driving on 4 narrow spare tires (or similar) to reduce rolling resistance

[oil pan 4]

I went from P235/75R15 firestone tires to 31x10.5R15 goodyear RT-S tires. The goodyears were nearly 2 inches taller and almost 20 pounds heavier per tire and truck fuel economy did not go down. I was fully expecting fuel economy to go down, but it stayed the same and may have went up slightly. The truck has a TH700R4 (overdrive transmission with a lockup converter) and 3.08 rear end gears.
If you don't bother to account for your new odometer readings with the larger size tires your fuel economy will appear to be going down a lot more than it really is.

[Ryland]

I have read about people having good luck switching from light truck to heavy truck tires, even with the extra weight of the tire, because the heavy truck tires are a stiffer tread and tend to be 100psi tires.

[euromodder]

Originally Posted by mans

this is most interesting. is this continental a low RR tire? it sure sounds like it from it's name of "ecocontacct ep"

It certainly was meant to be a LRR tyre back in the day.

can you show me a link where I can purchase such a tire?

I haven't got a clue wether it's (been) available in the US.

Wheel Machine 2000 - Felgen Tuning und Reifen in Germany sell them

(under menu option REIFEN, click Sommerreifen then select the size from the menues)

[mans]

"metrompg" has made rolling resistance tests with various types of tires by rolling down a small hill and seeing how far the car would coast. he repeated this with 5-6 types of tires including a set of spare tire doughnuts.

the doughnuts tested the worst. even worse then old junk tires.
here's the link http://ecomodder.com/forum/showthrea...eet-19126.html

HOWEVER here we have a young man who performed a mpg test on his metro, then replaced the rear wheels with spare tire doughnuts and repeated the test.

he gained 5 mpg with the doughnuts on his rear wheels. that would show that doughnuts are pretty good, way better than stock tires!

whats up with that?
thats a total conflict of results.

can anyone explain how this can be?

http://www.youtube.com/watch?v=b2K4j037TyY

[mwebb]

with smaller tires on the rear only the angle of the roof of the car relative to the road is changed

the rear of the car is lowered

these tests are apples and oranges
the variables are not the same

the comparison is therefore
invalid

this does point out something of interest to Geo Metro owners however

next

[Ken Fry]

Originally Posted by mans

he gained 5 mpg with the doughnuts on his rear wheels. that would show that doughnuts are pretty good, way better than stock tires!

whats up with that?
thats a total conflict of results.

can anyone explain how this can be?

If you use this calculator, (Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com) you will see that the tires he took off may have had a Crr of .012 (who knows to what pressure they were filled?). The donuts may have been .009 at 60 psi. If you put in good guesses for a Metro, you can get 55 mpg at 50 mph. Then change only the Crr to .009. You get a little over 60.

I think MetroMPG used better controls.

[oil pan 4]

Originally Posted by mans

he gained 5 mpg with the doughnuts on his rear wheels. that would show that doughnuts are pretty good, way better than stock tires!

whats up with that?
thats a total conflict of results.

can anyone explain how this can be?

How much more weight is on the front then compared to the rear on a metro?
Maybe thats it.

Some times stuff just works and it seems to go against what we think or what we believe should happen.

On a side note aren't little spare tires usually bias ply?

[Vekke]

Ok here are some results conserning wheel weigths which comes from my memory:
1. wheel weight affect fuel consumption
2. Effect depends do you do most of driving in city (bigger effect) or highway (smaller effect) What is your cars total weight. 10% lighter car gives 2 to 8% better fuel consumption depending on the source and test method city vs highway. Rotating weights affect 4 times their own weigth to overall weight. So if you save one pound you can calculate that you have saved 4 pounds... Also the car driving characteristics change to better response etc.
3. You will see much better effects in fuel consumtion if you go from your stock wheel size to one or even two size smaller rims. Your front brake calibers dictate how small rims you can put there. Note that there is big difference in the inner shape of the rims. Racing rims are can eat inside much bigger brakes. 3mm clearance is more than enough to brake calibers.
4. Usually this means that you have to add narrower and taller height profile tires to keep the revs at same level or even lower them which is good for FE. Both lower the fuel consumtion hugely. On my Seat Cordoba 1.9 TDI stock tire size was 185/65R14 and with 175/80R14 on from I saw immediately about 10% better fuel consumtion figures. Smaller rim and taller tire give always the best results. On my Lupo 3L I now moved from 145/80R14 to 175/70R14 and fuel consumtion get worse about 15% and the winter tires are best LRR tires out there. My winter wheels are also little bit heavier
5. Check that your weight limits on the tire narrower and taller is enough if you care about that kind of things.

Aerodynamic effect.

Smooth hubcaps lower the fuel consumtion about maximum of 5-6% so that might easily have bigger effect than the lighter wheels which you should always note.

Best setup is:
- about 30mm narrower tires than stock
- about 5-10% taller tires than stock.
- usually that setup means that you cannot get easily LRR tires but that is not the most important factor. Effect of LRR tire and worst tire is about 5% to fuel consumtion in tyre tests. You can get that 5% better RR by putting 50 PSI on the tires .
- Most lightweigth rims you can afford (weight saving will not give ROI never on rims) on most cars lighter rims cannot be seen at the pump.
- Try to buy aerodynamic rims or install smooth wheel covers:
http://tuneko.com/TuneCover---Alumin...el-hubcaps.php

Best setups in my cars:
at seat cordoba vario 1.9 tdi, stock size 185/65R14
front 175/80R14 toyo 350
rear 175/70R14 Michelin energy savers
This setup needs -60 front and -30 rear lowering kit or just -30mm in the front to get the stance correct

Lupo 3L stock size 155/65R14
145/80R14 Bridgestone B381 ecopia with Lupo magnesium wheels which are pretty light.
I also tried 135R15 2cv tires at the front but it did not give better FE on my Lupo

So for starters I would use the current wheels and put there narrower and taller tires. If you put there higher pressures you can use 1" wider wheels what is the recommedation. This gives also little bit better aero.

What comes to buses or other cars 1% difference is not measurable in normal driving on any vehicle.