Tires and fuel economy

[Mark]

For a while I've been wondering why the fuel economy of my little '81 Tercel is so much worse than my '84 wagon. It's been getting around 22 mpg in town while my wagon would get around 27. I recently got new tires (second-hand since I won't pay more than $30 per tire) for the wagon. They're winter tires since that's what they had at the tire place in good condition. I've noticed that the fuel economy on the wagon has now dropped down to around 22 mpg just like the '81. On the highway it used to get around 31mpg on the highway and now it's down to 27 or so. I didn't think there would be such a difference in economy. I found a link to a test done on a truck with similar tires: http://www.fourwheeler.com/how-to/wheel ... l-economy/ The side-by-side picture of the tires about halfway down the page look the same as my old and new tires. Their test had about the same results as I did, about a 15% drop in fuel economy. I've noticed that the tires on my '81 are M/S tires so I wonder if that's why it has worse than expected fuel economy.

[dlb]

when did you get the 22 mpg tank in the wagon? was it during that week or two of really cold weather we had? because i find ambient temps play one of the biggest roles in fuel economy.

tires, OTOH, haven't had any noticeable effect for me. going from 155/80r13 to 185/70r13 on the last tercel made no difference.

[Mark]

I got the tires for the wagon in early Nov. so I've been through a few $20 fill-ups since then. I don't think it's much to do with the tire size or profile, but the more aggressive tread pattern on the winter tires. The previous tires I had had the "grooves" or whatever you call them running in a straight line all the way around, while the winter tires have a staggered tread. I think the winter tires also are made of a softer rubber. I also noticed today while driving in the rain that the winter tires are much noisier. It sounded like I was driving through puddles when there weren't any. I think I'll leave these tires on for the winter, but I might end up replacing them with normal all-seasons eventually.

[irowiki]

The richer running when cold starting definitely has a huge impact on MPG, especially when making shorter trips.

[dlb]

i have some very aggressive winter treads on right now, i used them last year without noticing any significant change in fuel economy but i'll pay close attention let you know what i find over the next several tanks now that the temps here are back up closer to 10* C.

with all seasons and temps around 8* to 15* C, i was averaging 13 km/L. with the winter tires and temps around -2* to 6*, i got 11 km/L on the last tank. the winter tires could have played a role but we'll see.

[Petros]

it should not be getting only 22mpg with M+S tires, check timing, state of tune (spark plugs, wires, etc) and for a vac leak, or possibly leaky AAP in the carb. All are easy fixed.

[irowiki]

It is an Auto T4, so 22 MPG city isn't completely out of question!

[Mark]

Both of mine are autos, both have the AAP disabled (pretty much as soon as I got them). The timing on both is around 8-10 BTDC. I've changed plugs/wires every couple of years. They both idle very smoothly and accelerate without any stumbling. The '81 has M/S tires and the new ones on the wagon are winter tires with the mountain/snowflake logo. I've had the wagon for several years and the fuel economy has been pretty consistent (30+ on the highway, 27 or so in town). I know that's much better than the epa ratings. I'm only connecting the sudden drop in fuel economy on the wagon to the new tires because it happened at the same time. The only other thing I did at the same time was replace most of the exhaust system (and remove a non-functioning catalytic converter). I've heard of people having a drop in fuel economy when a cat is removed or cleaned due to changes in exhaust gas velocity and scavenging, but on mine I haven't noticed any change in power or anything. The link I posted above to that tire test had similar results to me with similar tires. Their 15% drop in economy is similar to mine.

[xirdneh]

when i am not getting 28-31 mpg with all around driving winter(27-29, summer 28-31)
first i check tire pressure
then i swap or re-build carburators

[Petros]

we had freezing icy weather here for 4-5 weeks so I put on the studded snow tires, my normal mixed driving of 28-29mpg dropped to about 22-24 mph. tires make a difference, the studs add a lot of resistance. It was particularly annoying since I only needed the studs to get home the last mile or two, compared to the 40 to 60 miles of driving each day.

After that I thought of a way to have retractable studs so I do not have to change the tires to get good fuel economy. My idea is a tire can have retractable studs, they are below the tread surface until you inflate separate chambers in the tread that push the studs to the surface. I could sell a lot of them.

the best fuel economy tires would be the skinniest tires you can get, smooth surface with no tread and inflated to 60 psi. Not good for wet weather, but it would have the lowest rolling resistance.

[Mark]

A month or so ago, I re-replaced the winter tires on my wagon with regular tread tires (used). I was hoping that the fuel economy would go back to what it was before I put the winter tires on, but it only improved slightly. I used to get a consistent 31 mpg on the highway and now I get 28. The only other thing I did when putting on the winter tires last year was removing the clogged catalytic converter. Now I'm thinking that the increased airflow through the engine is the culprit for the lower mpgs. I haven't noticed any increased power during in town driving, but last weekend I drove on a steep mountain highway and the extra power was obvious. I was able to accelerate up the uphills without the transmission kicking down to 2nd (automatic) like it did before. I'm thinking that since the jets in the carb are calibrated for a certain airflow and throttle position, reducing the airflow would decrease the vacuum and the amount of fuel being drawn into the airflow (Bernouli's principle). This might lean out the mix, increasing fuel economy. With modern computer-controlled fuel injection and MAP, MAF and O2 sensors, the fuel delivery system would try and compensate for this (keep the mix at an ideal ratio) and you would get a reduction in fuel economy. I don't know what mix the Tercel jets are calibrated to achieve. I assume it's the ideal 15-1 or so, so a reduced airflow/vacuum might lean it out to maybe 17-1? People on this forum have always been surprised that I used to get 30+ mpg on an automatic (now not anymore unfortunately). Now it gets closer to the lower fuel economy on my '81 Tercel (which also doesn't have a cat) Maybe the secret to hypermiling on carbed cars is to stuff a potato in the exhaust.

[dlb]

mark, i've been watching my fuel economy closely since this thread started. i put our summer tires on at the start of this month and have noticed a slight improvement in FE since then, but that also coincided with the start of this warmer weather. so it's tough to say. i'm considering throwing my snow tires back on for a few tanks in july, but only if i'm super bored and have absolutely nothing better to do with my time.