taller tire = MPG improvement?

[choij1]

What do you think of 205/75-15 on the stock steel rim, instead of 195/65-15?

Advantages I can think of:
a. equivalent to having wider 1st gear
b. equivalent to having taller 5th gear (plus on MPG)
c. 1” more ground clearance - less chance of having a cracked belly pan

Disadvantages I can think of:
a. Less power – Upsoluting becomes a must?
b. wider tire – more tire resistance (minus on MPG) I wish I could find 195/75-15
c. more air turbulence under the car – more air drag (another minus on MPG)
d. 10% speedometer error – ABS might work funny (would speedo recalibration help?)
e. 1” clearance is required around the wheel - might need stiffer springs
f. taller tire and higher center of gravity – worse handling

I always had this idea, believing it must be the cheapest way to improve MPG and adding 1” ground clearance. But now I think it might not improve fuel efficiency at all. Still, the 1" part sounds sweet to me. What do you all think? Jin.

 

[96Wagon]

If you could run on the flat all the time (you would have to gear down to make any hill), without start/stop traffic (takes more power with the lower gear ratio), with no need to accelerate or brake (more reciprocating mass, unsrung weight), also disregarding the apperance of your car (can you say "VW 4x4"). If these things don't include you then I would say Go For It! Not for me.

 

[The_Oz]

Your disadvantage list alone is enough to convince me not to go for a bigger tire

Also, don't forget that with a larger tire comes heavier weight...which in turn gives you bad handling, bad tire-to-road contact, loss of power, etc.

Regards,

Oz

 

[TDIMeister]

I can think of at least one other con: the car would look like crap, IMHO. A taller and wider tire will mean less RPM per MPH, but that's offset by (usually) greater tire weight, greater rotational inertia, etc. This will result in more than just a negative impact in fuel economy, but also worsened handling, acceleration and braking. I noticed this when going from stock 195/60-14 in OE steel wheels to 215/40-17 on alloy wheels (~3% increased rolling diameter). I lose approx 4 MPG, and acceleration and braking are just slightly worse, but certainly noticeable. Of course, many, many other factors are at play, such as tire width (aerodynamic drag), rubber compound (rolling resistance) and weight (total wheel and tire weight was greater with the 17s over OEM even though the wheels alone were considerably lighter than the 14" OE steelies.

 

[ertzog]

you want narrow, not tall/fat. Check out the mini tires on the prius, insight, echo (or almost any high mileage vehicle).

taller also dings you with increased rotating weight during roll-offs.

 

[choij1]

Thanks for all the opinions. Certainly, I missed the tire mass and the stopping part.

And I’ve a few more things to add to the list. I talked to a coworker today, who did some hot rodding and restoration work before, and he pointed out a few more things.

1. Turn the wheels all the way, and check the clearance to the inner wall.
2. The ride will be smoother. (not that I want any smoother ride)
3. Add front spoiler to reduce the air under the car

I have 2001 Golf GL TDI, and I am not planning on replacing the stock tires until they wear out. The idea is being cheap

, so I have to wait a couple of years before I can try anything. But I would appreciate more inputs. Jin.

 

[NSTDI]

The ABS is not affected, as long as the 4 wheels are the same. It works on the difference in wheel speed measured at the hub, so it thinks one wheel is slipping if you are using different tire sizes.
I have a NB with a stock tire size of 205/55/16. I run 15 inch winters at 195/65/15, the same as the stock Jetta size. The diameter is virtually identical. When I wore out my first set of winters and summers, I went to a 205/65/15 and a 215/55/16. They are probably as big as I could fit without major rubbing. After allowing for the different diameter when calculating fuel mileage, I wouldn't say I had any change in fuel mileage.

Don

 

[aaron]

Along these lines. . . has anybody increased the ride height of these cars? My 2000 Golf has horrible ground clearance, and either the spoiler will break off on minisucle speed bumps (eventually), or I can raise the car a hair. Is there any way to do this without really screwing up the entire suspension dynamics?

I'd probably never do it, it's just a mental exercise.