33 to 31 vs. 3.08 to 3.73 - The 1947 - Present Chevrolet & GMC Truck Message Board Network

kevinr1970 · 02-22-2008, 04:00 AM

On my ’86 GMC short bed 4x4 I have 33” tires and according to the glove box 3.08 gears. No reason to believe that gears have been changed.
I have a 305 (with headers) and a 700r4.

When driving the truck I can’t help but think the 305/3.08/33 isn’t the optimum combination.
I have a set of 3.73 axles on my parts truck.
I also have talked to a guy that said he would be interested in trading me tires for his 31’s. Tires are in about comparable condition.

What I am wondering is will either one make a bigger difference than the other?
My inclination is to go with the tires as we have a tire machine at work so it would theoretically not cost me anything (maybe a case of beer to the maintenance guy to grease the wheels if you know what I mean). I think that the 31’s would also be the same size as stock or closer anyways. This would get my speedo back right again (or closer).

The 33’s look better though and I think the 3.73’s would give it more “oomph” when driving it. There has been a fairly convincing argument for going with the taller gears as they would put the engine in a better power range at freeway speeds. Also require less effort to get the truck moving from a start. Theoretically netting better MPG; maybe not much but at $3.15 a gallon (and climbing) everything counts.
The 33’s would require more work and probably some money with brake fluid for bleeding the brakes. The front brakes on the parts truck need to be done too so turning the rotors and new pads, etc.
Also, not only is the speedo off because of the tires, but not sure how it would be affected by the gear change too?

Any thoughts?

Prerunner1982 · 02-22-2008, 05:18 AM

Go with the new axles. The 700R4 has a decent 1st gear so the lower gears would certainly give you a better start. My 32"s with 3.08s were a pain to get going and I think i only got abotu 8-10mpg around town because of this, with a th350 trans.

As far as ease... The tires would be the best route.

As far as getting the speedo close, changing the gear ratio could do the same thing. I dont have the chart infront of me to see exactly what gear ratio closest to the origial ratio but I am sure someone will pop in with it.

duallyjams · 02-22-2008, 05:23 AM

My vote goes for the 3:73

Texasdeere · 02-22-2008, 10:42 AM

3.42 gears are about as tall as you want to go with a small block TH700 combo. 3.73 are a good match for a SB/TH700

Here is a gear calculator for you to plug in some diff tire & rear end combos to check RPM:

http://www.xse.com/leres/bin/gearrat...mference=91.11

02-22-2008, 04:00 AM	#1
kevinr1970 Senior Member Join Date: Aug 2004 Location: Everett, WA Posts: 2,316	33 to 31 vs. 3.08 to 3.73 On my ’86 GMC short bed 4x4 I have 33” tires and according to the glove box 3.08 gears. No reason to believe that gears have been changed. I have a 305 (with headers) and a 700r4. When driving the truck I can’t help but think the 305/3.08/33 isn’t the optimum combination. I have a set of 3.73 axles on my parts truck. I also have talked to a guy that said he would be interested in trading me tires for his 31’s. Tires are in about comparable condition. What I am wondering is will either one make a bigger difference than the other? My inclination is to go with the tires as we have a tire machine at work so it would theoretically not cost me anything (maybe a case of beer to the maintenance guy to grease the wheels if you know what I mean). I think that the 31’s would also be the same size as stock or closer anyways. This would get my speedo back right again (or closer). The 33’s look better though and I think the 3.73’s would give it more “oomph” when driving it. There has been a fairly convincing argument for going with the taller gears as they would put the engine in a better power range at freeway speeds. Also require less effort to get the truck moving from a start. Theoretically netting better MPG; maybe not much but at $3.15 a gallon (and climbing) everything counts. The 33’s would require more work and probably some money with brake fluid for bleeding the brakes. The front brakes on the parts truck need to be done too so turning the rotors and new pads, etc. Also, not only is the speedo off because of the tires, but not sure how it would be affected by the gear change too? Any thoughts? __________________ 2009 Honda Fit CfC (bsf 44.9 mpg) 2000 Tahoe Limited 1991 GMC CrewCab Dually 2wd, will end up swb, not dually and replace CCswb below 1991 GMC CrewCab Dually 4x4, just going to fix things up for now 1982/1989 K5/GMC Jimmy 2wd 1987 GMC 1/2 ton swb 2wd Crew (sold)

02-22-2008, 05:18 AM	#2
Prerunner1982 Registered User Join Date: Oct 2002 Location: Edmond, OK Posts: 5,904	Re: 33 to 31 vs. 3.08 to 3.73 Go with the new axles. The 700R4 has a decent 1st gear so the lower gears would certainly give you a better start. My 32"s with 3.08s were a pain to get going and I think i only got abotu 8-10mpg around town because of this, with a th350 trans. As far as ease... The tires would be the best route. As far as getting the speedo close, changing the gear ratio could do the same thing. I dont have the chart infront of me to see exactly what gear ratio closest to the origial ratio but I am sure someone will pop in with it. __________________ Jon 1982 Chevy Silverado 350/th350.... RETIRED 1993 Jeep XJ 2 door(Cherokee) 4wd 4.0ltr/AX-15 (5spd)/NP231 .... Oklahoma Roll Call

02-22-2008, 05:23 AM	#3
duallyjams Registered User Join Date: May 2007 Location: Deale Md Posts: 4,663	Re: 33 to 31 vs. 3.08 to 3.73 My vote goes for the 3:73 __________________ James 63 GMC V6 4speed carryall 65 chevy swb bbw V8 auto 68 K20 327/4 speed, buddy buckets 2002 GMC CC Dually Duramax 64 GMC lwb 3/4 V6 4speed SOLD 66 GMC swb bbw buddy buckets SOLD IG duallyjams The only thing that stays the same is constant change!

02-22-2008, 10:42 AM	#4
Texasdeere Registered User Join Date: Sep 2006 Location: League City, Texas Posts: 732	Re: 33 to 31 vs. 3.08 to 3.73 3.42 gears are about as tall as you want to go with a small block TH700 combo. 3.73 are a good match for a SB/TH700 Here is a gear calculator for you to plug in some diff tire & rear end combos to check RPM: http://www.xse.com/leres/bin/gearrat...mference=91.11



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