Testing MPG with 17s vs. 15s

I would like to condense what information we know about the fuel efficiency effects of Plus sizing wheels on a GenIII into one thread for easy reference.

I know some of you have swapped back to 15s from 17s or 18s to perform similar tests. I hope to include your findings here here when you have completed your testing.

For my part. I have run my 2012 Three with solar roof for approx. 7,500 miles with the new Yokohama AVID Ascend 195/65/15 tires at approx. 44/42psi. A T-rated tire with max pressure of 51psi. My best tank calculated was 59mpg and my worst was 44mpg? During the winter/spring storms we had. I seems to average about 50mpg when I stop trying or 53-55mpg when I slow down and try. If I can avoid my commute uphill then I could easily pull 57-59mpg tanks.

This morning, I swapped my 15s for my old Centerline RPM 17x7 wheels. They weight approx. 15lbs and sport a 42mm offset. I currently have Michelin Primacy MXM4 LRR tires (215/45/17) with about 9,000 miles on them. Pressure is set at 44/42. A V-rated tire with Max pressure is 51psi. I only have one trip on these tires and it was slightly downhill from about 250ft. to 50ft. over about 55miles in 77F temps. I drove in the same manner I drive with my 15s and the result is curiously impressive. This trip resulted in 62.3mpg over 43.5miles. The tank total for the 55miles is at 59.8mpg.

Here is where it gets weird. The 17" tires are shorter than the 15" tires. Revs/mile for the 17" is 844. Revs per mile for the 15" tires is 828. I compared the tires side by side and the 17" is visibly shorter. See attached pics. Even more curious is the fact the speedometer kept its 2mph overread according to the ScangaugeII and was only about 1mph over the GPS reading (accuracy of 15'). The 17" tire should cause the. Speedo to increase its overread, not decrease it or remain the same. Stay tuned...

*UPDATE*

So I have pretty much concluded my testing and I have to a few conclusions. The 17" tire definitely decreases fuel efficiency over the 15" OE tire. That is something we already knew. I have observed an average of a 3mpg drop vs. my 15" AVID Ascend tires. This was about what I expected although it is a little more than what was reported by the UK test. I could see this a being a difference between testing protocol, tires and wheels. The OEM 17s are likely more fuel efficient than my Primacys. I decided to test a set of 15" Energy Saver A/S tires and see how those compared to my 15" AVID Ascends and the 17" MXM4s. I was astounded by how fuel efficient the Energy Savers were. They topped my Ascends by about 3mpg and bested my 17s by around 6-8mpg! Granted the handling and steering response was much worse as well. This new information also complicated my decision to continue running the 17' wheels. I could stomach a 3mpg loss but a 6-8mpg loss is almost too much to handle. If I drove less miles per year I could handle the loss but at closer to 30K a year the losses add up to quite a bit of money.

Summary:

MPG loss with OE or OE style 15" tires vs. LRR Primacy MXM4 17" tires = 2.5mpg - 3mpg
MPG loss with Energy Saver A/S tires vs. LRR Primacy MXM4 17" tires = 6mpg - 8mpg

Because the LRR 15" tires roll so much easier, hypermiling techniques can be enhanced to provide even better mpg. The drawback to the 15" tires is worse handling but in the case of the Energy Saver A/S you get a softer more pleasant ride.

• So for those of you who want better handling and a little bling but do not drive more than 15,000 miles a year I'd say go for the larger wheels but choose your wheels and tires carefully so as to minimize the mpg hit.
• For those of you who love to see high mpg numbers, want the lowest cost per mile and don't mind a mushy tire then get the Energy Saver A/S 195/65/15
• For those who want a better handling tire with a little more spirit but don't want to take the mpg hit of a 17" or 18" tire than check out the ProContact or PureContact with EcoPlus

To save everyone the effort, I know these wheels are not everyone's cup of tea but I used to like them and they are all I have for testing. Ooh and the car was dirty and the wheels have been in storage since last fall AND they are cell phone pics.

Would be nice if someone here could lend you their OEM 17".

So are you convinced now that your installer put on the 40%?

1) Good luck with your testing, it will be interesting to see your results.

2) Wash that Vette!

3) We were in your area yesterday, on our way up to my Aunt's memorial in El Dorado Hills. That last hill prior to the Latrobe exit off of hwy 50 (exit 30) killed my MPGs. UGH! In less than 2 miles, it sliced off 1 mpg off a 300 mile tank (I was holding my tank average at 68 mpg prior to that hill. It was 67 on the other side, and that was after the downhill run to the exit.

I understand why you hate that hill in your commute. I understand it even better after yesterday. (But I hate wind more)

32Kcolors, what do you think? It's pretty dark.

Codyroo, I hate that hill too but my daily commute is on I 80. It's the hill from Roseville to Auburn. Very similar but even higher I believe.

I'm confused. Are you expecting the relationship between SCII reported speed and speedometer speed to change? Aren't they both based on wheel revolutions? I would expect a change relative to GPS.

I was expecting a change. I noticed a change in my GenII. I did find it odd that the GPS went in the other direction. Very confusing. The GPS and Scangauge used to read 2mph slower than the speedo. Now the scangauge is 2mph slower and the GPS was barely 1 mph slower. It's weird.

*edit*

I re read your question. No, I did not expect the Scangauge and speedo to change their readings but I did expect them to change in relation to the GPS reading.

Thanx for your thread, I will be piggy-backing w/ my data soon when I switch to my 17's as well. BTW, I would of loved your rims back in the day, especially on a black car as chrome or anything shiny tends to stand out more.

What does shorter mean ? If you mean less vertical height, perhaps it changes when mounted and inflated ?

I though 15 and 17 inches are diameter measurements. Incorrect ? I do not understand how a larger diameter circle would have more revs/mile.

The rim diameter is the tire's inner diameter. He's looking at the outer diameters.

Thanks! Although I doubt our numbers are going to differ much after the results of this morning's commute.

The 15" tire was a taller sidewall thus making the overall tire outside diameter larger. This is what effects revs/mile. The smaller the outside diameter the more revs/mile. If you look at the first picture in the first post you will see the 17" tire on the left is noticeably shorter than the 15" tire on the right. Contrast this the with same 17" tire on the left and a 185/65/15 Yokohama dB Super E-Spec tire on the right. See attached picture.

This morning's commute is telling. Yesterday's fuel economy was great but it wasn't part of my usual daily commute that I track rigorously. I make that same trip quite often, however, and I see anywhere from low 60s to low 70s depending on how I drive and the environmental conditions. Since I cannot remember any hard numbers for that trip with the 15s I resisted making any assumptions other than 62mpg was still pretty good.

My morning commute is less variable and I have been tracking it everyday since the purchase of my 2012 thanks to the trip summary screen that comes on when you turn the vehicle off. I have been averaging 49-50mpg during this part of my commute using the 15" tires with temps in the low 50s to high 50s. I drive at approx. 60mph for most of the 47 miles since all but 1 mile of it is freeway. With the 17s on I noticed I could not maintain speed on the down side of overpasses where I normally could with the 15s. Instead, I found that once traveling on the downside, I would enter a glide and start losing speed. Obviously the 17" tires do not roll as easily. The result of this morning's commute was 46.5mpg.

If I get lucky and do not have to make any other stops on this evening's commute I will do my best to match my record of 81mpg over the same 47miles but downhill. Even averaging 70mpg will be an accomplishment and close to my normal mileage.

So far it looks like there may be a 3-4mpg hit. For the record I hope I am wrong. The car corners soooo much better on the 17s.

Didn't you get a bigger mpg hit on the Gen II? That doesn't seem too far off than OEM 17" (~1.8 mpg hit per European version of EPA testing), especially with worse tires for mpg.

Aye. In the GenII with Kumho Ecsta ASX tires I felt like it was a 4-6mpg hit vs. Integritys and Nokian i3 15" tires. When I swapped to the Primacys it seemed like the mpg got worse (of course they were brand new compared to worn Kumhos) but the mpg never really recovered. The Primacys vs. the dB Super E-Spec was more like a 6-8mpg difference depending on how much I tried to hypermile. That makes sense since the 185/65/15 dB is a lower rolling resistance tire than the 195/65/15 AVID Ascend.

I think I can salvage a 55mpg (indicated) average on this tank which would put me in that -3mpg ballpark. Remember though that I am also comparing them to the AVID Ascend which seems to be close to the OE AVID S33D but not nearly as good as the Energy Saver A/S and according to Yokohama, not as good as the dB Super E-Spec.

I would love to see someone with 16s jump in. That would be my preferred upgrade.
Thanks

TOTALLY off topic.....would that happen to be a 70 LT1
in the background ?

So I tried my best to keep the mileage up but I didn't want to stray away from what I normally do on my 15s. The trip total is now 56.?. While I know now that I am definitely dropping in mpg I will say that I never could have achieved even 56mpg indicated in my GenII. Even with a 5-6% calculated number I am still higher than the GenII with these exact wheels. This leads me to believe the GenIII doesnt suffer quite the mpg hit the GenII does. I think this may have to do with the GenII wheels/tires being more fuel efficient than the 15" GenIII wheels/tires. So there should be a slight hit with the OE equipment on the GenIII but since the car is much more efficient in general, it masks the reduction in tire efficiency. That is my line of reasoning at the moment anyway. It would be interesting to test the results of a 185/65/15 tire on a GenIII.

In all, if I were to continue driving the same way for the rest of this tank I could manage a 55mpg or even 56mpg indicated average which isn't bad considering my uphill commute. Granted the same driving with my 15s usually nets me 60mpg to 58mpg.

To really mix things up, The Critic and I are going to swap wheels. He'll use my 17s and I'll use his 15" Energy Saver A/S. lol

It's a '72 454. It's my friend's car. His dad was saving it to work on with him but he passed away when ny friend was young. So now my friend is saving it to restore with his son.

Thanks for the info ! I wish them well on the father/son restoration. We've restored more than a few. Takes a lot of time, work, and the green stuff. The 72 454 is a good Vette to collect !

I told him about your post and he was impressed you asked.