Quote:
Originally Posted by Andyinchville1
Hi All,
I am adding larger tires to my MK 4 TDI Jetta wagon shortly. I mostly drive highway so the upgrade will cut RPM's at speed.
Factory size is 195 65R 15
I am currently running 205 70R 15 tires
I am looking to upgrade / upsize to 205 75R 15 OR if they'll fit 215 75R 15 tires
I am not modifying the car to accept the larger tires just adding the tires, so nothing extra to hang into the wind.
I have spoken with some folks on the other site I am a member of and they think raising my car with the larger diameter tires will kill aero and negate any effective gearing advantages. (NOTE : interestingly the steel wheels with the factory sized tires are the same weight my new Aluminum wheels with the current tires .... I'll probably pick up 2 to 3 lbs weight per tire going up to the 75 profile tires).
I cannot see how raising my car 2 inches all around (no additional rake etc ) would negatively affect aero since the car was not scraping the ground to start with. What do you think ?
on the flip side , is it possible , that raising the car can actually help aero because there would be less effect from swirling air reflecting off the ground etc... I know its only 2" or so so maybe any changes would not even be measurable?
I figured I would ask here as well since there was a section specifically for it and I think this site is much harder core into mods for fuel efficiency which is great ! (yes, I know I NEED to make my kamm back and underbody tray .... I got some ideas but now I just need to do it!).
Thanks in advance for any and all input on this.
Andrew
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For a 'rubber gearing' experiment, I went to a 235mm section, with the same aspect ratio, from the original 205mm section, on the rear tires only, and registered my highest ever mpg, despite the aggravated frontal area.
30% of streamlining benefits can be lost without gear-matching, which far overshadows the extra width.
The change in body inclination ( higher boot ) no doubt affected the greenhouse airflow, but I've no way to parse it out, and isolate the effect.
If carmakers are smart, all future production will have active suspension, which can maintain a car's attitude, and we'll never have to consider this challenge.