Gas engines

Nobody can give you an estimate on a "bigger gas engine". What engine? 454, 440, 502? What tranny and what rear end ratio? You leave to many variables.

Which engine is the "bigger one"? Are you asking about the 6.0 as compared to a 5.3 or an 8.1 as compared to a 6.0? Or, an older truck comparing a 7.4 to a 5.7?
Anyway, the 8.1 will get 10 mpg no matter what you do. The one in my dually was like that. Empty, loaded, towing, didn't matter. I got 10 mpg.

My fathers first truck had a 262 cid in-line six. It very often got 18-20,000 lb put in a single axle trailer. Then he had several 352, 351 and 460. They all pulled a good load before he got his first diesel. They might have been a little slower but they got the job done. Yes a gas will work but back then gas was less then 20 cents and there was no DOT.

Thanks TomG2' I am jus weighing all the options. We don't tow but three or fout times a year now. Not much over 300 miles. Trying to justify the extra cost of the d/a like I drive now. With the cost of fuel and it being a daily driver. Want to make sure I get what we need.

As expected, the diesel guys will tell you all the bad about towing with a gasoline fueled engine. This time, they are right. But you asked about gasoline mileage. You will be in second and third gear a lot of the time, averaging about 5 mpg towing that much.

Been so long since I had gas engine I am not even sure of the options

About 10mpg when hooked up. Are you talking a 8.1 or a 6.0? About the same when towing. The 8.1 will be quicker.