Taller Tire Test (5.7% MPG increase observed on Honda CR-Z @ 55-65 MPH)

[mcrews]

Originally Posted by rmay635703

It is my belief that long term proper testing is the only way to really know how a tire will affect FE on any given car.

I think the tires load rating (and loading), PSI and aero drag likely play more of a role than their size in terms of FE. (although they are also related)

Here's my take (and experience), In the day to day reality world of a slight upsize on a car (trucks are another discussion),
1. the load rating is higher so it's not a factor.
2. the width is actually SLIGHTLY narrower, but even it slightly wide, aero gain or lose is non-mearsurable and not an issue.
3. PSI - you should run the desired PSI (40+) regaurdless of size. Since you would run the same psi, then psi is not an issue
4. Size - need atleast a 5% increase in diameter for it to statisticly matter.
5. If you go from a high performance tire to a LRR tire you will see an increase even w/ same size. That is why it's important for all posters to state the full size and type of tire.
6. Downsizing to gain areo is UNSAFE. Period. running narrower (defined as 90% or less) tires is just too risky
Let me give you an example. driver A is got his car all stripped down, scangauge in the ashtray and running donut tires. He is in an accident w/ driver B in a normal car. If I am driver B's attorney......I'm going to own driver A. I wont settle for the insurance claim. I'm going after criminal malice, neglect, ........ I'm sure you see where this can head. When you step out and 'mod' a vehicle uotside the standards, you are setting yourself up for truoble, ESPECIALLY if you didn't notify your ins company of the mods.....

[California98Civic]

Originally Posted by rmay635703

I think the tires load rating (and loading), PSI and aero drag likely play more of a role than their size in terms of FE. (although they are also related)

Interesting. How might load rating affect FE?

[rmay635703]

Originally Posted by California98Civic

Interesting. How might load rating affect FE?

It can go either way, if you run the tire at rated pressure at much less weight than it is rated its RR is lower than the same tire loaded up to its rating.

This can mean a smaller tire can reduce aero+RR on the rear of many FWD vehicles because the weight isn't there. (per experience)

[mcrews]

Originally Posted by rmay635703

It can go either way, if you run the tire at rated pressure at much less weight than it is rated its RR is lower than the same tire loaded up to its rating.

This can mean a smaller tire can reduce aero+RR on the rear of many FWD vehicles because the weight isn't there. (per experience)

can you be more specific?

[euromodder]

Originally Posted by California98Civic

Interesting. How might load rating affect FE?

Less sidewall flexing and less tyre deformation is good for FE - that's what you get with a higher LI tyre when it's only lightly loaded - provided most factors remain the same, i.e. profile, pattern, rubber composition, pressure.
Tyre weight will normally be a bit higher on the higher LI tyre though.
OTOH, higher LI tyres often also have higher allowed pressures to cope with the higher loads.

Wether the effect is measurable on the road when you're using say LI 94 tyres instead of 91 ?

[California98Civic]

Originally Posted by euromodder

Less sidewall flexing and less tyre deformation is good for FE - that's what you get with a higher LI tyre when it's only lightly loaded - provided most factors remain the same, i.e. profile, pattern, rubber composition, pressure.
Tyre weight will normally be a bit higher on the higher LI tyre though.
OTOH, higher LI tyres often also have higher allowed pressures to cope with the higher loads.

Wether the effect is measurable on the road when you're using say LI 94 tyres instead of 91 ?

That makes a lot of sense. (I take it "LI" is a way of referring to "load rating".)

[roosterk0031]

Load Index, after the 195-65-15 there's a 89S or something 89 is Load Index number S is the speed rating.

89 good for 1278 lbs, 91 is 1356, 95 is 1521 lbs for that size, don't know if same for all sizes.

[Chris D.]

What are you doing for speedometer and trip odometor totals?
If your speedometer reads 60, wouldn't your actual speed be around 63ish +?
How do you calculate your fuel economy?

[mcrews]

Originally Posted by SecondWind

As I am the OP on this thread, I'll say that the ScanGauge was re-calibrated between testing the OEM tires and the "tall tires". As I recall, the OEM tires used -3% setting, and the "talls" +7%. In both cases, the ScanGauge error was less than 0.1 mile in 10 miles. AFAIK, an improperly adjusted ScanGauge will affect both MPH and MPG readings.

chris D, pls read post #22 or re-read it above.

[ps2fixer]

This is an interesting thread. I too thought about increasing the size of my tires (needed to be replaced anyway), but oped to go with factory since the gearing isn't bad.

On past experence prior to ecomodder... my dad owned a 1992 Buick Century with a 3300 v6. His tires were about shot, and going on the cheap, a I had a 92 chevy lumina I was scrapping out with good tires. The lumina's tires were taller and quite a bit wider, enough to hit *something* when turning really shape around corners or parking lots. He claimed a MPG loss and it took off slower. He wasn't exactly an eco driver though, and I don't have any good tracking/numbers to give. He probably didn't count for the tire size change, and the mpg drop wasn't enough to bother getting different tires (like 2-3mpg drop, car normally got like 26mpg). I'm pretty sure the tires were 8-10% taller.