What mpg do you get at 70 mph cruise on

I just ran up the motorway , flat ish, zeroed mpg at 70 on gas got 23.5 mpg instantainiois result. I get 24.7 on petrol at the same speed. What are yours like.

Mine is a 96 4.6 with a bigas gas conversion. Fresh engine rebuild but on 124k.

My average on a 14 mile round trip is 17 mpg gas/petrol.

Very interested to see what new petrol 4.4 rangies do if any have gas/petrol.

At 70mph both of ours sit in around the 18 /19 area. Computer shows just over 20.

Right, most computers must be slightly out then. Mine reads ..23.5mpg thought it was high.

cljcars said:

I just ran up the motorway , flat ish, zeroed mpg at 70 on gas got 23.5 mpg instantainiois result. I get 24.7 on petrol at the same speed. What are yours like.

Mine is a 96 4.6 with a bigas gas conversion. Fresh engine rebuild but on 124k.

My average on a 14 mile round trip is 17 mpg gas/petrol.

Very interested to see what new petrol 4.4 rangies do if any have gas/petrol.

Click to expand...

How does the trip computor know how much gas you are using is it psychic?

Explain psychic? What am I missing?. Surely they ain't far off (or meant to be resonably accurate).

The Wammer is asking how the system knows how much gas (LPG) you are using when it isn't plumbed into that system, it will only measure petrol/diesel usage.

Also the P38, isn't fitted with an instantainous MPG display (that I know of)....it averages the MPG based on fuel used and the mileage run since last reset.

Not sure about the science behind how the RR calculates MPG but lets assume it knows how far you have driven and also how much petrol you have used. Divide one into the other and bingo - mgp. With lpg I guess the ecu does not know how much lpg you are using thus any MPG calculation will be skewed. I don't have lpg so don't know much about it.

not enough

Now I remember from years back that mpg calcs on fuel injected motors where made using fuel used in injectors, and miles travelled (why it needs time to calc). Now if the injector timing Tinj petrol stays the same when you are on gas, ie you have the gas system tuned correctly so the Tinj gas gives the same combustion products as a petrol (read by the lambdas) then the Tinj petrol timing shouldn't be out by that much, and therfore the mpg reading shouldn't in theory be that far out. So therefore in theory the extra little bit you open the throttle should be accounted for.

Thoughts?

cljcars said:

Now I remember from years back that mpg calcs on fuel injected motors where made using fuel used in injectors, and miles travelled (why it needs time to calc). Now if the injector timing Tinj petrol stays the same when you are on gas, ie you have the gas system tuned correctly so the Tinj gas gives the same combustion products as a petrol (read by the lambdas) then the Tinj petrol timing shouldn't be out by that much, and therfore the mpg reading shouldn't in theory be that far out. So therefore in theory the extra little bit you open the throttle should be accounted for.

Thoughts?

Click to expand...

Trip computor works as far as i know, on fuel quantity being used at any particular throttle setting set against miles covered. As when on gas there is no recording of fuel being imput, computor can only use last known petrol usage for it's calculations. So basically it is guessing. That's what i think. Unless anyone knows different.

Totally agree with how the fuel input is calculated, then used to get mpg. Lpg systems run independantly of engine ecu's, in fact they piggy back them and sense the signals the petrol ecu send to the petrol injetors to control the lpg injectors. So the ecu never stops calculating the mpg and averaging it (in the p38's case). So the lpg mpg is still depending on the petrol ecu for its figures. So what happens is you stick you foot down a wee bit to compensate for the lpg's lower energy content during combustion, and the ecu compensates and increases the Tinj petrol timing to get more oomph. (then need to consider gas volume delivered to the cylinder).

My 4.2 doesn't have any mileage computer but me. After doing some rather difficult and intricate mathematical calculations my computer says I get 18 mpg going that speed not stopping for anything.

Starting and starting changes it to 16.5 mpg.

Colonial gallons? Add 20% for Queen's gallons...

70mph for a longish motorway run !?!?

Hang on I'll ask

Ere Ratty, what mpg would you get at a steady 70 with my P38 on the back?

Nice, had mine since august, 50% of that time I've been soaked on the bike going to work, whilst fixing it.

28mpg at 70mph on my diesel

70 mph....whats that ?

I don't know wot mine does I fill it up computer says 36 then slowly drops down to 12 so I just put the stuff in when gauge say 1/4 it's a 4.6

Hi clj, thank god someone gets about the same as me!! Steady mway cruising about 24mpg petrol/22 gas. These were full to full measurements, not the computer, although the computer only gave me 1.5 mpg more on petrol, didn't ask it on gas cos I've got a single point system which only asks for engine rpm and load, then stuffs gas into the plenum chamber. If you floor it going up a hill and you're in a high gear you get gassed before kickdown!

Nice one Dave, good to know its ok hey. Sounds the same as mine.