Inflating tyres - effects on MPG
 

I'm sure others have come across this, but still worth considering.

Overinflating tyres (within safety limits... usually!) is known to cause an increase in MPG. However, it also has an effect on the recorded mileage that will actually 'hide' some of these gains. This is simply because the effective diameter of the tyre increases, if it inflated at a higher pressure (since there is less deformation at the contact patch).

I worked out a few weeks ago that my replacement speedometer was 4% optimistic in the distance travelled, but now I have my tyres at a higher (but within the safety limits) pressure, it actually underestimates the mileage by about 1% :) (worked out by digital map data + an actual journey I did yesterday). I won't be correcting my MPGs at the moment as that would make
things more complex, but for those who have changed their tyre pressures, it might be worth rechecking the accuracy of their mileage meter. This is because I have noticed a nice increase in MPG, even when using the speedometer mileage readings that appeared to 'lower' my MPG by 5%.

Rrreeeeaaaallllyy? Wow... Anyone with a GPS see the same thing?

You may want to try marking your tires with something similar to the sidewalk chalk kids play with and try measuring how far the car rolls per revolution. With steel belted radials, the change in diameter likely would be negligable. The contact patch with the road would be smaller with higher pressure, but the diameter would not get bigger.

That's what I mean - the 'effective' diameter is that of the bottom of the contact patch to the centre of the axle, and this gets less with overinflation. If the tyre was at 0.5psi pressure, then the effective diameter would only bit slightly more than the wheel rim itself.

When I changed the speedo a few hundred miles ago, it was clearly optimistic, but now that I pumped my tyres up, it is almost perfectly accurate.

That's interesting I've done this on 3 cars and have not noticed any change with different pressures. What size tires are on Bluey?

At present, with a nominal 40psi front / 38psi rear (vs the 'standard' 32psi and sidewall max 44psi), my Scangauge-reported distance is within a fraction of a percent of GPS-reported distance. The odometer report, however, is around 1.5% fast. (*sigh* I should adjust my gas log down some day.)

Its semantics. Doesn't amount to a bean, much less a hill of beans in the real world when over all mpg, miles and mph are looked at.

My OD,,, trip A/B are all on spec with the Navi in the Civic. The SGI doesn't agree. Unless I adjust it it wants to read long by 15 miles or so on a 600 mile tank. Do the basic math and its not worth the time or energy. Since the ECU replacement in my car. Ive been within 1 or 2 tenths in my fuel useage from pump to SGI. Before ECU replacement... it could be way long or short. I wanted to run over the SGI before I got to the meat of the issue.

If your car has a good reverse p-trap for the evap system. Do full top offs and work from the OD if its close to right and call it good. Anything else is mind games with no clear cut result...

psy

Ill check with my gps

Yet another thing to test with a SG: Someone do some runs @ 45psi compared to the same route @ 32psi. Check out the differences in odometer readings with the trips.

"Sure the diameter doesn't grow appreciably BUT the distance between axle and road rises with increased pressure, effectively increasing the "diameter"."

I guess I should have been clearer, the effective diameter (distance between axle and road) may change, but the radial belts keep the circumference from changing, effectively meaning there should be little change in the accuracy of the odometer/speedometer by changing tire pressure.

I'm going to have to disagree a little here. I can notice my car rising a little when the tire pressure is low. How can the circumference not change if the diameter is changing? Doesn't make sense to me. Not trying to start an arguement here at all. It just mathmatics since diameter is used to calculate circumference, there must be a change in circumference if diameter changes.

-- Scott

Tire curcumference and pressure
 

At first I got in an argument and then lost when confronted with what actually happens. I know it is a steel belt so the change has got to be really small. However, my Impalla looks at the wheel speed indicators and can conclude if a tire is low. It has set off the warning light twice. The tire was down around 15 psi when the warning came on. To put this in perspective after the tire is fixed and the warning is reset it can take up to 100 miles to again infer the size of the tire according to the manual.

Just a side note.

Ernie

Yes and there is a different amount of slip when a tire is low vs high in pressure. I have noticed that both my GPS readings are off from the SGII readings at 60mph i.e. 60.9mph on Garmin GPSIII+ is reading 60 on SGII when usually it would read 61mph - wondering if I should be rounding up past .5mph or drop down to lower reading. It would affect the distance traveled on the SGII if I increased the speed by 0.5 mph to be closer to the GPS.

if the tire pressure is lower than the sidewall pressure you can increase the diameter of the tire by increasing pressure. So if the tire says 44psi max and you go from 32 to 38 then the tire effectively becomes bigger. If you go from 44 to 50 then the diameter does not change enough to be measured.

I think run the highest pressure you are willing to ride on and don't worry about it. Just make sure you run at least what the tire is rated for. I run 48psi cold in mine :thumbup:

Originally Posted by mrmad View Post
"Sure the diameter doesn't grow appreciably BUT the distance between axle and road rises with increased pressure, effectively increasing the "diameter"."

I guess I should have been clearer, the effective diameter (distance between axle and road) may change, but the radial belts keep the circumference from changing, effectively meaning there should be little change in the accuracy of the odometer/speedometer by changing tire pressure.

I'm going to have to disagree a little here. I can notice my car rising a little when the tire pressure is low. How can the circumference not change if the diameter is changing? Doesn't make sense to me. Not trying to start an arguement here at all. It just mathmatics since diameter is used to calculate circumference, there must be a change in circumference if diameter changes.


No worries about not agreeing with me. In old bias ply tires, the overall diameter and circumference would increase as you increased pressure, like the tire was a ballon. The reason the circumference doesn't change (or at least by any significant amount) in radial tires is there is radial belts running the circumference of the tread that do not allow the rubber to act like a ballon. A tire with load on it, is not a perfect circle, it is deformed at the contact patch. The change you are seeing as you increase pressure is how much deformation occurs.

Well, I went from quite a low pressure, to the rated pressure - so that is probably why I saw a difference!

As a side note - if a tyre is very flat (e.g. 8psi), imagine the shearing stresses it must undergo - it is no wonder that they blow out when this happens!.

Edit - the difference was 6% in my case, so it did make a difference. This is because I went from 28PSI (car specs) to 45PSI (tyre rating). Reading something elsewhere, I'm sorry if I caused any offence in this thread? I didn't mean to if I did.

Reading something elsewhere, I'm sorry if I caused any offence in this thread? I didn't mean to if I did.

This forum is far too polite (maybe that's why I like it). Other forums I'm on, someone is bashing you if they disagree.

Pressure Allert !

Hey the other day I check my tire pressure now that the temps are in the 60's and found they have climbed up to 45psi and I was running them at 40-42psi and had not checked them for about a month. You all may want to double check your tire pressures.