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From my understanding, an unmodified gas engine running on pure propane will give around 10% less fuel econ, it's not super common here, but some vehicles are swapped over to save on fuel costs.
If you switch over to the diesel world, propane injection is known to act like putting NOS in the diesel engine, but it's much cheaper and it doesn't have to be floored if it's metered going in. I've seen claims of around 30% better fuel econ. I've read up a fair bit and that 30% most people don't account for the propane used. The propane is used as a 10-20% mix with diesel and the theory is the propane helps ignite and encourage a more complete burn of the diesel. The other added benefit is more power which is the most common focus for people wanting those systems. I've seen figures of propane vs NOS and they were real similar, propane + NOS had just at tiny bit more power but separately it was a huge jump over just diesel. Of my understanding, the only extra power from propane over NOS is propane is also burning as a fuel while NOS isn't. The guy doing the tests found even at 10% the jump was massive, I don't think he tried under that figure.
The setup for those tests were a ford power stroke turbo, I think 7.3L if I remember right, with a tuner. I think they were around 400ft/lb of torque on diesel, propane or nos was around 600ft/lb and combined it was like +10-20 ft/lb. I don't recall if there was a 10% mix figure as well or not, been a while since I watched the vid.
I've seen those claims all over, and never really seen anyone debunk those claims. So if my 7.3L powerstroke normally gets 20mpg, a 30% increase in mpg would be 26mpg + 260mpg of propane (10% target). 1000 miles normal would be 50 gal diesel vs 38.5 gal diesel + 3.9 gal of propane (almost a 20lb tank). Today's price for diesel is roughly $6/gal, so $300 vs $231 + propane (should be like $3-4/gal in bulk, even a tank swap out place is $20-25). So MPG alone it saves money, plus you get extra power if the figures are true and accurate.
The OP doesn't say if their vehicle is gas or diesel, but I think for diesel it makes sense, for gas I'd say full conversion is probably the best route if you can get a high compression engine that's more ideal for propane use.
I've always wanted to modify a gas engine to be more ideal for propane and compare the econ of before and after. It's been a while since I researched it but I think the ideal compression ratio for propane was something like 12-14:1. Compressed natural gas engines should be somewhat similar I'd think.
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