Smart Car with 155/6R15 ContiPro tires all around
 

http://i714.photobucket.com/albums/w...g?t=1320366521

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.

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. :)

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.

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.

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

Thanks , Peter.

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.

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.

Tail happy? Not that I've noticed!
 

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.

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.

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.

Smart car tyre setup
 

You should try bigger tyres in the rear, 175/65 R15 fits well, which produces 6,1% taller gearing. With these tyres the engine might feel a bit weak in 5th gear.

But when you're drafting a fast truck or a bus you should get pretty good MPG.

I've tried that with a lot of different vehicles. Once you exceed the optimum diameter, your mpg starts to go down. Also, I've found on the Smart Car that it gets the very best mpg at 25mph in 4th gear - not 5th. That tells me that the car is very close to perfectly optimized already, so any changes should be small or, at least, obvious - like fender skirts or chopping.

The wider tyres are fitted to the rear of the ones outside the US too - most "modding" owners go the opposite - wider tyres both ends for obvious "looks" and grip reasons.

I did once watch an older Smart understeer and ping into a kerb in Edinburgh on a cobbled road during a typical summer day (i.e. lashing with rain :D) and wondered about the narrow front tyres. Apparently Austin 7 (1926-39) owners use Smart front tyres as the originals are hard to get hold of these days.

Handling - My Aygo has 155/65 x 14 Continental EcoContacts which seem OK for grip. When I first got the car in July I ran at the factory tyre pressures (34-36) until I got used to how the car handled. The one thing I did notice was that the front tyres felt like they were rolling off the sides when cornering really hard - kind of like tyres felt when I was driving in the mid-80s on 135s ;) So I upped the pressures to 45 and now it feels pretty secure.

My car is the opposite of yours - front engine, front wheel drive.

I don't rotate my tyres as I prefer to replace them in pairs - the fronts go first, so I replace those and put the older rears onto the front and the new ones on the back - understeer is preferred.

Good luck with the change.

Light front end
 

Once I hit a rough patch of asphalt while accelerating through a left turn from a stop. My front tires came loose and the car jumped to the right. Technically, i don't think this can be described as understeer because the tires have actually left the pavement but it is something that people who run sidewall max psi in tiny rear-engine plastic cars need to be prepared for. In my Smart, for instance, when travelling without a passenger, I try to carry heavier cargo on the floor in front of the passenger seat because anything placed behind the seats is pretty much carried exclusively by the rear wheels, offsetting the car's balance.

A couple of thoughts
 

A couple of thoughts:

My understanding of why there is a difference in the front and rear tire size has to do with the "Moose Test" - a reference to an incident involving a sudden apprearance of a moose during one of the early test drives, where the rear end of the vehicle suddenly broke loose in the avoidance manuever. I also think the difference in track derives from this incident as well.

It's a little known fact that the smaller the tire size (while holding the load on the tire the same), the less linear the steering response - meaning less predictable. The term is called "saturation".

In this case, the front tires are 74 Load Index with a placard pressure of 29 psi, and rears are 77 Load Index with 36 psi (max load carrying capacity pressure). I would think that would mean that installing the front tires on the rear is in the direction of less predictablility even if you compensate by using more inflation pressure. I think the vehicle darting around is an indication of this instability.

Smart car safety things
 

I've had 3 different rear tyres, 165/65-15, 215/35-16 and the current 175/65 R15. Still the car refuses to go faster than 90mph ( 145 km/h). It can show engine load of 70% momentarily at the top speed, but won't go faster than 90 mph.

Also the ESP can't be turned off. They really learned something from the moose tests.

The "Moose Test" comes from a Swedish magazine which managed to get a then new Mercedes A-Class to tip over in an avoidance swerve. It was around pre-Smart Car so you would think they would have tested it and the J-test.

Moose test - Wikipedia, the free encyclopedia

The moose didn't appear during the testing, it's a basic obstacle avoidance test.
It got its name from the moose as that's what they do: stand on the road at night ignorant of approaching traffic - or in tunnels, which is wildly scary ! - forcing drivers to swerve around them.

IIRC, that was indeed the case.
The prototype or pre-production Smart in the Brussels Autoworld has less flare on the rear fender than the later production versions, and what look like same-sized tyres front and rear..

My thoughts exactly.
Pushed hard(er) it'll lose the rear end.