Guy did rolldown tests specifically on the RE92s mounted to a geo metro...
Thanks for the link. Seems to suggest that there is minimal if any gain from going over the sidewall max. I've always filled mine to 300 kpa (44psi). I've wondered how much fuel economy I was giving up by not going higher. This link seems to suggest, not much, if any.
Very interesting results. I'd be curious to know what caused the coasting distance to drop as the pressure increased.
Edit: According to the chart he shows, it looks like the optimum pressure is actually in the 60 psi range instead of the 45-50 range.
__________________
Silver '00 MT
85.5 LMPG
80+ psi in RE92's for the past 2 years without incident
Calpod switch
Warm air mod
Grill block
EPS fuse removed
FAS/fuel injector interrupt circuit
grid charger
OBDII C&C gauge
Last edited by diamondlarry; 12-30-2011 at 08:58 AM.
According to the chart he shows, it looks like the optimum pressure is actually in the 60 psi range instead of the 45-50 range.
The difference is minimal and actually seems to between attempts. In fact, one of the 60 rolls was less than the 50 rolls. Above 44PSI, it seems the difference is probably within the margin of error.
Very interesting results. I'd be curious to know what caused the coasting distance to drop as the pressure increased.
Edit: According to the chart he shows, it looks like the optimum pressure is actually in the 60 psi range instead of the 45-50 range.
There was possibly a bit of wind at the end. It would take a mere breeze to effect the results. Also, the difference at the top is so small that decrease/increase is probably within the margin of error. 3wheeler, a very smart engineering type, has had problems with coast down repeatability.
It would also be interesting to know the span of time over which the data was gathered. Looks like it would have taken several hours, and I suspect tire temperatures would have changed within that timeframe. There is another subtile question in the data. One has a tendancy to read the data top to bottom, BUT, the data was taken the reverse order with the high pressure data taken first. I kinda wonder if the tires actually warmed up a bit during the test cycle due to rubber hysteresis?
I'm going to repeat the test on an Insight when we get a bit warmer spring weather, but I'm going to do it by driving my long test course. That would be more representative than the cold stuff we are having right now. Looks like MetroMPG gathered his data on a warm day.
In any case we known that the improvement with increasing pressure was a diminishing effect. At some point the improvement has to get too small to measure, though I have some doubt that it actually goes negative.
I just wish I could put a set on my HCH... but the total diameter is quite a bit different (1.8" less)... I think they could handle the load though (937 lbs rated vs the 1200 lbs stock tires).
__________________
2004 Honda Civic Hybrid @ 53 mpg so far!
RIP 2000 Insight, 40k miles @ 69.2 mpg
I tried these tests myself and got a slightly different result, but only slightly.
I think that one potential problem the OP had was that his method did not adequately warm up the engine and transmission, so that viscous drag changed somewhat during the tests. I thoroughly warmed up my engine and transmission before my tests, and I left the engine running during the entire cycle so that engine heat would stabalize the transmission, final drive and inside CV joints. All my test runs were engine on, transmission in neutral. Of course I tested the RE92's on an Insight Gen1.
I only tested the very top of the pressure scale because I was intensely curious about the reversal at high pressure. My results are:
@80 psi
739.5', 740.0', &743.0'
Ave=740.8'
@70 psi
739.0', 739.5', & 740.5'
Ave+739.7'
@60 psi
736.0', 737.0', &740.0'
Ave=737.7'
@50psi
729.0', 731.0', 733.0'
Ave=731.0'
Steeper hill, longer runout.
I have very little confidence in this kind of "splitting hairs" on an experimental basis, but at least MY data showed a continued increase even to 80 psi. I'm not bragging because I think all this top end tire pressure stuff is clearly in the noise and various errors probably swamp any significant findings. If I did the test again tomorrow, the results might be different, and I don't think either result would mean much. When I get a nice day, I intend to drive my long test course at 70&80 psi and see if the fuel economy differs. After all, that is the real test. JMO
Awesome, thanks for the data! Seems like I should probably bump my pressure up to 60PSI.
I, too, question the reversal of gains at high pressures. I think the real point is that you get into diminishing returns after about 60PSI.
50 to 60PSI = 20% increase
731.0 to 737.7 = 0.92% increase
60 to 70PSI = 16.5% increase
737.7 to 739.7 = 0.27% increase
70 to 80PSI = 14.3% increase
739.7 to 740.8 = 0.15% increase
__________________
Insight #1 - Silver '01 5MT @ 158,388 as of 7/11 - Best Tank: 84.5MPG over 807mi
Insight #2 - Silver '01 5MT @ 450,000 as of 1/12 - Best Tank: 86.0MPG over 800mi
Insight #3 - Silver '00 5MT, MIMA #163P, BCM Gauge, OBDIIC&C Gauge, BetterBattery @ 228,869 as of 1/12 - Best Tank: 78.4mpg over 687mi
I tried these tests myself and got a slightly different result, but only slightly.
I think that one potential problem the OP had was that his method did not adequately warm up the engine and transmission, so that viscous drag changed somewhat during the tests. I thoroughly warmed up my engine and transmission before my tests, and I left the engine running during the entire cycle so that engine heat would stabalize the transmission, final drive and inside CV joints. All my test runs were engine on, transmission in neutral. Of course I tested the RE92's on an Insight Gen1.
I only tested the very top of the pressure scale because I was intensely curious about the reversal at high pressure. My results are:
@80 psi
739.5', 740.0', &743.0'
Ave=740.8'
@70 psi
739.0', 739.5', & 740.5'
Ave+739.7'
@60 psi
736.0', 737.0', &740.0'
Ave=737.7'
@50psi
729.0', 731.0', 733.0'
Ave=731.0'
Steeper hill, longer runout.
I have very little confidence in this kind of "splitting hairs" on an experimental basis, but at least MY data showed a continued increase even to 80 psi. I'm not bragging because I think all this top end tire pressure stuff is clearly in the noise and various errors probably swamp any significant findings. If I did the test again tomorrow, the results might be different, and I don't think either result would mean much. When I get a nice day, I intend to drive my long test course at 70&80 psi and see if the fuel economy differs. After all, that is the real test. JMO
Thanks for the data. This seems to be closer to what Right Lane Cruiser and I've seen. From 50 to 60 there was a 6.7' jump in the average distance, from 60 to 70 was 2', and the 70 to 80 jump was just 1.1'. I suspect that going on up to 90 would likely show only 6"-8" or less. I think your data seems to show that somewhere around the 80 psi mark is where "the point of diminishing returns" begins which is right in line with what I've seen; going above 80 shows no significant jump in mpg numbers. I stick with around 85 or so just in case I go too long between remembering to check the tires.
Edit: Larry once again types slow.
__________________
Silver '00 MT
85.5 LMPG
80+ psi in RE92's for the past 2 years without incident
Calpod switch
Warm air mod
Grill block
EPS fuse removed
FAS/fuel injector interrupt circuit
grid charger
OBDII C&C gauge
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Finally: proof the best pressure is 45-50 PSI!
