Hey guys I have a question for all of you with a M6, 3.73s, and 315 rear tires. How much power did you lose when going from the factory small rims and tires to a 17 with a 315? What about gas mileage? I'm not overly that concerned, but it's something that I've been wondering. I just got some 17in ZR1s with some Nitto 555r's in the rear and some guy was trying to tell me I'd cut my power and mileage by at least a third. The car is not a daily driver unless the weather is great, but I'd like to not have to fill up every time I go for a nice cruise. This is on a 1986 Iroc-Z that I'm dropping an ls6/t56 into. So for now I'm just pushing the tired sluggish 5.0 that's in there now. Any help will be appreciated, thanks in advance.

 

Um, really not very much difference at all. It shouldn't effect gas mileage at all, if it does you're looking at maybe 1-2%. Wider tires will give you better traction and therefore your car will actually be faster. Not sure wth the guy is talking about haha.

 

uhh a third? next time you see him, punch him in the ***** and call him an *******...

 

no need to do that, hes one of my bros customers so i cant do that. i never said i believed him, i was just wondering what kind of actual percentage would be lost. i am also aware that i'll have better traction, there'd be no point with buying wider and stickier tires if I wasn't seeking out traction right?

 

The main disadvantage to wider wheels/tires is the extra unsprung weight and larger rotational inertia. If the car is not traction limited, then it is not a performance upgrade. More mass/larger radius of gyration means more drivetrain loss.

The gas mileage will decrease due to the above and the larger coefficient of friction. A third is obviously a significant amount. If the original MPG is 21, 21/3=7; so the new mileage would be 14 according to that.

As far as measuring the MPG, to many factors come into play to be able to accurately tell a difference while filling up at the gas station. It's not easy to keep all of the other variables (temperature, throttle position, city/highway driving) constant.

I don't know any equations to calculate drivetrain loss with respect to drivetrain weight. Hopefully someone has done the dyno comparison with different wheels.

 

I'd say 1-2% loss in power and 1-2% loss in fuel mileage is a good ballpark estimate. No more than that. A 33% loss is simply idiotic.

 

I felt no loss in power and I'm still getting about 18-19 mpg in the city with no highway driving.....

 

shouldn't it stop costing you power once you get to speed. since the increased size only pertains to accelerating the mass...not keeping it at speed?

my physics is a bit rusty.

i saw no difference in mileage with my 315s...that was going from stock 245s.

try this for rwhp loss. cost a few hp and torque