3:42 Ratio ?

[TDSECK]

I have a 1999 Chevy Suburban 6.5, 2500 4x4 with what i believe to be a 3.78 axle ratio.
I am thinking of changing the 3.78 ratio to a 3.43 or lower so that at 65 mph the 6.5 is running at 2000 rpm or lower. I also have the stock size tires.
I am not completely sure of my rear/front axle ratio, how can I tell what it has? By the vin # or stamp some where on the axle?
I was told to call my local Chevy dealer and they could tell me info about my Sub if I give then the VIN #
I do not plan on hauling any thing heavy with it, mostly trips to New England around 500 miles each way.
The last trip I took I did get 18+ mpg by keeping the rpm around 2000 rpm this made the trip very slow going, at that rpm my speed is around 55 mph. (i am guessing that i have the 3.78 ratio by this rpm/mph combo, am i right? or do I have the 4.10?)
I did read in the one of my Diesel Place books that the torque peak is 1800 rpm for the 6.5. and that that the higher the rpm in a diesel the lower the mpg will be, even a 5 mph increase in speed will lower the mpg.
If the cruising rpm is between 1500 to 1700 rpm at 60 to 70 mph will I get less mpg?
I have read many posts and it looks like my best bet is the 3:42 ratio for my needs. Any feed back from owners that have a 1992 to 99 year suburban with this ratio would be appreciated.

[okhotair]

Go to your dealer and give them the VIN. They will printout an RPO sheet that will tell you all the options your truck came with from the factory.

If you pull anything you will not like the 3.42 gears. The 3.73 is better for hauling.

If you never pull anything then the 3.42 gears will help.

[joed]

If you still have the sticker on the floor of your glove box, the RPOs (reg. production options) will be listed.

GT4=3.73, GT5=4.10, GU6=3.42 ratios.
G80=locking differential.

Hope that helps.

Joe.

[More Power]

When choosing gear ratios for best fuel economy you want the engine torque peak (at ~1800-rpm) and chosen vehicle speed to coincide. This is achievable with a 3.42 ratio at 65-mph.

I don't recommend someone pay to swap gears when the vehicle is already equipped with 3.73 gearing. The cost of the gearing change will be harder to recoup when compared to a vehicle equipped with 4.10 gears.

Both the front and rear differentials in a 4x4 vehicle will be equipped with the same gearing, and both must be changed when moving to a different gear ratio. It can be costly on a 4x4 if you pay to have it done right. Less if you find a salvage set of axles.

Jim

[Edahall]

I've got a 1996 Cummins powered Tahoe with 3.42's. It gets very good fuel economy and the diesel engine seems really happy with those gears.

[john8662]

Sounds like 4.10 to me.

Check the RPO codes in the glove box as mentioned above.

I vote 3.42 gears. I bought a front differential that was 3.42 and a ring and pinion from Yukon gear for my burb, then, it got wrecked. I'm now gonna see how many years it takes for those parts to turn to complete iron oxide, lol.

I did the change from 4.10 to 3.73, got me 18mpg at 70MPH, granted it was a modified 300HP 6.5.

[greatwhite]

My 4:10 equipped, stock tire sized 1998 k2500 spins 2100-2200 at 100 kph (approx 62-ish mph).

If you're turning 2000 at 55mph I'd say you've probably got the 4:10's.

Nessesary evil for me, I pull a 35 foot/8500 lb travel trailer in the summers.

Don't feel too bad about 18 mpg, I'm lucky if I can break 14......

[arveetek]

As was mentioned, don't hope too much for better fuel economy. I have a 95 Tahoe 4x4 with 3.42 gears and 31" tires. I have yet to top 18 mpg. I usually average 17.5.

Casey

[phantom309]

My truck is a 94 2500 xcab 4x4, it has 3.42's, i tow stuff with it it just fine,.it rolls up hills with weight on better than my dually with th 4:10,s
I have 265-75-16,s on it,.people put big tires on then sometimes don,t account for the difference in the odometer in their mileage calculations,.,.and really big tires take more torque to turn so they will burn extra fuel,.
INMHO the 3.42's are just right,.keeps the drive pressure down on the turbo on the highway,.summer time gets me close to 21mpg.
I have a set of 3:42's for the dually but i,ve been looking for 3:23's

[JohnC]

I have a set of 3:42's for the dually but i,ve been looking for 3:23's
Don't press your luck! I had 3.42's in my '93. It would have been fine with an automatic, but with the stick it was too long. My guess is 3.23 would be about the same with the automatic; a little too long.