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Old 08-02-2020, 02:07 AM   #1 (permalink)
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//// Will 2" lift hurt aero ? ////

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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Old 08-02-2020, 04:13 AM   #2 (permalink)
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Quote:
Originally Posted by Andyinchville1 View Post

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 ?

Hello!

I am still quite new to the world of aerodynamics, but to me it seems that a 2 inch lift (that's 5 cm!) will affect your MPG noticable. Think of the extra air that can get under the car, where it's a mess without adjustments. A good undertray will probably help you a lot.
But also don't forget the extra air resistance against the tires. The frontal area is enlarged and that's never good for MPG. For this you can make an airdam or deflectors for those wheels.

But but does the car really go up 2 inches because of new tires? 2 inch higher tires will only raise the car 1 inch.
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Old 08-02-2020, 12:25 PM   #3 (permalink)
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When I swapped from 205 50 R16 to 205 65 r 16 on a car I used to have (7th generation celica), mpg shot up but that was due to tire choice not size. I later swapped on 195 65 r15 and mpg basically stayed the same. Michelin energy saver tires both latter sizes. I had to remove these thin plastic inner wheel wells to fit the larger tires. I also had more aero wheels the second time. Those seemed to cancel out the better gearing.
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Old 08-02-2020, 12:36 PM   #4 (permalink)
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My short list from dozens of tire experiences on cars and trucks has led me to the following priorities. First select your tire tead and type that fits your application, eg LRR Highway tires unless you have a need for something else. Then pick the best brand in that type. Then pick the most aerodynamic factory alloy wheel available. Then and only then select your tire size that fits those wheels going as tall is you can but as narrow as possible. From my experiences wider and taller does not help mpg but just taller does. If you are willing to fabricate custom wheel spats perhaps the width would not matter, but without going to those measures it certainly does.

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Old 08-02-2020, 09:21 PM   #5 (permalink)
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Going a little taller might not be as bad as going for wider tyres. A friend of mine recently wanted to raise the ground clearance of his truck through changing the stock tyres, but he got even more convinced toward such change once I explained him it would also have the effect of a taller gear ratio and the fuel-economy benefits on road. Width of the tyres was the same.
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Old 08-05-2020, 10:50 AM   #6 (permalink)
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tire width

Quote:
Originally Posted by Andyinchville1 View Post
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
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.
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Old 08-05-2020, 03:58 PM   #7 (permalink)
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Hi all,

I took the plunge today and ordered 4 new p205 75 r15
Tires (hankook kinergy st h 735).

I was debating about going with the even larger P 215 75 R15 Tire but decided against that for fear of rubbing...

I figured once these tires are on I'll check it out to see how much clearance I actually have and maybe next go-around I'll try the bigger tires.

I'll try to keep everybody up to date on my findings.

I know it's not optimal to test out miles per gallon with new tires until they break in but I'm also having a exhaust system put in and some other work so hopefully I can get a tank or two in before the changes.

I didn't want to wait too long to get some of the other work done because the same mechanic that's doing some of the upgrade work is doing some maintenance that needs to be done like a ripped inner CV joint boot ... I don't want a relatively new CV joint going bad because I drove around a lot on a ripped boot.

Thanks for all the previous input and advice

Andrew
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Old 08-05-2020, 04:21 PM   #8 (permalink)
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tire break in

Quote:
Originally Posted by Andyinchville1 View Post
Hi all,

I took the plunge today and ordered 4 new p205 75 r15
Tires (hankook kinergy st h 735).

I was debating about going with the even larger P 215 75 R15 Tire but decided against that for fear of rubbing...

I figured once these tires are on I'll check it out to see how much clearance I actually have and maybe next go-around I'll try the bigger tires.

I'll try to keep everybody up to date on my findings.

I know it's not optimal to test out miles per gallon with new tires until they break in but I'm also having a exhaust system put in and some other work so hopefully I can get a tank or two in before the changes.

I didn't want to wait too long to get some of the other work done because the same mechanic that's doing some of the upgrade work is doing some maintenance that needs to be done like a ripped inner CV joint boot ... I don't want a relatively new CV joint going bad because I drove around a lot on a ripped boot.

Thanks for all the previous input and advice

Andrew
The experts say that, it'll will be after the first 4,000-miles that you'll see the rolling resistance at minimum ( ignoring tire wear late in their life).
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Old 08-05-2020, 04:31 PM   #9 (permalink)
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Back when I was driving an '88 Ford Escort Pony as my daily driver I've ran tires as large as 185/70/14 on it. Stock tire size was 165/80/13. After figuring the odometer correction for the larger size tires I did see an increase in mileage. I don't recall exactly how much increase it was but, it was a noticable amount. At least a couple extra MPG I'm pretty sure. At the time I lived close to a self serve junkyard (Pull-A-Part) and would often find used tires of a larger variety already mounted on wheels that were mounted, balanced and ready to bolt on. I could often buy steel wheels with tires that were still good for 25-30K miles for under $20 each. Awhile back I decided I wanted an extra steel wheel for my Versa to use as a full size spare and went to Pull-A-Part and found one with a tire that's good enough to use as a spare for under $13 including tax. The tire probably has close to 50% tread left on it. They only charged me for the wheel and gave me the tire. Newer model Nissan wheels are hard to find in this area and other local junkyards were wanting $50. for a wheel alone.
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Old 08-05-2020, 05:36 PM   #10 (permalink)
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The Cd may be the same but you just added 2" by 5 feet of frontal aera, so the CdA goes up. Almost one more sq foot. Like a Pizza Hut sign on the roof.

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