compared to my Dad's 96' running 265/75's on it. Stock tires for both trucks were the 245's. I put new rims and tires(235/85's) at the next change and then swapped with my dad. He wanted the narrower tire and he couldn't with his 8" wide rims. Besides I liked his rims better$$$. So to make a long story shorter, I'm now running the 265/75's and he is running the 235/85's both about the same height and getting the 22+ fuel mileage.

 

[Blakers]

What's the OD difference between 265/75 and 285/75? It can't be too much.



I run 265/75-16 in late fall/winter/spring and 33-12. 5x16. 6 in the summer. I don't see much difference in rolling resistance.



Now my truck is bombed, so maybe it doesn't care as much as a stock truck would about the difference rolling resistances.



Blake

 

[CumminsPower98]

It's not the tires

I very strongly doubt it's the tires. I run BFG ATs in 33x12. 5x16. 5 size with my timing set at 16º and I get 20. 5 to 22. 4+ all the time. My cousin runs the same BFG in the 285 size with timing at 16º and also gets awesome mileage as Jim S. stated earlier. I would personally be checking other things out such as timing, boost leaks, fuel leaks... I don't know, I'm sure others will have better ideas on what to check.



Has there been any noticeable change in performance at all? I just feel that everyone is getting too hung up on the tires when something else might well have changed close to the same the tires were changed leading you to think it's the tires. I agree that tire size can have an affect on mileage but the size difference in this case is not going cut your mileage nearly in half.



Mark

 

[Oil Ape]

The difference is only a little more than an inch but I know that it was enough to make a difference on my non-bombed truck(soon to be bombed). I think being from Wyoming that the 285's and the thinner air ate my lunch when going up hills. Almost as if I was partially loaded. The engine was working too hard and my overall speed and mileage would suffer. The truck definately ran better with the stock 245's but they look stupid on these trucks. The 265's seem to be the best of both worlds. They are big enough to fill in the fenders a bit yet small enough not to rob me of precious power. I've got the gauges, modified the intake and exhaust, new transmission pan, just waiting for a few $$$ to put a plate in.