9.4 mpg with 37s and Gobi rack - whats wrong

[dna]

I really never tracked or cared about my mpg since I've had my 2003 H2 over the last 4 years. Though I recall when I first got it that I was getting 12.5 mpg, and that was when drivin' it hard (as I always have and continue to do so) - if I drove it like an old fart I'd get almost 14mpg.

Anyway, a few years ago I put on a Gobi rack, and recall that the rack caused quite a change in the rpm to get to the same speed - especially on the highway. Also about a year ago, I installed 37s on the truck.

Now with the higher gas prices, I'm starting to track my mpg. No matter how light or hard I drive it, I'm averaging 9.4mpg. Does this sound right for my setup? Could the larger tires be causing false readings - do I need to compensate for the larger tires??? I know that speedo are affected when you increase the tire size, so am I maybe getting better than 9.4mpg???

Lastly, anyone ever use a Superchips module in their H2? A buddy installed one into his Duramax Dodge truck and claimed 11mpg increase while 100+ hp increase. Hard to believe for a cheap ($500) plug and play, but I'm now seriously thinking of getting one. My bud said the Superchips module will have less effect on a gas engine, but their website still says 20+ hp gain while getting better fuel ecomony.

Thanks

[HummerJim]

The problem sounds like your odometer is reading incorrectly with the larger tires. They make compensation devices for that, but they are expensive and not worth the money unless you are running monster truck tires. Also, with larger tires you in essence numerically lower your axle ratio, which can help on the highway, but hurt mpgs under lugging condtions.
As for the superchip, they just work by recurving the various engine O2, fuel, timing, injector duration etc. The factory uses a balance between power and durability. The after market chips all lean towards power and away from durability. I had one in my 1988 Corvette and a year afterwards a hole appeared in the #1 piston. When they tore down the engine they found several burned valves, but the power was definitely increased while it lasted.

[timgco]

You are getting better mpg than what is shows on your dash. Teh 37's throw it off a little. If you have the full height GOBI you will notice about 1 or 2 mpg difference...unless you have the tire mount, tool boxes, fuels cans, and all that stuff up there.

Sinc eyou have an older truck, it may be time for a good tune.
I ran teh superchips on my last H2. You can adjust for your tire height and change the shiftpoints. Those 37's, although not much bigger than your almost 35" stock tires, will cause your truck to shift more up longer grades and shifting in and out around town or cruising around 55mph to 60mph. That $400 Superchips programmer helps with that quite a bit.

Check your tire preassure too.

Good luck.

[timgco]

Also just saw your other post about your engine not running properly. There's another issue contributing to your problem.

[HumminNBoatin']

I have 37's on mine. My average gas milage over the past year has been a steady 10mpg. However, I did some math at one point to acutally see what my milage really was. Turns out that when I says im getting 10mpg im really getting 10.5mpg. So not alot of differnece.

[Kerr]

your mph and mileage are off.

Lets say you went from stock 35's bfg's they rotate 601 rpm if you stayed with bfg and just went to a 37" they rotate 574 rpms. the stock 35's are 34.5 tall and a "37" is 36.00 tall. Spedo wise at 65 your really going 67.9 mph, 70 is really 73mph, 75 is really 78.3

If my math is correct ever 10 miles is really 13, every 100 is 113, ever 1000 miles is really 1113 miles.

so if you go 300 miles on 30 gallons, = 10 mpg, you really going 339 miles on 10 gallons which is 11.3 mpg.

also try some lucous fuel injection cleaner, i run that in my cars about ever 4-5 tanks. Its $10 Bucks for the large bottle. all it does is clean the junk in the tank. In the suburban i gained 3 mpg after the fist bottle went threw. 2 mpg on the armada but its a 07 so it shouldnt have to much dirt in the fuel system.

As for the rpm difference the taller tire lowers the over all gear ration.. ( taller tire turn the gears less in a nut shell) So 4:10 gears with 34.5 inch tires now feel like 3:73 gears with 36 inch tires.


I just adjusted my sububran. i went from a 30 to 33.5 in the computer tuning once the tire size was recalculated it said my gear ratio was 3:44 instead of a 3:73... So the game plan is to insall 4:56 gears so the truck will run like it has 4:10...

[brad_p]

i have a full gobi with tire mount, 37's on 20's and a rear tire carrier. And i get 9.5