Trying to work out the error in diastance

[StanTheMan]

OK I just courious how far out my speedo is & how econonmical my engine is as i've noticed a drop in fuel efficiency. I know in the last 2 1/5 years its only doing trips down the road to the shops which is not helping. but i used to get about 420km per tank. on the weekend i got 350km......and i wasn't pushing it.
It also needs a tune & I'm mixing fuels. I've always had better economy on 98Ron than the standard. But i'm just courious.

Now the standard diff for the NA6 is 4.3:1
My diff was upgraded to a 4.1:1

recently my wheels went to 205/45x16.
OEM is 185/60x14

The tyre sze calculator tells me the wheels alone make 2.3% slower or less distance travelled.

[JBT]

With those wheels and tyres you're actually going faster than before for the same indicated speed on the speedometer. Likewise, the diff ratio is higher which means more distance for the same engine rotations. So theoretically you're going about 5% further per engine revolution than you were with the standard setup.

You may have lost some around town economy because of the present higher gearing but it should be doing a bit better on the highway. My speedometer over reads by 5% (105 indicated = 100 true). If yours was the same it should now be indicating about right.

edit/ If your odometer was correct, the actual distance travelled now will be about 5% more than indicated.