Changed Diff/MT Trans Fluid to Motul 300 and super low MPG
 

Just wondering if anyone else had effects such as this... I changed out both my MT fluid and differential to Motul 300, and went from 29 MPG to 23... Is this just due to a break in period of the new oil? Or just characteristic of this particular oil? Thanks!

I saw no change at all in mine. 30 MPG solid.

It's not the oil causing a 20% reduction in your gas mileage

Tire pressure, driving habits, bad fuel, different weather can all change your MPG, but a bit of oil certainly wont! :P

Just curious, I just checked tire pressures, I did bump up to 8 inch wide wheels and 255-40 tires... But doubt that's going to do that either... And I actually have been driving more conservatively

Still running Primacy HPs or something else?
If you're running something else, you've found your problem. The Primacy HPs are low rolling resistance tires.
'Usually' going to a thinner MTF fluid will increase fuel economy.

Heavier wheel and tire combo can definitely affect MPG...and a non LRR tire too.

I think this bit of info should have really been in your first post - the gear and differential oil is not really causing your decreased fuel mileage... LOL


That's like someone saying they switched from Exxon to Shell fuel and noticed a decrease in the fuel mileage...then later posting "Oh yeah I also installed a turbocharger system with higher capacity/flow injectors at the same time...." LOL

Except looking back, I did my wheels and tires first, and hadn't noticed a difference but had been driving highway only... then did the fluids change and have been driving local+highway... only noticing it greatly now... I am currently on 17x8 Enkei RP03 wrapped in 245**-40-17 Dunlop ZIIs... not LRR, but lighter combo than oem setup.

Well, there's your explanation. You're comparing apples to oranges. City and highway driving, low-rolling resistance vs high grip tires. :thumbsup:

It's the tires. Going from the Michelin Primacys to Bridgestone RE11As brought me down to about 23-24 as well.

Your setup is over 3lbm heavier a corner than stock...with the tires ~6lbm heavier than the stock ones and in the highest performance level for street tire categories (Extreme Performance - which is designed for track use first, street use second.) Physics is physics... I'd easily wager the MOI is higher with that setup than stock, so it will take more energy to accelerate and stop than with the stock setup.

You added unsprung weight and increased your rolling resistance.

Result is too be expected. As least you can sleep well knowing nothing is wrong.

I noticed there are difference between different brand of gas. I am getting much better fuel economy with Shell V powaaaah.

Tires and wheels over stock will make a HUGE difference. Therein lies your issue.

-mike