rear gear

[towerpower]

i've had my truck for only a few months and the best mpg i've gotten is around 17. that's usaully between 65-70 mph trying my best to drive conservatively. the truck has after market wheels on it and oversized tires, a fiberglass shell, a visor, and a chrome bumper up front. it has 27000 miles on it, and it's a '97. i'm on the verge of investing in some fuel conditioner and maybe getting different wheels for it, but before i do i was thinking about putting in a lower rear gear. i just found out today that it has a 3.42 in it at the moment. my question is this: will going down to say a 3.23 or 3.08 help me much? i travel around the country a lot for work at around 75-80 mph and it drives me nuts to go any slower. i don't tow anything at the moment. what will be the setbacks to lower gears when not on the highway?

anyfeedback at all would be great. thanks.

ps- i think the site is really great. this is my first truck and first diesel engine and i've learned so much about it from all the material on the site. please keep it up. thanks again.

[patrick m.]

if fuel economy is you goal, then the 3.42 ratio is probably going to yield as good as you can get.
changing to a 3.23 wont make any noticeable difference, and a 3.08 would drop the rpm at 80mph to 1500-1800 rpm, and in many situations, traffic would only allow 65-75mph, which rpm would be too low.

you may want to try some add-on stuff to increase the engines air flow, and efficiency, keep the 3.42 and check mileage again

[badburban]

You have the gears mixed up. Gears are backwards. Higher numericall numbers are lower. What size tires are you running. Im going to guess a 285/75/16 or 305/70/16. Both are 33" tall. Granted my burb isnt a diesel but its still all the same concept. I used to run 3.42's on the 33's and got terrible mileage and the thing couldnt get out of its own way (dont laugh but 23sec 0-60). So i put in a set of 4.10's (I cruise at about 2300rpm at 70-75) but I picked up 2-3 mpg all the time. My city jumped from 9mpg to 12 mpg and highway to 14-15mpg. This was all behind the stock tbi 350. If you were to go with some 4.10's (assuming your running a 33" tire) Im sure you would be pleased. And alot of people dont like to have gears changed because they are expensive. But they were the single best mod I made. Just going to 4.10's put my at a 12.5 sec 0-60 (real world numbers). Plus the mileage I picked up. Plus gears are not that hard to set up. If you want a instructions let me know Ive set up a few rear ends. Just my $.02

Josh

[Scooby]

Josh- your mileage went up because you put your motor back in it's proper rpm range. I have done that before too, after changing tire sizes, go to lower gears to get the rpm's back up. mileage is better because you are operating the engine in its best torque range. On the diesels, you wont see much, if any difference in mileage by going lower, unless you are towing something real heavy and cant use overdrive due to excessive rpm loss.
I have 3.42 gears in my c1500, and I also get around 17 not towing, and driving between 70 & 75 normally.

[Marty Lau]

TowerPower;
If who ever went to larger tires didn't recalibrate or replace VSSB then you oddometer is off. Take a time on a messured mile at 60 mph and time should be 1:00 minute under minute your running faster and show less miles than acutal traveled. At 75 with 3:42 I show 2100rpm. 3:23 is about 6% higher ratios so at 75 you would have about 2000 rpm. 3.08 is 11% higher so you would show about 1900 at 75 which is the "sweet spot" for MPG on my truck. At 75 mph at 2100 rpm with my c2500 3:42 auto, topper I get 19+mpg so you might look at cleaning up the exhaust and intake then gearing. You might call Bill Heath he has a 1/2 ton with a 3:08 and see what he can tell you.
When I dumped the stock down pipe and went to 3 inch mandrel bent exhaust I picked up 6% MPG and going from 3:73 to 3:42 I picked up another 5% I would bet if you went with a good exhaust and 3:08 gears you would pick up 15-17% increase fuel economy. which should give you about 2-2.5mpg. Whats your pay back at current fuel cost? I figured 60,000 miles on the exhaust and gear change. I have put on over 60,000 miles on past break even on the truck.
Larger than stock tires often times adds added rolling and wind resistance and may not improve mileage that much compared to stock size tires and higher gearing.
Hope this helps. Also what is you truck besides a 1997?

[Dvldog 8793]

Howdy
This has been discussed countless times, try a search on gear ratio or MPG.
That being said.....
I have 4.11 rear gears in my truck. I also have a Gearvendor aux overdrive. My truck is the longest heaviest truck they made w/ the exception of a dually. The last time a went across I tipped at about 8800lbs. So..... I like to drive at highway speeds of around 70-75mph, I have bigger tires but the speedo has been corrected, I get around 20MPG sometimes better sometimes worse. Mytruck runs 1850-1900 at 75mph. It really likes that rpm range. I think any faster and you really start to battle the wind. (something else you might think about is the visor sucking a little MPG) Also those big tires we all like are fuel suckers, air and rolling resistance. The biggest thing I notice is fuel quality. In the winter when I run winter blend I can normally take 2MPG off the average. A good additive can help that, make sure what you use is ALCOHOL free and also a lubricant. If you are going just for MPG than you want your rig to be as sleek as possible and as low as possible and try to keep it in the 1700-1900 RPM range. As stated earlier, opening up both ends of the engine really helps also. Synthetic lubes can add up if use enough of them!
Hope this helps!
L8r
Conley