Mechanical Problems & Technical Chat - Weird gas mileage and various issue

My question is:

How come when I fill up my gas tank at half mark and calculate MPG I get different result than if i were to run the tank to E mark?? Shouldn't the gas mileage be the same being gauge both ways? My brothers civic is like that too?

I calculated app. 31mpg on my last tank(normal driving) AC on...I have a 98 Civic HX cvt to EX mtx swapped with 166k miles. This is kinda low for a civic isn't it?? Especially with it being the D16Y5 Vtec-e engine.??? I do have a check engine light right now but thats because of the tranny swap. One thing during the swap... I forgot to bolt the O2 sensor onto the transmission bolt but i don't think it is grounded this way? Also I do not have any CEL reguarding the O2 so it cant be it. Before the swap though..I did get a CEL which indicates insufficient EGR flow, but have not seen it since the swap.

My oil is old (I think 5 months...) but it barely have any miles on it so i dont think it is the oil.

Another weird thing is sometimes when i accelerate it feels like the engine surges at certain rpm, which could vary from @ 3k rpm to 4k rpm. Somtimes i think it could be because the mounts are not all in???So the engine rocks.... HX requires different mount for side of tranny so i just have the front and back on right now.. But all other mounts are in good condition except for the front bottom driver side mount which needs changing but does it have to do with this surging??

How come when I fill up my gas tank at half mark and calculate MPG I get different result than if i were to run the tank to E mark?? Shouldn't the gas mileage be the same being gauge both ways? My brothers civic is like that too?

When you fill the tank has no discernible effect on gas mileage. Instead, the type of driving you does. You probably did more city driving or idling when the gas mileage lower.

I calculated app. 31mpg on my last tank(normal driving) AC on...I have a 98 Civic HX cvt to EX mtx swapped with 166k miles. This is kinda low for a civic isn't it??

31 mpg is probably okay mileage.

Especially with it being the D16Y5 Vtec-e engine.??? I do have a check engine light right now but thats because of the tranny swap. One thing during the swap... I forgot to bolt the O2 sensor onto the transmission bolt but i don't think it is grounded this way? Also I do not have any CEL reguarding the O2 so it cant be it.

I am not aware that the O2 sensor has a ground on the transmission.

Another weird thing is sometimes when i accelerate it feels like the engine surges at certain rpm, which could vary from @ 3k rpm to 4k rpm. Somtimes i think it could be because the mounts are not all in???So the engine rocks.... HX requires different mount for side of tranny so i just have the front and back on right now.. But all other mounts are in good condition except for the front bottom driver side mount which needs changing but does it have to do with this surging??

Is this the VTEC kicking in?


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Just to make sure, how did you calculate the gallons of fuel used. The midway point of the fuel gauge doesn't mean you used half of your fuel. I just want to make sure you didn't use that to calculate the MPG.