Taller rear tires

[no_one_of_consequence]

I have a few questions with this mod, but anyways it’s done.

I changed the rear tires with taller ones. The swap was: Michelin Hydroedge 205 65 R16 T94 instead of Michelin Pilot HX MXM 205 55 ZR16.

I needed to replace one tire but the cost of the Pilot was TWICE as much as the Hydroedge. Then I remembered that gdr703 suggested that "if the car is encouraged to sit with a nose down tail up attitude, by adjusting weight distribution, or suspension, this will help reduce the underbody drag". So I went for the tallest tires available for the width (205) and rim (16) classes.

I still have to find out if this tire fits in the forward position, but if this configuration yields better MPG then I will keep the 205 55 R16 size in the front.

Another question is the rotation. The best I can figure out is to dismount-mount (because the hydroedge is directional) and rotate left to right both sets (front and rear). The alternative is no rotation.

The diameter of the Pilots are 642 mm and the Hydroedge are 668 mm the difference is 1.026 inches. This means 19 rotations less per kilometer. Now this will influence the speedo or the trip meters? My guess is no considering that the taller tires are not at the front.

Any thoughts?

 

[Chuck_Norris]

I also am interested in this swap, It is time for me to buy tires too, if the 205-65-will ride well all the way around I'd consider that.

 

[no_one_of_consequence]

Apparently the taller rear tires have no influence on the tripmeter, here are some numbers.
GPS 253.5 miles
TRIP METER 251.7 miles
Scangauge 251 miles

Here is a preview on FE, the MPG has gone into steroids!
The scangauge reports MPG a little bit lower than actual.
Lets see how it fares at half tank.

 

[Fix_Until_Broke]

NOOC - Yes, as you found out the rear tires do not effect speedometer or odometer readings. Interesting about changing only the rear tires. Looks like you're on track to be in the 70's if you maintain your driving style.

I may look into what's available in 15" tires when I need another set.

 

[GoFaster]

couple things to think about here.

Tire rotation - Swapping left to right will probably have absolutely no effect. If the rear wheel alignment is good then the left and right wheels should wear equally (give or take something for differences between making left turns versus right turns).

ABS / ESP function - If there is a significant difference in wheel diameters, it can interfere with ABS and/or ESP because it will affect the wheel rotation speeds at a given road speed. If the warning lamp stays off, it's probably okay, though. There is some room in the system to allow some variation between wheels.

And don't try this on an all-wheel-drive vehicle ...

 

[Brock_from_WI]

I was going to say if you have ESP that could well effect how it responds. But as GoFaster said if you don't have a light it might be within allowances for the system.

A good friend of mine works for a company that makes ABS systems for the big auto makers. He said miss-matched tires will really confuse an ABS system because it expects all 4 tires to respond the same way. So even if the diameter is the same if you have two different sets it he said would affect this. I had never believed this until after I talked to him, now I always keep 4 matched tires on the cars.

I am not saying don’t do it, just passing along what I have been told. After having kids that whole safety thing actually means something now.

 

[no_one_of_consequence]

Brock_from_WI and GoFaster I appreciate very much your concern. So far I have no warning lights and “normal” braking performance is as usual. I will go to the raceway and do some braking tests, probably by the end of the month. I agree with you, ABS or any other safety device should be in working order whether you use it or not.
What appears clear to me is that pumping up all tires to the same high PSI is no good, front tires should have a lower pressure, so they sit lower. Instant FE when P&G was about 1.1-0.8 L/100km with 205/55 @50 PSI whereas now (205/55@48PSI & 205/65 @39PSI) is from 1.0 to 0.6 depending on speed; therefore overall rolling resistance is proportionally related to FE (L/100km) at P&G. When I pumped all tires to 50 PSI, fellow drivers traveling in the opposite direction often complained about high beams, even with no load in the trunk. This means that my car was facing a bit upwards and probably had a higher rolling resistance!!

 

[timeline]

What would the recommended max tire height tire size for best milage be for a new beetle?

 

[BioBob]

New Rules?

Have the rules been set for the 70 mpg sticker?

Having just completed a 1000 mile tank (on I95), I'm
not ready to go for a 1200 miler anytime soon. I would,
however like to try to achieve 70 mpg.

I have seen Ernie's proposal, and Fix's counter proposal,
but have not seen a resolution (unless I missed it somewhere).

Is it 5 gallons or 10? Is it 400 miles or 700? Or is the 5
gallon standard for the 80 mpg level?

I'd like to try for the sticker (when Fix get's them printed)
but what are the rules?

I'm sure there are others interested in achieving this level, where is the carrot?

BioBob

PS: I'd be willing to chip in for the cost of the stickers Fix.

 

[straightliner]

You guys got it all wrong!!!

The taller rear tires increase mpg because the car is actually always rolling down hill!!!!!!

LOL,
Chris

 

[Fix_Until_Broke]

BioBob

I don't know that there are any formal "rules" or anything - Ernie and I were just discussing different criteria.

I'd like to see 10+ gallons just to minimize measurement error and add credibility to the achievement. This does take longer and a more dedicated person to make it happen. This is hard enough as it is

I think any well documented run where you fill up at the same location (station/pump/nozzle/direction/etc) for the tank before, the tank evaluated and the tank after etc would suffice.

Thanks for the offer on the stickers - Once I get off my bum and get them designed/ordered I'll see about soliciting some re-embursement if the costs get excesive. Don't worry about it for now.

 

[no_one_of_consequence]

Well, here is my evaluation of taller tires using my records of the last ten months. I selected full tanks, those with used more than 13.7 gallons. There were nine records of each before the swap and after the swap of taller tires. I present the data and a statistical analysis that is interpreted as "it is": the MPG after swapping to taller tires was higher with a 95% confidence. The MPG means were 65.85 for before and 68.89 for after.
about 4.6% improvement.

before after
62.4822 69.4879
59.1640 62.5828
63.9380 74.3065
67.2034 62.6642
67.0361 70.2849
67.0931 70.4095
68.9476 69.4404
69.0032 69.7093
67.6444 71.1500


Two-sample T for before vs after
______N_____Mean__StDev __SE Mean
before 9_____65.83__3.30 ____1.1
after__9_____68.89__3.85____ 1.3

Difference = mu before - mu after
Estimate for difference: -3.06
95% upper bound for difference: -0.09
T-Test of difference = 0 (vs <): T-Value = -1.81 P-Value = 0.045 DF = 15

 

[Fix_Until_Broke]

NooC - Would you be willing to put the shorter tires back on and see if the mileage goes down again?

I'm not saying that the above data is not true, There's just so many variables that could contribute.

Either way - excellent work and a nice bump in mileage. You're consistently putting down some very good numbers - way to go

 

[bakdoor]

This is definitely an interesting concept. As a long-time "4WD" guy, you would never consider putting different sized tires on your rig (my Jeep would go into convulsions if I did this.) But with the TDI, and the rear wheels having no influence on the drivetrain, I can't see any negative effects (except for the aforementioned possible issue with the ABS.)

One thought to consider about tire rotation: if your tires are radials (and what tires for passenger cars today aren't?) --they have an internal carcass that wears in when the tire spins in one direction for the first tire rotation interval. Now, if you rotate the tires from R-L, L-R, you're causing that same tire to rotate in the opposite direction from the way it did when it was "broken in," and it will wear out faster. Normally, you want to rotate radials from front-back, back-front only (but you obviously can't do that with different sized tires.)

Good luck with the rest of your mileage experiment, I'm intrigued.

 

[HopefulFred]

With a directional tire like the Hydroedge the OP bought, he'll have to flip the tire on the wheel - there's no changing direction. It's jsut s lot of work for a tire rotation... blah. I've been thinking about doing it on my Exalto's because I tweaked the alignment and shouldn't have. Now that I've fixed it, I'm looking for ways to get them back smooth.

 

[SmudgePot]

I don't understand the concept of taller rear tires improving fuel economy. TDIs are front wheel drive so changing rear tires cannot have any effect on gearing. ??????

 

[whitedog]

Taller rear tires, will lift the rear of the car, thereby providing a different, possibly better aerodynamic profile.

Or, we could be seeing a placebo.

The reason I wonder about the placebo is that NOOC is already trying for very high mileage, so putting on the taller rears to test this would mean that possibly the mind is set to get even better mileage.

I hope that I'm wrong and feel free to correct me.

 

[nchapman]

I don't understand the concept of taller rear tires improving fuel economy. TDIs are front wheel drive so changing rear tires cannot have any effect on gearing. ??????

x2
What is the logic behind this?

 

[DPM]

I'd guess aerodynamics as WD reckons, plus a taller (and narrower if possible) tyre will have a lower RR. Slightly less bearing drag at the slower speed of rotation too?

 

[mrGutWrench]

I'd guess aerodynamics as WD reckons, plus a taller (and narrower if possible) tyre will have a lower RR. Slightly less bearing drag at the slower speed of rotation too?

__. The guy here in NC who altered his profile with "spring spacers" reports positive results, too. This wouldn't affect tire profile; it also wouldn't affect bearing drag. That leaves aero effects.

__. But the scientist in me is with Pup on the placebo issue. I'm not saying that people who report positive results are wrong or don't know what they're doing, but I'd like to see a wide range of test results.

 

[Joe_Meehan]

Taller rear tires, will lift the rear of the car, thereby providing a different, possibly better aerodynamic profile.

Very true. I wonder what would happen if they switched those tyres from the back to the front? That should be an interesting test of both the placebo effect and aerodynamics. Generally raising the front will increase drag.

 

[SmudgePot]

From what I understand about aerodynamics, which admittedly isn't much. When a car goes down the road the air passes over it and creates a drag/vacuum at the rear of the car. Basically the same as the drag/vacuum created by water passing over a dam traps a log, a car is trapped by similar drag/vacuum and cannot escape without some increase in energy. I also think the increase in fuel economy is probably a placebo effect.

 

[whitedog]

I hope that No One can give us some more convincing that it's not a placebo effect. My mind isn't made up, so talk to me and show me it's not.

 

[Berniem]

I agree with the spacer idea. You can keep the same tires, not worry about rotation & ABS issues, & have better aerodynamics if that helps. Does anyone know of other trials with these issues? I would like to know what NOOC has done to improve his mileage. I'm game to improve my 53mpg's. Bernie

 

[40X40]

Air also passes UNDER the car. If the car is higher in front it is trying to compress the air and ride over the 'wedge' of air. (more drag)
If the car is lower in front it creates a low pressure under the car instead of a high pressure one. (less drag)

Speed is critical regarding aero drag.... Doubling the speed quadruples the drag.
(right? corrections please!)

At the speeds we travel and the aero designs of our cars, I am in the placebo camp, with the understanding that someone here will actually make it work for them.

Bill

 

[Fix_Until_Broke]

Speed is critical regarding aero drag.... Doubling the speed quadruples the drag.
(right? corrections please!).....

Drag goes up at the square of velocity. Doubling is true at the right numbers, but only there.


I don't know much about aerodynamics either, but, I do know that the majority of the gains are to be had at the trailing edge of whatever is moving through the fluid. For example imagine a triangle moving through the air (left to right) |>. It has the least aerodynamic drag with the blunt edge leading. I don't have the foggiest idea if the nose down/tail up attitude will help or not.

 

[whitedog]

Don't know much about history,
Don't know much biology,
Don't know much about science book,
Don't know much about the French I took.

(I'm leaving the rest blank so no one gets the wrong idea...)

 

[no_one_of_consequence]

??

A double blind experiment is the only way to completely remove a placebo effect. It will be required that the driver (preferably drivers) would not know what is the treatment that is being applied for a given run (stock or taller tires, in this particular case). Also, not knowing the nature of the treatment is important. Each driver should test-drive each of both treatments at least once. All test runs should be randomised, at the worst case this implies taking off a set of wheels-tires and replace them for another set wheels-tires every other run. Both sets of wheels should be similar. 20 runs of each would be enough. Venting the car after at the end of each run would be cumbersome, but required. Weighing the car with a scale is more adequate but its availability complicates the location of the experiment. As you see, a proper experiment requires some money, the involvement of several subjects and some teamwork as well. More importantly an independent party should test such a study.
I wish I could present some better evidence. The list of factors confounded and the probable sources of error are endless. This is a common problem with every modification that is reported in this forum: additives, oils, chips, RR tires, you name it. That’s the reason why I reported my evaluation as "it is". Pulling together the data of some other drivers who choose to test the idea could be an improvement to what I did, but it would not remove the placebo effect because these drivers would be aware of the modification.
I could go back to stock tires and try as hard as I could to get high MPG for say 10 tankfulls, then test this new data set against the early days nine tanks with stock tires that I presented. If both sets are equal it would demonstrate that with stock tires 65ish is as good as it gets, or someone could finger that different climate, engine mileage, tread wear, or mind set were different. I could not argue that conditions were the same.
Going to the top a hill and allow the car to roll down and measure the distance traveled could give an indication of the benefit. In doing so the mind set of the driver has little influence but the distance traveled is very small and the initial speed would not be representative of the range of speed were an aerodynamic mod would be more effective.
More ideas??

 

[mrGutWrench]

(snip) More ideas??

__. Yeah, don't think too much. It gives you indigestion and makes your head hurt. Just keep an eye on your MPG and report it. I'm sure that more of us will give it a try, too. If there's a trend, it will show up.

 

[whitedog]

I could go back to stock tires and try as hard as I could to get high MPG for say 10 tankfulls, then test this new data set against the early days nine tanks with stock tires that I presented.

Right. I'm doing the same thing with a guy from work. I am trying to avoid the placebo effect of B20 Vs D2, but in my analytical mind, I will never be happy with my own results. (I wonder if the XXX had much effect? Was this year colder than last year?)

So, yes, many, many variables, but if you are trying hard to drive the same and avoid the placebo effect, all is well.

I did give up on my experiment for the Winter knowing that my mileage would dive with more cold driving and Winterized fuel, but if you continue to see improved numbers, that says a whole lot.

Good luck with your tests.