K3500 Gearing

[PlayingWithTBI]

With software on a lap top via OBD port? PW TBI - is that how you do it also?

Yes, keep in mind this is the older OBD1 system, but I'm using TunerPro RT and an ALDL 12 pin to USB cable. YMMV

http://www.tunerpro.net/features.htm
http://aldlcable.com/aldl.asp

 

[88GMCtruck]

I think Dave @88GMCtruck had them on his K3500; @DanteGTZ owns it now so either of them can chime in; and I also think Devin @83GMCK2500 runs the same brand/size Dave did.

slow to respond, but a dually stock runs a 225/75R16. TINY tire. You can go up to a 235/85R16 without the need for rear spacers between the duals, or you can go for spacers and run a 255/85R16. That is what my 98 had, what my 06 has, and what @83GMCK2500 has.

I calculated it out, me switching from a 225/75R16 to a 255/85R16 was overall like moving my truck from 4.10 gears to 3.73s. It helped my unloaded highway mileage a good amount and lowered my RPMs. In your case it would be like going down to a 4.10 gear.

 

[Supercharged111]

235/85/16 without a spacer on my flipper is TIGHT unloaded. They'd surely rub together loaded. It may depend on the tire manufacturer.

 

[bugdewde]

My '95 TBI 454 Suburban K2500 with 3.73s got 12 mpg unladened (Phoenix Arizona to East Tennessee) 1,800 miles.

Here in East Tennessee, unladened, 10-12 mpg (lots of hills).

Towing a 2k lb rally car ('91 Miata) with a 20' steel deck open car trailer (~2500 lbs?) plus tools & race stuff .... totaling around 5k lbs ... I get 8-9 mpg.

Towing would be much better with 4.10s around here because of the hills. I'm using a LOT of throttle getting up to the race track (110 miles one way).

3.73s will not improve your towing mileage an appreciable amount.... in my opinion. .. .. .. because you'll be using more throttle. .. .. .. especially if you have hills like I do.
3.73s may help mileage unladened.... maybe up to 12 mpg like mine, depending on driving style & conditions.
The Vortec engines get better mileage than my TBI .... so you may fare better.

Unrelated: My '99 6.5L turbo diesel K1500 Suburban had 3.42 gears (14 bolt SF 8-Lugs). It got 16.8 mpg towing on flat land (not East Tennessee) .... Here in the hills it got closer to 14-15 towing or unladened. Towing at higher speeds (80+) it got down around 9-12 mpg. But remember, the turbo diesel made more power & torque than the gasser motors and only turned 2000-3000 rpm.