Smart Car with 155/6R15 ContiPro tires all around

[Arrow]

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.

[Ptero]

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.

[Arragonis]

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 ) 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.

[Ptero]

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 ) and wondered about the narrow front tyres.

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.

[CapriRacer]

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.

[Arrow]

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.

[Arragonis]

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

[euromodder]

Originally Posted by CapriRacer

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.

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.

I also think the difference in track derives from this incident as well.

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..

I think the vehicle darting around is an indication of this instability.

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