Im a little confused whe you say petrol time to be the same on gas as petrol, is that not the same as injector time.
You have two sets of injectors, one per cylinder for petrol and a second one per cylinder for gas. The gas ECU doesn't replace the petrol ECU but piggybacks onto it, the car doesn't realise it isn't running on petrol. The petrol ECU sends a pulse of a predetermined duration to fire the petrol injector but this pulse is intercepted by the gas ECU which adds a fiddle factor to the duration and fires the gas injectors. So if at any given revs and load the petrol injector needs to be open for say, 5mS, at that same revs and load the gas injector needs to be open for 6mS, then it is the job of the gas ECU to add the additional 1mS. If the gas ECU has been programmed correctly, the lambda sensor reports to the petrol ECU that the mixture is correct and everything is happy. If however, the gas ECU is only adding 0.5mS then the lambda sensor reports that the mixture is lean so the petrol ECU will increase the petrol injector pulse to 5.5mS to get the mixture correct. For this reason you need to be looking at the petrol injector times when on petrol and gas to ensure that they are the same and the petrol ECU isn't having to make any corrections. What the gas injector times are is pretty much irrelevant. If they are much higher than the petrol times (meaning the gas ECU is adding a huge fiddle factor) then that is down to gas pressure or nozzle size rather than programming.
Sounds to me like your installer is fiddling in the dark rather than actually looking at the data and working out what the problem is. If the car is indeed running lean then the programming is so far out that the petrol ECU cannot adjust the pulse duration enough to get the mixture correct.
I don't really have a trip computer rather a mpg gauge
Same thing. You have two factors that affect how much fuel the engine is using. Fuel pressure, which is fixed and injector pulse duration. The mpg meter knows what the petrol pressure is so by looking at the injector pulse duration can work out how much petrol is being used and display an mpg figure. If the gas ECU programming means the fiddle factor is wrong then the petrol ECU will adjust the pulse duration which will make your mpg meter show a different mpg figure between fuels.