Tire size and gears

[CJ4BAJA]

O.K. folks. I have an 85' Suburban with a 6.2 and a banks. I do not have a tach. I have 4.10 gears and 33 inch tires. I understand that for gas mileage purposes the ideal rpm for freeway speeds is 1800 to 1900 rpms. I do not tow anything over 2000lbs. To run at 65 to 70 mph with maximum fuel efficiency, how high in gears could I go? I currently get about 14 mpg with the 4.10's. Could I go to 3.42 with the 33 inch tires?

[ 04-14-2005, 06:22 PM: Message edited by: CJ4BAJA ]

[NH2112]

I think you'll find that the height of the vehicle off the ground will preclude you from improving your mileage very much, regardless of gearing and cruise RPM. There's an awful lot of turbulence and wind resistance underneath a vehicle, especially a lifted vehicle. Still, if you can come up 2mpg it'll be an improvement, but how long will it take to offset the cost of 2 gearsets, installation, and new tires (assuming you'll have to buy the 33"s.)

[Ratau]

What transmission are you using? If you running 1:1 you will get 2715rpm @ 65mph and 2923rpm @ 70mph. With the 700r4 in OD running 0.7:1 you will get 1900rpm @ 65mph and 2046 @ 70mph (that

[Ratau]

Go to member's area and copy Paul Rowland's calculator.

[jcomp]

Check out the gear ratio calculators at www.4lo.com (http://www.4lo.com)

Knowing what transmission you have is vital to estimating your cruise RPM.

FWIW, I get 17-20 MPG in the mountains with this truck (link) (http://jcomp.home.bresnan.net/P82940001a.jpg). The 4.10 gears are a little high for my 37s, the next set of tires I get will be 35s. I spend a lot of time in drive or second gear going over the passes around here. I get roughly the same mileage in drive as overdrive.

IMHO, your gearing is probably fine. You should be looking into a turbo setup if you want a real gain in MPG. I saw a 5 MPG gain with the Banks kit.

[More Power]

http://www.maximumdiesel.com/images/jims1982GMC.jpg

This red/silver GMC was equipped with 3.42 gears, 31" tires and a non-OD auto when I first bought it. It routinely delivered 17-19 mpg running empty at no more than 65 mph.

Installing a 700R4 jumped fuel economy to 22-24 mpg at 65-mph. This was 1800 & change rpm at 65.

Installing an OD was the single best mod I made to that truck - mostly for how nice it was to lose the noise and be able to drive faster. The fuel economy bump was an added bonus.

MP

[jbell]

I had the EXACT same decision to make. 3/4 ton 85 suburban, th400, 4.10 gears 32" tires. It was almost $1000 to regear, but I bought a 700r4 from a rusted out diesel burb for $150. Had local shop cut driveshaft and install HD yoke, and bolted everything up. Best thing I ever did. RPM's still seem a little high, but I get 20mpg @75, and it 'drives' alot faster. I can accelerate or out accelerate the 'crowd.' and I always seem to have enough power (non turbo) probably because I never tow with it. I really think in my case, 245-85r16's would help get my rpm's down just a little more -- and will probably be the tires I install next. (narrower profile, ribbed tires get better mpg...)

[CJ4BAJA]

Thank you for all the responses. I collected a little more information. I bought this suburban a couple of years ago. I had it gone through by Diesel Tune in Downey, CA. It leaks oil but runs strong. I have 33x12.5x16.5 tires, Gear Vendor O.D. and a Banks Turbo. It has a TH400 tranny. I guess I should be happy with my mileage but I get jealous of this talk of mileage over 20mpg. The odometer only has five digits, so I have no idea how much mileage was on it. Diesel Tune added the O.D. and the turbo for me. I love this truck, I just wish I could get 20mpg, or find $1.50 diesel again. Thanks again.

C.J.

[jbell]

Wow, you've got what most folks would call the 'ideal setup.' At least for towing that is.

However, I don't know anyone with a th400 or a 4l80e that is getting 20 in a burb. They just burn too much heat. You have too many $$$ in the GV to want to change trans, and changing rear ends isn't going to help any.

Your best bet. Amsoil, Redline, or mobile1 gear lube in the rear end, Redline or Amsoil in the tranny, narrower profile (less than 12.5 wide) tires with long ribs, not lugs, and a front air dam with a pair of sagging front springs is your best bet for better mpg. (Try the air dam first, it makes a HUGE difference, got 2mpg better on my toyota on 32's that way)

You might actually get a return on investment with those mods (probably get you close to 16, maybe 18 if you're lucky) You'll never get a return on investment with gears.

The only other thing you can do, (and I don't know that it's a great idea) is change out your 1200rpm stall converter for a 800-900 stall, it'll generate less heat cruising, just be ready on a cold morning when you throw it into reverse, you'll be across the driveway and into the neighbor's quick....

[moondoggie]

Good Day!

Maybe this doesn't apply to what you're talking about here, but our 95 Sub with 4L80-E tranny ALWAYS returns > 20 mpg on the highway. (It has 3.42 gears.)

My 95 pickup gave best of ~ 20 mpg highway B4 the Gear Vendors Aux. OD (it has 4.10 gears & 4L80-E); I have made three long trips with this truck since the GV was added, getting 22.5 - 22.9 each time. :D This truck averaged 19.6 for all of 2003, 20.1 for all of 2004.

(All of these numbers are corrected for odometer error. Yeah, I should find a real hobby. ;) )

Blessings!

Brian Johnson, # 5044

[ 04-20-2005, 05:46 AM: Message edited by: moondoggie ]