Just purchased 2008 3500 Express. Took a recent 100 mile road trip and had around 9 MPG. The trip was on a super slab and large parts of the trip were at the legal speed limit which is 80 MPH. I don't know which engine it has and don't know how to identify the engine. At this point all I know is that it's a V8. I have a 2002 1500 Suburban with a V8 and it get's way better milage than that.

Where do I start to diagnose the problem? I complained to the seller and he said that it probably needs a tune up. Thanks, Jim

 

It's a big brick pushing a lot of air.
This is mostly around town and lots of ethanol in the fuel hurts mileage too (often run E85). Do all the normal checks but also just be ready to slow down if you want better milage.

 

What engine, what mileage, what trans, what gears, what tires etc.? GPS verified mileage? Full tank to full tank at same gas pump gallons? Do you have a scan tool? Look up the torque app on this site.

 

The original equipment questions can be answered using the RPO code sticker inside the glove box compartment. It will be a sticker that has a bunch of 3 letter codes on it. Take a picture of that and post it here. You can also go to a website like https://decoderpo.com to figure out what they mean.

The most important code will be Lxx (engine), Mxx (transmission) and multiple Gxx codes that identify the rear axle. I am pretty sure a code Qxx will tell us tire size. My son in law's 2008 has QSS for the tire size P275/55R20, while my 2006 has QPR for P245/70R17.

But as others pointed out - that van is a big square box pushing a lot of air. At 80mph, it will not be very efficient.

 

My 2011 1500 with the 5.3 V8 averages around 17 or so on highway trips doing between 75 to 80. That's with over 200,000 miles on the clock. I've been doing a 4 hour trip pretty regularly. Pulling a 2,000 pound trailer on the same trip brings it down to around 13 or so.

I would start with a basic tune up if you just got it. Plugs, wires, air filter etc... can all hurt your mileage. The previous owner could have put off a lot of maintenance knowing he was going to sell it.

Check what tire size is on there and what tire size it calls for. If it has taller tires the van will think it's traveled fewer miles than it actually has. That will throw off the calculation for sure.

Also make sure the tranny is shifting into top gear. If it's not you'll definitely burn more gas.