I have a 1994 Chevy 2500 4x4 6.5L Turbo Auto with 4.10:1 ratio.
i am only getting 12mpg on highway going 60mph.
any advice on how to improve mpg?




1994 Chevy Silverado 2500 4x4 6.5L Turbo Automatic 4.10:1 ratio.
300,000 Miles
1000 miles on Rebuilt Engine 98 Block,New Rings,New Bearings,New Gaskets

 

No real good way other than changing axle ratio or adding a gear vendors overdrive.

 

Should be getting at least 16-18 highway, possibly you injectors are bad/dirty (possibility with 300k on it). Mine get 22-23 running 55, it is a 2wd manual, but there shouldn't be a 10 mpg difference either.

 

Once you get over or approach 2400 rpm the turbo loses most of it's efficiency so poor mileage at high rpm is pretty much normal.

You can't compare apples to oranges,3.73's @ 50 would likely be about 1700rpm vs 4.10's @ 60 being closer to 2700rpm- a 10mpg difference is not outrageous at all.

With my 5 speed @100kph my tach reads 2700-there may be some calibration error but my truck is well beyond the turbo's ability to breath at highway speeds,one day I need to compare the tach with a scan tool to get an idea what rpm the ECM sees vs the tach?

 

So if a 3.73 runs at 1700 rpm, doing the math, 4.10/3.73, to get the ratio of the two, =1.0992 times faster that the 4.10 run. so when a 3.73 runs 1700 rpm 1.0992*1700 = 1869 rpms. This says that it should run 1950. Speed vs RPM with 4.10 rearend - TheDieselPage.com Forums. I know @65 my tach says 1950-2000 so 1.0992*2000 = 2198.4 so 2200 rpm. 2200+1950/2 = 2075 rpm @ 60 mph. According to the math, that's what it should run. His is a 4wd so taking 2 mpg off is necessary, and an automatic with lower gears so take another 2 mpg off, there is still about 5-6 mpg that are missing in the equation. Mine in 4th doesn't even run 65 mph @ 2700 rpm, Mine is probably a little off, I have 265/70 or 75 I don't remember on the back and 235/85 on the front, but it is pretty close according to the speed checkers on the highway. +-1 mph @35. I know real life situations, just "doing the math" isn't 100% accurate, but still 1000 rpm difference at 50 mph is hard to accept.

 

There is another difference you pointed out,mine has 225/75/16's.

Like I said,I need to one day compare the tach with the ECM to see how distorted my tach really reads.

Play with this calculator for fun just to see how close it comes:
RPM / Speed / Gear Ratio Visual Calc

When I try my setup with the ratio for a 5 speed it works out pretty close.


Your truck MontanaTechPetroleum @ 60mph should be roughly 1830 rpm. 0.75 overdrive,yours @ 110 kph (67 mph)would be roughly 2066 rpm.
My truck @ 60mph should be roughly 2400 rpm- 570 rpm difference. 0.85 overdrive,mine @ 110 kph would be 2700 rpm-common cruising speed on the highway.

 

Both transmissions should be geared the same, the only difference should be the rear gears, Mine won't even run 2700 @ 85 mph. Why is mine .75 overdrive and yours a .85 overdrive, both being nv4500 they should be geared the same, but if 67 mph runs 2700 rpm on yours, what kind of mileage does yours get, being a dually it got to be close to 14 still. And there's no way that a dually should get better mileage than 3/4 ton pickup with the same engine and gears, don't know how the automatic is geared however, but it should still be getting better than 12. Had the taller tires since I bought it, I don't know how big of difference they make, but I can believe that GM would make a vehicle that at 65 mph would be running 2700 rpm, especially a diesel where it's not good to exceed 3000 rpm, just basic science says that it wouldn't last very long.

 

My mistake-thought you had the auto trans.

Never took the time to check mileage on mine,it was a dual tank truck that now only has the frt tank,that and mine has 2 large saddle tanks so mileage calculations are more effort than they are worth-it is what it is.

Ours should be nearly identical,strangely today using the linked calculator I get different results.