Smart Car with 155/6R15 ContiPro tires all around

[Ptero]

After three years of messing around, I finally did the NUMBER ONE THING to get better mileage in my Smart Car. I bought a couple front wheels with new tires on eBay for $200 or so and pumped them all up to 42 psi.

I had already installed an UltraGauge so it was easy to tell if there was a change from the STUPID, TOO WIDE OEM BACK TIRES that come on U.S. Smart Cars, lower the MPG and make it impossible to economically rotate your tires. My fronts were wearing nicely with 42 psi but the rear set was wearing more to the outside. Now that's fixed and I'll save money on tires, too.

They fit perfectly and there is now enough room to fabricate a fender skirt. They are also about 15 pounds lighter than the two wide tires.

My first question was, "Will it handle okay?" I pulled onto the windy canyon road that I commute on and immediately noticed a difference. The car wanted to dart a little more than usual. What it was, I realized later, was enhanced sporty handling. It is more maneuverable. Once I got used to it, I completely forgot about it. The wide back tires had actually hurt the Smart Car's handling!

My lifetime average is 52 mpg without pulling the trailer, but I was surprised to see 65 and even 75 mpg popping up on the UltraGauge. Nothing I have ever done to the car has been this noticeable.

You nutcases who like to drift around corners in your Smart Cars please let me know how it responds if you try this. I don't drive like that but I bet it drifts better with a more equal co-efficient of friction. But the configuration is very much like the track of an older VW. The rear track seems about 1.5 inches wider than the front, now, maybe less.

My last comment. Why would Smart put wide tires on the back, anyway? I think it was to appeal to a buncha dumasses who wanted Cobras and couldn't afford 'em so got tricked into buying a deceptively racey looking plastic car. I'm gonna do a 200 mile mileage run this weekend and I'll let ya know how it goes.

[NeilBlanchard]

Are they also a different diameter? Have you checked the odometer calibration?

I've heard good things about some Continental tires -- they were used on the Edison2 VLC's; though I think they were 14" rims. So, basically you are running the front tire on all four corners? How wide are the rims?

I'm asking because I am going to use the Smart ForTwo windshield in my CarBEN EV5, and if I buy a rear-ended Smart for the windshield (and wipers, too) then I can use the front rims, as well.

[Peter7307]

Originally Posted by Ptero

My last comment. Why would Smart put wide tires on the back, anyway?

Possibly as a result of the weight distribution?

Not sure how much was on each end of the Smart but if the bias was rearward then the larger rear tyres may have been to reduce the load rating on the tyres at the rear.

Peter.

[Ptero]

The 74t 155/60R15 tires are load rated at 827 lbs.
The 77t 175/55R15 they replaced were load rated at 908 lbs.
A 74t is 2 lbs lighter than a 77t. 2 x 2 = 4
Narrow wheels are 4 lbs lighter than wider wheels. 2 x 4 = 8
(2 x 908) - (2 x 827) = 1816 - 1654 =
162 lbs lower load rating - (4 + 8) = 150 lbs lower load rating
Rated load capacity for the 2008 Pure with a full tank = 507 lbs
New load capacity = 507 - 150 = 357 lbs
Still okay for me, a passenger and light cargo - or me alone and a lot of stuff.

I usually hypermile alone with a half tank and about 20 lbs of stuff.
(4.35 gal x 7 lb/gal) + 357 = 387.45 lbs

I can pull an additional 850 lbs on the 150 lb trailer with a 30 lb tongue weight calculated at 2x for dynamics of 60 lbs
That gives me about 100 pounds of gear inside on long trips with the trailer, which is still within range. (I've traveled this way from California to Houston, Chicago and Laramie on separate trips.)
http://www.cleanmpg.com/forums/showthread.php?t=24446

So I expect the primary result will be significantly improved mileage with few drawbacks.

[Peter7307]

Ptero,
Post what you find. This should be a worthwhile comparison.

Thanks , Peter.

[euromodder]

Originally Posted by Ptero

Why would Smart put wide tires on the back, anyway?

Stability during the moose-test - an avoidance test at speed.

If you're hypermiling it, you should still be OK.
If you're pushing it, you could be in for a surprise when it gets really tail-happy, which in such a short car would likely be nasty.

[F8L]

I assume you also compared the revolutions per mile for both sets of tires? Even two models of the same size tire can have large discrepancy in revs per mile. Small change in revs per mile could mean and extra 2,000 revs per 400 miles. Not large numbers but it could calculate out to an extra 2+ miles per tank.

[Ptero]

If you're pushing it, you could be in for a surprise when it gets really tail-happy, which in such a short car would likely be nasty.

There are a couple of important factors in avoidance maneuvers. One is the balance of the car. Another is the co-efficient of friction of the tires. If we assume the front tires will stick and the back tires will break free on hard cornering (oversteer), we should examine the co-efficient of friction (kinetic). Wider tires present a lighter footprint. If wide tires are better, are wider tires even better? Not necessarily. Grip is a function of weight vs. footprint area vs. rubber compound. A hard wide tire will not grip as well as a narrower tire with softer compound. Even with a softer compound, a light car may not exhibit inferior abilities in cornering or balance compared to a car equipped with wider tires. Therefore, a very light car may not find an advantage in wider tires - particularly if it is of lower horsepower. This is anathema to the popular belief that wide tires are better and therefore look cool. I seriously doubt that narrower tires will break free earlier or more suddenly than wider tires on the rear of a Smart. If you look at history, well balanced cars have primarily run the same size tires. I really suspect a Smart Car with 155/60 R15s could prove superior to the OEM configuration in a salom because I believe the wider tires were installed to feed a popular misconception. Also, if this misconception was the case, wouldn't wider front tires be du rigor? And if short wheelbase aggravated the balance equation, wouldn't go-cart racing be nearly impossible? That said, I do agree that there is no substitute for empirical testing.

[Ptero]

I assume you also compared the revolutions per mile for both sets of tires? Even two models of the same size tire can have large discrepancy in revs per mile. Small change in revs per mile could mean and extra 2,000 revs per 400 miles. Not large numbers but it could calculate out to an extra 2+ miles per tank.

Thanks for pointing that out. I had measured the new fronts vs. the worn rears and was unable to tell any difference. Below, I'm using the specs supplied by TireRack for the OEM ContiContac Pro 155/60R15 and 175/55R15.

Continental ContiProContact

Note that the supplied revs per mile are incorrect and are recalculated here.

Front 22.3 Rev/mi (5280 x 12) / (22.3 x 3.14159265) = 904
Rear 22.6 Rev/mi (5280 x 12) / (22.6 x 3.14159265) = 892

Percentage change 100 - ((892 / 904) x 100) = 1.327 % higher indication
Induced error 52 mpg average x 1.01327 = 52 - 52.69 = -0.69 mpg

Tank 400 x .69 = 1 mpg error every 276 mi
Good guess!

In real world terms, my rear tires were quite worn when I replaced them and the outsides were wearing more than the insides of the tires. I think the big factor will be rolling resistance. The rears were one inch wider each, so a two-inch strip of rolling rubber has been removed from the previous configuration.

In my hypermiling style, I try to run in 5th as much as possible, averaging 50 mph. The gearing will now be at a slightly higher ratio. Remember, the Smart has two overdrives, 4th and 5th. The only way to tell if this is beneficial to mpg or not is to perform mileage runs. But I'm not sure I'll be able to tell. For one thing, having more tread on the back tires (all the tires are new with 10/32" tread. Mileage will improve as the tread wears. I may run the tires up to 50 psi, although I have been told there is little noticeable difference between 45 and 50 psi. I am tempted to try it myself, now that all the tires are the same.

[NeilBlanchard]

Calibrating the odometer is reasonably easy to do on a road/highway with distance markers at say 0.1km or 0.1m. If you drive 20-30 miles you will probably see that the odometer either under reports or over reports the distance vs the actual distance. If you have a good GPS unit, that can also work.