LRR Tires and Speed vs Effect on Fuel Economy and Overall Cost - What do I Buy?

[plus 3 golfer]

There are a lot of threads on tires and what effect do LRR tires have on FE.   Recently, CR published a short list of tires (see attachment) from their testing  to determine the Rolling Resistance force of the tire in pounds on a dynamometer.  CR then, based on the dynamometer results, rates rolling resistance of tires in 5 categories: poor, fair, good, very good, and excellent.  However, we don't know the force in pounds which define the categories but can deduce what they might be from the published list and rating categories.  This data allows one to compare FE effects of RR, all other things being the same.

 

Second, we know the Road Load Horsepower coefficients that Ford determined for the C-Max to use in EPA required emissions testing. I assume Ford used the Michelin Energy Saver tires which are the OE tire.  The equation for RLHP is shown in the chart below.  One can research how RLHP is determined.  I took the Ford RLHP coefficients determined by Ford and produced the attached chart for various speeds.  The blue bars are the RR component of RLHP.  For example, at a speed of 30 mph, RR accounts for about 50 % of the power required to maintain 30 mph and at 75 mph RR accounts for only 16 % of the power to maintain 75 mph.  So, if the ES tire has 20% lower RR than another tire, the FE effect if one switched to the other tire would be about a 10 % decrease in FE because RLHP has increased 10% (assume C-Max FE is a linear for a +- 10 % change in RLHP).  Now at 75 mph, the same non-ES tire will result in about a 3.2 % (20% X 16%) lower FE than the E/S.

 

So, when looking to buy tires, one might want to take into account how RR differences affect fuel economy at different speeds.

 

Lets, take an example: my General Altimax RT43 (9.4 pounds) and BFG Advantage T/A Sport (10.6 pounds) which are both on the CR list.  The Altimax are 1.2 pounds (about 11 % lower) in RR force than the Advantage.  At an average speed of say 50 mph, I should expect about 3 % better FE with the Altimax than the Advantage.  CR rates the Altimax RR as Very Good and the Advantage as Good.

 

Let's next assume the Michelin E/S has an 8.4 rating (not tested by CR recently but in older tests of RR, the E/S was rated excellent).  So, going through the same calculations as above the E/S RR is about 11% lower than the Altimax.  So, at 50 mph, I might expect to lose about 3% in FE with the Altimax.  This is right in the ball park of my actual FE hit with the Altimax vs the E/S.  The E/S RR would be 21% better than the Advantage or the Advantage will lose about 6% in FE vs the E/S at 50 mph.

 

So, what tire to buy based on overall cost?  I estimate that 70 % of my miles are at 65+ mph.  When I go back East (over 4000 miles RT) my average moving speed is above 68 mph.  Most around town trip miles are on freeways - 65+.  So, at 65 mph, my Altimax tires take about a 2% hit in FE vs the E/S.  So, the E/S being a lot costlier than the Altimax will never pay for its additional cost over the Altimax with fuel savings.  Being extremely optimistic with a 4% hit, 70 k miles of life of both tires, gas at $3 a gallon and 38 mpg with the E/S, I would optimistically generate $230 in savings with the E/S (which is below the initial cost difference).  A good base case: a 3% hit, 55k miles life, $2.75 gas and 37 mpg, my savings would be $123.  It's not intuitive but the higher ones overall FE, the lower the FE savings of LRR tires.   If I change the 37 mpg to 50 mpg, my savings with the E/S is only $94.  In order to match the optimistic $230 if I got 50 mpg in the E/S, my FE with the Altimax has to be 7% lower than the E/S.  

 

In summary, to achieve maximum savings benefit of higher cost LRR tires, one has to drive fairly slow.  The best LRR tires would likely be ideal with respect to savings for slow, long commutes but not cost effective for higher speed driving.  :)   

 

[C-MaxA2]

Thanks for the analysis.  At this point I'll stick with the Michelins since I've had exceptional reliability from them (on several cars).  You probably can't 'cost out' the no flats, no issues of tires.  I guess it's the old 'if it ain't broke, don't fix it' analysis. 

 

All your comments are very factual and we all learn a lot from what you say so thank you again for posting this kind of stuff.  I've learned so much from this forum and it makes me a lot more comfortable knowing what other people have found. 

 

I assume your analysis is based on all the tires at the same pressure.  Or is that another issue to be considered?

[plus 3 golfer]

My guess is that the CR tests use a standardized test pressure for all tires when measuring RR. There are standards for virtually everything. EPA likely requires Ford’s RLHP coefficients to be based on their recommended tire pressure. The point is that the CR data allows one to better compare different brands of tires RR and the RLHP data is specific to the C-Max. So, we have a good estimate of the likely % difference in FE of two different tires on the C-Max at ones average driving speed.not some FE test of tires on a Prius for example. The assumption is that all other things that can affect FE would be the same including tire pressure. I’m going to look at CR category ratings of the tires in the list compared to the RR pounds and see if I can estimate the category ranges.

 

There are many variables that can affect the % distribution of RLHP in the real world like road surface, environmental conditions, driving techniques and so forth. But these would generally just offset the RR percent of RLHP. For example, assume rough road surface increases RR force by 10%. So, total RLHP goes up as does the RR component of RLHP. The % RR RLHP would increase, FE would go down and so forth. It would be very easy to run sensitivity analysis on the effect of rough roads, hypermiling changes in FE and RLHP.

 

Also, I buy tires first based on handling, second on mileage ratings, and lastly on overall cost. Others may put overall cost first. I replace tires well before the tread is down to the wear bars. So, my choice of tires will likely be different than others.

[plus 3 golfer]

I looked at CR tire ratings compared to the RR pound values in the list and found the following:

 

Excellent RR <= 8.2

8.8 <= Very Good RR <= 9.8

10.1 <= Good RR <= 10.9

11.1  <= Fair RR <= ?

? <= Poor RR

 

Attached for comparison are RR values of some of the most popular tires in 2007 from a NHTSA study published in 2009.  

 

 

I also want to point out an example of TireRack's testing of FE of tires.  Here is a link to a FE test which includes the Altimax, the Advantage, and the Pirelli P7 for which we have CR pound data and the Firestone Champion Fuel fighter which is rated Excellent by CR.  The tire which gets worse FE than expected based on CR tests compared to the Altimax and the Advantage is the P7.  

 

But, as TireRack concludes: 

 

"Larger differences in consumption between tires may indicate a difference that might be experienced on the road, while smaller differences should be considered equivalent. As they say, your mileage may vary." :) 

[ptjones]

What was the difference in MPG's between Michelin ES and  General Altimax RT43 on your car? My Daughter replaced the OEM Goodyear tire with Coopers at the Toyota Dealer and lost upwards of 10 mpg. She just replaced the Cooper tires with Michelin Defender T+H which seem to improved MPG's by a couple MPG's.

 

Paul

[plus 3 golfer]

Thanks for the analysis.  At this point I'll stick with the Michelins since I've had exceptional reliability from them (on several cars).  You probably can't 'cost out' the no flats, no issues of tires.  I guess it's the old 'if it ain't broke, don't fix it' analysis. 

 

All your comments are very factual and we all learn a lot from what you say so thank you again for posting this kind of stuff.  I've learned so much from this forum and it makes me a lot more comfortable knowing what other people have found. 

 

I assume your analysis is based on all the tires at the same pressure.  Or is that another issue to be considered?

I used to be a Michelin Man too. :)    But, tires have come a long way since my first set of Michelins in the early 1970 (Michelin Radial).  I really don't believe Michelins have any advantage with respect to "flats".  I get just as many screws in Michelins as any other tire.  The only blow out I had was with a Michelin tire in the early '80 but that's because I hit a large pot hole. I will say: about the only thing that hasn't changed with Michelin's is their high price. ;)

[plus 3 golfer]

 

What was the difference in MPG's between Michelin ES and  General Altimax RT43 on your car? My Daughter replaced the OEM Goodyear tire with Coopers at the Toyota Dealer and lost upwards of 10 mpg. She just replaced the Cooper tires with Michelin Defender T+H which seem to improved MPG's by a couple MPG's.

 

Paul

 

It's hard to tell because I had Ecopias in between and I paid more attention to achieving higher FE early on with the C-Max than I do now.  I also don't live in the same area now as I did with the Michelins.  So, there's no comparative, repetitive trips. But I'd say around a 3 % difference overall. 

 

If one does research on Cooper tires (which I did when I bought the Altimax), most Coopers perform poorly in FE. Look at the Cooper CS5 in the CR list, their RR is 11.2 pounds. So, if one would drive mostly city at slow speeds, I could see I don't believe the Defender's strong point is FE either but better than the Coopers.  You can't beat the E/S for FE. 

 

What Goodyear tires did she have on the car?  The Goodyear Assurance Fuel Max is rated excellent by CR in FE.  So, there would likely be a very large FE hit replacing worn Fuel Max with New Cooper tires.

[ptjones]

I think you are right.

 

Paul

[fbov]

There are two tests with the P7... both with similar overall results for that tire, so no change in design. I would conclude that the Firestone Champion Fuel Fighter is very low RR, and that's about it given the variability.  

 

8/12/16 test

% vs. Most Efficient

BFGoodrich Advantage T/A Sport  -4.5%

Firestone Champion Fuel Fighter  --

General AltiMAX RT43  -2.5%

Pirelli Cinturato P7 All Season Plus  -3.8%

and 

8/29/2017 test

% vs. Most Efficient

Goodyear Assurance WeatherReady  -1.0%

Michelin CrossClimate+  -0.7%

Pirelli Cinturato P7 All Season Plus  -0.7%

Vredestein Quatrac 5  --

 

Plus 3, how did you like the Ecopias? I looked at a 2020 Escape and it had Ecopias, so I'm wondering what you thought. 

 

Have fun,

Frank

Edited October 23, 2019 by fbov

[plus 3 golfer]

I won’t buy the Ecopias again. Positive: ride comfort and very quiet. Negative: wet handling like the E/S. The Ecopias absorbed bumps very well as the sidewalls seemed to flex a good bit. It seemed I had to corner slower as the tires might start to slide. Might be more slip than The E/S and AltiMax. Rear tire cupping / feathering of Ecopias was more severe than the E/S as tire noise at 75 mph was unbearable at just under 42 k miles compared to just under 48 k with the E/S. FE was slightly less than E/S but still good.

 

When I bought the Ecopias EP422 Plus, the tire had just come out replacing the original EP422. I thought the “Plus” should be “better” than the non-Plus in its poorer rated categories. But, I guess not. Marketing wins again to boost sales. I guess you can’t design a tire to max. FE, great handling, and tread life in the same tire. Throw cost into the mix and what to buy.

[fbov]

... you can’t design a tire to max. FE, great handling, and tread life in the same tire. ...

Amen!

[ptjones]

I put Ecopia H-T 422 on my wife 2013 Explorer and they didn't get any better MPG's than the OEM tires, but cost a lot more. :sad:  If I get a new Escape NRG I will put Michelin ES on it if they have the right size.

 

Paul 

[fbov]

Then get the base model; ES/AS only fit in 17". No 18's are close and they don't list a 19" version. And remember this product line is over a decade old. I suspect that Michelin has engineered LRR into most of their product line. For example, I see no difference when running snow tires. 

MPG vs Temp Rural 2014 only.pdf

 

It looks like there's a -2 MPG shift with the snows, until you look for the green points (ES/AS) at low temperature, and they overlay. The full curve becomes non-linear at low temps because the vehicle occupant requires some heat at 15F ambient. The tires make no difference. 

 

I've posted before about Michelin's new CrossClimate product line. The wife's noticed no loss in mileage in her Sonata Hybrid, but she keeps no data. The non-Ecopia Escape OEM tire in the videos is the Michelin Premier LTX, which looks promising....

 

Wow, I found the full ratings at Tire Rack for "Crossover/SUV Touring All-season" tires. The CrossClimate, Premier and Ecopia placings are telling. 

https://www.tirerack.com/tires/surveyresults/surveydisplay.jsp?type=CSTAS

 

Note the Premier "life" correlates with "would recommend" at Tire Rack (both poor), and they note a new, extended-life version is marked "DT" on the sidewall. There's a similar situation with the CrossClimate (early Euro-only tire) and the CrossClimate + we got. It would appear Michelin is having issues achieving comparable life with the new compounds. 

 

Have fun,

Frank