Lookin for gears.....

[danhercules]

Heres what I got.......

'97 GMC 4X4, 6.5 Diesel. With 33' tires. Towing 32' 5th wheel. Toal trailer weight, 12k.

My speedo is off. When it shows 60, GPS says 65. My RPM's are at about 1800. She is a dog now.

Before the tires I was pulling a little smaller trailer, and she was great. With the new tires slowed way down. New trialer, slowed a little more. I saw to look at the glove box for the codes, but what do I look at? The VIN? Or the other numbers that are there?

What gears do yo suggest I run?

Thanks for all the info. I have Gauges on the way and gonna get a chip too.

[tanker]

Well I assume you have a 3:73 axle ratio, and it should be 4:11, or even 4:56. But you have two differentials to change out, which can be a little expensive. Your other option would be a "GearVendors" or "US Gear"underdrive gear box behind your tailshaft, but it will only work in 2 wheel drive. Now your looking at about $3000.00 till your done with cutting the drive shaft etc. Do you have a lift kit that changed the driveline angle? That would be a problem when adding the "GearVendors" You could look to swap differentials with a salvage yard, or someone looking for better mpg. Lastly, you could go back to 245 tires :eek: , But I guess you wouldn't want that. :rolleyes:

[danhercules]

Where exactly in the glove box do I find the current gear size?

I wont be going back to smaller tires, I go in sand, I need the big foot print to get my big trailer through the sand. No lift on the truck.

Plus, its accually the wifes truck, she like the big tires. tongue.gif

[DmaxMaverick]

Have a look at this link:
RPO Codes (http://www.thedieselpage.com/members/rpocodes.htm)

It should have what you are looking for. Just match up the code.

[Heartbeat Hauler]

There is a SPID label, usually on the door of the glove box that has all of the RPO codes for your truck. Here are few codes to look for:
GH0 : AXLE REAR, 3.54 RATIO
GH2 : AXLE REAR, 2.29 RATIO
GH3 : AXLE REAR, 2.77 RATIO
GH4 : AXLE REAR, 2.92 RATIO
GK9 : AXLE REAR, 4.63 RATIO
GL3 : AXLE REAR, 6.17 RATIO
GM1 : AXLE REAR, 2.59 RATIO
GM3 : AXLE REAR, 3.45 RATIO
GM8 : AXLE REAR, 2.56 RATIO
GS4 : AXLE REAR, 3.70 RATIO
GT4 : AXLE REAR, 3.73 RATIO
GT5 : AXLE REAR, 4.10 RATIO
GT7 : AXLE REAR, 3.33 RATIO
GT8 : AXLE REAR, 4.10 RATIO
GU1 : AXLE REAR, 2.41 RATIO
GU2 : AXLE REAR, 2.73 RATIO
GU4 : AXLE REAR, 3.08 RATIO
GU5 : AXLE REAR, 3.23 RATIO
GU6 : AXLE REAR, 3.42 RATIO
GV2 : AXLE REAR, 5.83 RATIO
GV4 : AXLE REAR, 3.36 RATIO
GV8 : AXLE REAR, 2.72 RATIO
GW4 : AXLE REAR, 3.31 RATIO
GW6 : AXLE REAR, 3.27 RATIO
GW9 : AXLE REAR, 2.93 RATIO
You will notice there a several codes for a particular gear ratio. This is because my list includes PU trucks, cars, and med. duty trucks. Hope it helps.
JP

[Scottpearce]

danhercules,

What size tires were stock on your truck? What rpm's were you running before the switch to the 33's? It sounds as though you have 3.73's, in which case 4.10's would put you back in the ballpark of the same rpm's-for-speed. You might already have 4.10's though, which would mean you might need 4.56's
Then again, if you do a lot of towing of that trailer with the truck, you might want to go to 4.56's even if you currently have 3.73's If you ever go back to a stock-size tire though, you will not like traveling down the highway with the 4.56's

The following is assuming stock 4L80E ratio of .75:1 for OD. For towing in Drive, add approximately 33% to the rpms:

33 inch tires with 4.56:1 gears:
approximately 2275rpm at 65mph

30 inch tires with 4.56:1 gears:
approximately 2500rpm at 65mph :eek:

33 inch tires with 4.10:1 gears:
approximately 2050rpm at 65mph

33 inch tires with 3.73:1 gears:
approximately 1850rpm at 65mph :cool:


I tow with a total combined weight of 14,000 lb, which is about 5,000 lb less than you. My truck had 4.10's, and I went to 3.42's I haven't realized any noticeable drop in power, however my towing mileage went from around 9mpg to 12+mpg. Of course, I don't have as tall as tires as yours, mine are 235/85's, or 31.7" This gives me about 1775rpm at 65mph ;)

[Marty Lau]

Originally posted by Scottpearce:
danhercules,

I tow with a total combined weight of 14,000 lb, which is about 5,000 lb less than you. My truck had 4.10's, and I went to 3.42's I haven't realized any noticeable drop in power, however my towing mileage went from around 9mpg to 12+mpg. Of course, I don't have as tall as tires as yours, mine are 235/85's, or 31.7" This gives me about 1775rpm at 65mph ;) Scott; Did you run the stock tires size 245-75-16 before going to the 235-85-16's ? If so how much differece did the change in tire size make with the 3.42 gears? (i went to those also)
Thanks.

[Scottpearce]

Scott; Did you run the stock tires size 245-75-16 before going to the 235-85-16's ? If so how much differece did the change in tire size make with the 3.42 gears? (i went to those also)
Thanks.

16ga SxS,
When I bought the truck, it already had the 235/85's on it, although it was still running 4.10 gears. The stock size tires (according to my highly accurate GVWR sticker ;) ) for my truck were 225/75-R16's however, not 245's? Maybe because it is a two-wheel drive? The 225's should be about 29.3 inches, and the 245's should be about 30.5 inches.

The rpm difference with the 225's would be about 7.5% (higher for any given speed in OD), and the rpm difference with the 245's would be about 3.8% (also higher for any given speed in OD)