Breaking Issue & Fuel Economy Problem

[scot5146]

First off thanks again for everyone’s help on the last issue...

1999 GMC k2500, Suburban 6.5 diesel, 3 pillar gauges, oil cooler line upgrade, oil filter relocation, new stock injectors, AFE intake, 2.25 crossover pipe, 4" exhaust kit, 2 inch lift kit/shocks/ steering stab, 285/75/16 tires... Many more upgrades to come... 141 k miles all regular maintenance has been done by me...

OK so my new problems...

1] I keep trying to calculate my fuel economy, but it seems to be lower than I remember before I lifted the truck... So it donned on me that the lift and larger tire probably threw off my Speedo and odometer so I am not accurate in my calculations...

So how do I fix the Speedo and recalibrate it... I am not finding much online?



2] Just last weekend I took the family to Eastern OR for a camp trip, the truck ran great but coming back over the pass [Mt. Hood] every time I hit the brakes it was like I was trying to stop a fire engine... Both feet on the pedal and almost standing on them...

Since I got the truck almost 2 years ago I always noticed that the pedal was soft, but never thought much about it... The brakes look great front and back, even wear nothing out of the ordinary and no ceramic pads so I am sure they are grabbing...

To me it feels like the front is doing all the work... Could the proportioning valve be out of whack, or maybe an issue with the reservoir and the dual piston pump?

I am not very familiar with this system because this is my first diesel....

Any help would be greatly appreciated!

Thanks Again

Mike Scott

Firefighter
Van/WA

[DmaxMaverick]

It could be a valve issue, but most common is a failed master cylinder. Usually, it goes unnoticed when only the rears fail....until you get a trailer load on, as you've noticed.

[greatwhite]

Generally, larger tires and a lift rob some fuel mileage in weight and aerodynamic penalties.

With larger tires, you put the weight in the worst possible place, at the outside edge of the wheel assembly. It takes more leverage to get 'em spinning and more bake to get 'em stopped.

the taller a vehicle is, the more aerodynamic drag it experiences.

Now, that being said, 285 75 16 isn't that much bigger. I had them on my 98 and swapped back to 245's and didn't really notice anything other than my speedo and ODO were bang on the gps.

They put my speedo off by about 5 kph.

If you want correct your speedo though, google DRAC or VSSB mod. I think TBIchips.com has the info. There's a module behind the glove box that processes the speed signal from the trans sensor and sends it out to various systems (IE: speedo, PCM, etc).

You have to do some measuring, calculations and soldering to change some jumpers around on it's PCB....

[sammeir]

I am experiencing same problem. Does the load affects the fuel consumption? I'm not so familiar with how to balance the tire,load etc.

[AKMark]

I fit 285/75R16's with no lift on my 99 K2500 Burb, maybe a slight rub at full lock if you hit a bump. With the 4.10 gears I have, they actually improved highway mileage compared to the 245/75R16's that were on before. Before I got 13/16, now I get 13/19. And that's with BFG A/T's which in my opinion are heavier than most A/T's and have usually not been helpful to my fuel bill.

As far as fixing your speedo, have Kennedy Diesel retune your computer and fix it at the same time. More expensive than just getting it adjusted, but I've heard of folks who have had to buy a few of the little calibration things before they find the correct one for their tire size and gear ratio. Plus you gain a few ponies, torque and if your lucky a few mpg in the process.

Brakes, sounds like you need to start by performing a good bleed all around. When I do it, I really bleed it, meaning I put at least two bottles of new fluid in in the process of ensuring I don't have any bubbles in it. I find if I go into a bleed knowing I'm putting a bunch of new fluid in, it seems to work better. I know a few folks that just do three sessions of pumping and holding per brake, I do closer to 8 or 9.

[DennisG01]

Adjusting the speedo, as mentioned above, is done via the VSSB (little tiny computer, near the main ECM). All you need is a soldering iron. I purchased one of the reference books that available here and it had the info it... along with a ton of other stuff.

Bleed the brakes... yes. But also you will likely need a rear adjustment. It makes a HUGE difference for me when I adjust the brakes. Sometimes the "backing up really fast and slamming the brakes on" thing works, but manually adjusting them is better.

[rustyk]

Don't overlook the fact, too, that the larger tires decrease braking efficiency, as well as affecting fuel economy in some cases

[16gaSxS]

aerodynamic drag on larger tires can be a fair amount doubt me? okay a 245 75 16 tire is 245mm wide vs 285 75 16 is 285 mm wide. That is a 40 mm increase in with presented into the slip stream for added drag, how much drag? Try this: A 285 75 16 inch tire is 32.84 inches tall. 40mm X 4 (# of tires)= 160mm / 25 (mm per inch)= 6.4 inches cut a piece of scrap 3/4 plywood 32 inches by 6 inches. Have some one drive you in your truck on a Rural road at 65 mph. Open passenger window hold on very tight to your plywood board and stick it directly into the slip stream and see how hard it is to hold into. That is how much extra aerodynamic drag your truck has too over come as you drive. It takes fuel to over come it. This doesn't even account for the extra 6+ inches of rolling resistance that the truck has to over come or the extra structural drag on the lifted undercarriage. :eek: