Gears VS Mileage help

[timfretwell]

Okay, I realize that this is a really dumb question for most of you out there, but help me out on this. What is the relationship between diff. gearing and gas mileage?
Thanks

 

[Alec]

Generally, a lower final gear ratio (higher number) will make you run higher RPMs on the highway. Higher RPMs means more fuel consumed. Thus, two trucks identical in any way, one with 3.73 gears and one with 3.27; the one with 3.73 gears will be running about 600 RPM more on the highway, and thus getting worse mileage.

 

[Kampy]

Alec, I understand what your'e saying in theory but alot of times it doesn't hold true. In the situation you gave the difference in engine speed should only be about 250rpm.

Anyway, the best example I can give and it's usually (not always) what the factory engineers strive for also, is to achieve a final ratio of around 10% of the tire height. That seems to work best around the target speed of 60 mph.

Here's an easy example to follow. You have 31" tires, the final drive ratio should be around 3.1 (10% of 31) To get there multiply your gearing by the overdrive ratio. In this case 4.10 gears X .75 OD (a4ld) = 3.08 .

All of this has variables, if you like to burn down the interstate at 85 it will get better mileage at 2500 rpm than 3500 rpm so you gear it a little higher, (lower numbers).
If you are lifted alot and pushing alot of air at highway speeds you might want to gear it a little lower to overcome the resistance.
If you are lifted 6 inches, running 35's and like driving 85 all the time don't worry about it, your mileage will suck no matter what you do.

That's my experience, and my .02, hope it helps.

 

[sink]

I just wanted to add to Kampy's $0.02.
The reason for it is because engines don't strictly follow a direct RPM to consumption ratio. The point of best efficiency could be any RPM, usually around 2000 to 2500. So a truck pushin high gears might be doggin at 1800 RPM and the same truck with lower gearing is 'in the power band' humming at 2300 and running lower air/fuel intake volume and better efficiency.

 

[Alec]

Yup, thanks guys for adding that power band stuff. I just tried to really generalize it. BTW, you're also right the RPM difference wouldn't be 600, it would be about 475 at 65 mph in third gear. That's what most differential shops use for comparison: 65 mph in third gear.

 

[timfretwell]

Originally posted by Alec
Generally, a lower final gear ratio (higher number) will make you run higher RPMs on the highway. Higher RPMs means more fuel consumed. Thus, two trucks identical in any way, one with 3.73 gears and one with 3.27; the one with 3.73 gears will be running about 600 RPM more on the highway, and thus getting worse mileage.

Thanks Alec,
I knew the answer would be simple. I guess the 4.10 gears I have are part of my poor mileage problem and is part of the "towing" package I presumably have. I suppose its not feasible to replace the gears? This thing really sucks gas!

 

[jimbo74]

if you put on bigger tires like 31-33s you will get better gas mileage on the highway

 

[Alec]

You could replace the gears with higher ones, maybe 3.73 or even 3.55, but I don't think you'd want to. You'd lose a lot of the "oomph" you're used to at the low end. Also, it would cost about $500 to do. If you ever got larger tires, as James alluded to, you'd want the lower ratio gears again to get back into the powerband. If you got bigger tires now, you'd see a drop in RPMs due to a change in the final gear ratio caused by the tire diameter change. However, you might not see better mileage because your larger tires would cause more air and frictional resistance.

My advice: Sorry, you're driving a brick and most of the time crappy mileage is a fact of life. On the other hand, improvements in engine power might actually make your mileage better. I threw the supercharger on my truck and saw a definitive increase in highway mileage (slight decrease in city mileage). More power = less work to cruise and thus less fuel consumed.

But like I said, swapping gears is an option. You'd definately see a mileage gain (esp with 3.55 or 3.27 gears) but might not be happy with low end performance.

 

[TDG]

My 92 has the 3.08 gear ratio. It gets awesome gas mileage provided that I keep the engine in the power band, and since my normal speed is somewhere between 85 and the 2 on the gear indicator it suits me just fine. However it has absolutely no power through any gear at any speed, (having 200,000+ miles on the engine probably has a lot to do with it) and there is a nice big hole between D and OD that leaves me wishing I had one more gear.

The 93 on the other hand has the 3.73 gear ratio. This ratio gives it a decent amount of zip for a 100,000 mile well maintained stock vehicle. When I use the freeways I don't feel comfortable (and neither does my wallet) driving it over 75. However unlike the 92 I never have to manually shift it out of OD.

Just my .02 on my experience with various gear ratios.