gear ratio selection.

[SUPERSUB87]

long story short. i have a 3/4ton suburban with 37" mud grapplers and an ungodly high gear ratio of 3.42, and since all of my driving is highway driving would it be a good decision mpg wise to keep the high gears or switch to 4.56 gears.

do higher gears really get better gas mileage on the highway????

how much better gas mileage would i be getting???.

would you keep the higher gears or would you swap gears???

keep in mind that my highway to city driving is in a ratio of 36:4 on average.

 

[Hogback Fabrication]

What transmission?

 

[SUPERSUB87]

i believe sm465, isn't that the manual with a low gear and then 3 gears?

 

[SUPERSUB87]

i'm guessing that was stupid huh??? figures. i haven't touched the sub in so long i forgot the transmission.

 

[Hogback Fabrication]

IMO, go with 4.10's unless you plan on going to bigger tires later.

I had 4.56's with 37's and felt it was too low for the highway.

 

[SUPERSUB87]

4.10's would be a good compromise, but money is tight and if my gears are good for highway then i would like to keep them, they seem fine from what i can tell, has enough power (never going to set speed records), but it's enough.

 

[jonrpick]

4.10's would be a good compromise, but money is tight and if my gears are good for highway then i would like to keep them, they seem fine from what i can tell, has enough power (never going to set speed records), but it's enough.

Too high of a gear can hurt mileage too. Fuel systems are designed to richen up the fuel/air mixture under heavy load. My 3.08's with 31's sucked, so I can't even imagine 37's and a 3.42

 

[abig85]

Too high of a gear can hurt mileage too. Fuel systems are designed to richen up the fuel/air mixture under heavy load. My 3.08's with 31's sucked, so I can't even imagine 37's and a 3.42

my first truck had 273's and 38s about 2000rpms at 80mph. couldnt even spin the tires in reverse in 4low when i got stuck with the 305

i later put in 373s and going about 80 went up to about 2600 at 80mph.

no i got 411s and they seem perfect with a 400 trans and strong engine. havent hit the expressway yet

 

[HarryH3]

While accelleration suffers with the 3.42's and 37's, it's actually a pretty decent combo for highway cruising with a non-OD transmission.

RPM at 65 MPH is almost the same as using 4.56's, 35's and a 700R4; which is considered to be a good setup for all around use.

3.42's, 37's, no overdrive= 2018 RPM @ 65
4.56's, 35's, .7 overdrive= 1991 RPM @ 65

A measley 27 RPM difference...

Indeed, you're losing a lot of mechanical advantage to get that beast rolling, but it seems that the setup isn't bad in terms of highway cruising efficiency.

 

[user]

I'd suggest keeping what you've got if you're doing mostly highway driving. It's a pain to rebuild a differential, and if you don't do it exactly right, you'll screw it up really well, not to mention the expense. If you're going to pay someone else to do it, you have to find someone really good, 'cause the average mechanic can't think in three dimensions to thousandths of an inch precision. I'd suggest upgrading the transmission instead. If you don't have a TH700-R4, get one; if you've already got one, at least put a shift kit in it or get an upgraded one from one of those custom-rebuild/re-engineering places.

This advice is especially for if you live in Iowa, or someplace just as flat. If you live in West-By-Gawd-Vuhginya, or in the neighborhood of the Rockies, then you might need a lower gear (higher gear ratio number). I, myself, have a TH700 with 29" tires, and I switched up to a 3.08 from 3.73, 'cause the truck was all torque and no action. I was spinning wheels taking off from stoplights, but when I stepped on the gas, I could watch the gas gauge needle move down. I figure I get about a third better gas mileage now.

 

[obijuan]

you just needed bigger tires.