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We're new to the Safari journey and on our maiden voyage! We're cruising a 1999 (model 3876, CAT 3126B) and learned we have an 88.5 gal tank, but not before we realized the gauge wasn't moving below half (200 -220 miles in). We filled up twice at 20-22 gallons each... We know that's not half of 88.5... so anyone have a solid experience in mileage on their coach at an average speed of 65-70? The math says it's 10mpg but our Dodge work truck gets 10-12 and is much lighter then our 'Zanzi' TIA

Welcome to the Safari world!
Usually those get between 8 and 9 mpg, but if you have mountain grades and/or a head wind, all bets are off. If you go 60 to 65 your mileage should improve.
Enjoy your new journey!

I've got a 96 30 foot with a 250hp Cat 3126, and not towing anything I can get close to 10mpg. Towing it goes down to about 8 - 8 1/2 (I do a steady 62 for my speed). As far as your tank, I'm thinking you have the same 90 gallon fuel tank that the vast majority of us have (you should be able to crawl underneath on the drivers side and see a decal on the tank with the information on it). Now - the fuel gauge is a whole issue on its own. Our rigs have issues with poor dash grounds, which as a result can cause the gauges to be inaccurate. I replaced my gauge a few years ago - it lasted all of one season and it was back to the old game of not moving or jumping all over the place. I use my GPS to keep track of my mileage on my rig and normally try to fill around the 400 mile mark, guestimating that I've got about a 650-750 mile range on a full tank. I've ran my tank down to the point that it took around 75 gallons to fill it and will never do it again unless it's an emergency and I need to get somewhere ASAP!

There's a wealth of information here in various messages and files - you just have to play a bit with the search to find the right method for what you're looking for.

It's also common that the tank won't fill all the way due to venting problems.
I end up raising the front with the leveling jack and it will fill all the way just fine.