Wheels

[denguy]

I took off my steel 13" wheels and went back to the stock 12" . My gas mileage climbed from 40mpg up to 43 on both my festys.

[Safety Guy]

Did you take tire diameter into account when figuring your fuel economy?

Did you use tank fulls (7+ gallons) instead of short hops (2-5 gallon refills)?

For my 175/70-13s, I add another 4% to my miles travelled when figuring mpgs.

For my 155/80-12s I think I added 2%.

When I was running my stock 165/70-12s on Twinstiva I did it up straight.

Karl

[rollertoaster]

when I went from 145r12 to 175 70 13 it was a 6% change in ratio.

[Aaronbrook37]

When you're thinking about wheels and fuel economy, there's far more to it than mere rolling diameter. There's wheel width, tread width, tire rolling resistance, tire life, tire size, tire pressure, and wheel weight to consider. Account for the variables and go from there.

I bet you could get better fuel economy on a bigger, heavier, and wider rim/tire combination that had a lower rolling resistance, proper tire pressure, and better tread compared to a bald yet light and narrow 12" that is low on pressure and high in rolling resistance.

Obviously if you have the money, custom forged lightweight 12's or 13's with a grippy, new, and narrow tire will give you the best in terms of your fuel economy. The law of diminishing results applies though... at some point, you have to remember that you're driving an un-aerodynamic brick through a wall of air with a motor that expends up to 80% of its energy on heat, light, and sound rather than propulsion. The very shape of this car kills fuel economy far more than 10mm of tire or 1lb of wheel.

This is a reality of passenger cars that are practical though. You can mitigate this by reducing weight (this only improves city fuel economy), tuning and maintaining the motor to perfection, but most of all, through driving habits.

[cobra29svt]

when I went from 145r12 to 175 70 13 it was a 6% change in ratio.

So did you ad 6% to the mileage total when figuring your mpg's w/ the 13's?

[denguy]

All of the mileage checks on mine were taken after fillups when I would stop pumping at the first kickoff of the pump handle. All of the driving was done in the same area at speeds ranging from 25 to 60mph with a lot of stop and go. I would fill up when the gage showed under a 1/4 tank and the odometer was over 350 miles. This was in stock 91 [250k miles] and 92 [160k] stick festys.

[Clay]

When you're thinking about wheels and fuel economy, there's far more to it than mere rolling diameter. There's wheel width, tread width, tire rolling resistance, tire life, tire size, tire pressure, and wheel weight to consider. Account for the variables and go from there.

I bet you could get better fuel economy on a bigger, heavier, and wider rim/tire combination that had a lower rolling resistance, proper tire pressure, and better tread compared to a bald yet light and narrow 12" that is low on pressure and high in rolling resistance.

Obviously if you have the money, custom forged lightweight 12's or 13's with a grippy, new, and narrow tire will give you the best in terms of your fuel economy. The law of diminishing results applies though... at some point, you have to remember that you're driving an un-aerodynamic brick through a wall of air with a motor that expends up to 80% of its energy on heat, light, and sound rather than propulsion. The very shape of this car kills fuel economy far more than 10mm of tire or 1lb of wheel.

This is a reality of passenger cars that are practical though. You can mitigate this by reducing weight (this only improves city fuel economy), tuning and maintaining the motor to perfection, but most of all, through driving habits.

I would agree that all of those things mensioned do affect fuel economy, (weight also changes highway fuel economy because it translates into changes in friction)

I believe the best thing to do would be to install a odometer that you can calibrate, but even that would be hard to do. The input is usually rollout, but that even changes at speed a little bit. Getting an acurate odometer reading should be step 1 in doing a fuel economy study, maybe run 2 seperate ones. Without an acurate distance, the end mpg doesnt really mean anything.

I have only had my festiva for about 6 weeks, and have 4 readings, 40.5, 42.3, 44.1, 45.3. somewhere between 40.5 and 42.3, I installed plugs-wires-cap-rotor and filled up the tires to about 32psi. (one of them was less than 10psi I think). I am using the stock odo, but as long as I dont change tires, and keep them at the same diameter (pressure) it should be consistant. I would contribute most of the difference to change in driving speed. I usually run about 4-5 mph above the speed limit (depending on the in dash speedo, I may make adjustments) but often I get stuck behind someone going 5 under. it is nearly impossible to drive consistantly, which makes it nessesary to get multiple fillups and average. Even colder temperatures can cost you a couple mpgs.

If you are serious about doing an accurate study of fuel economy, here are some things I would start with:

A programable odo: TrailTech makes some neat computers for motorcycles and atvs, it would probably work pretty well, I think they can set a tire rollout of up to 2000mm. I would get on flat ground, attach a pencile to the tire, roll 3 or 4 revs of the tire and calculate circumferance that way. This would reduce measurement error.

Consistant Fueling: Somewhere there is a procedure for topping off a gas tank, it goes something like: Go full speed until the pump stops, wait 30 sec, go again, and then click it 3 times..... or something like that. Also, fill up at the same station, at the same pump if you can. Different suppliers have different % of Ethonal in their gas. Also, runing the tank as low as you can decreases the inacuracy of measurement error when filling up.

Try to drive consistant, and keep the vehicle as consistant as possible, with respect to maintanence. You will most likely notice a rise in fuel economy in the summer because when it is cold out, there is more O2 in the air, and you need to inject more fuel to make the engine run correctly. (you also get more power as well)

I am sure there is a lot more that can be done increase the acuracy of checking fuel economy, but this post is long enough.

Edit: I dont even know if this should be in this thread, but I wanted to say it anyways.

[Zanzer]

^^ Instead of a programmable odometer, many of us on here use GPS units to keep an accurate log of mileage. It may be easier and cheaper than installing an aftermarket odometer.

[Clay]

^^ Instead of a programmable odometer, many of us on here use GPS units to keep an accurate log of mileage. It may be easier and cheaper than installing an aftermarket odometer.

The TrailTech "Endurance" is about $80, not sure if it will require extension cords to reach the sensors. On the GPS note, TrailTech came out with a GPS this year (Voyager), and they actually have a input for wheel speed/odo because it is more acurate than GPS. The power is also automatic on these units, and they turn on with either a engine rpm sensor or wheel speed sensor. In the case of a dirt bike, they are in the back of a truck or trailer a lot, and you would not want to rack up miles that way (in the case of GPS).

GPS may work fine for cars though, since they do not turn very sharp, and I am sure the newer ones are pretty accurate. Doesnt work so well if you go on a ferry and forget to turn it off, or in tunnels.

GPS is for sure easier than installing anything aftermarket.

[htchbck]

My GPS power themselfs up and down with the car (plugged into cig lighter or hardwired on the ACC circuit). So that pretty much eliminates counting unnecessary miles, unless you turn the ignition on while its on a trailer/ferry, but I don't trailer mine very often. Car ODOs are very consistent at measuring revolutions of the wheel, since they are direct drive. When I get a new car, I first run about 200 miles with the trip odo and GPS both running. Then I get my % error from that, and just use that along with the ODO when measuring mileages. The speedo itself my fluctuate, but the ODO % error will be the same as long as the wheels and tires are the same. In the case of a wheel mounted sensors, you would have to adjust anyway if you changed wheels and tires, just like you would with a GPS, so its not really any extra work.

[rollertoaster]

So did you ad 6% to the mileage total when figuring your mpg's w/ the 13's?

Not exactly the stock speedo is calibrated to 155r12 ,so it's actually only a 3% error. I verified it with gps on a highway trip. But yes I do correct for the difference when I log my mileage.


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[Pu241]

Not exactly the stock speedo is calibrated to 155r12 ,so it's actually only a 3% error. I verified it with gps on a highway trip. But yes I do correct for the difference when I log my mileage.

How would the OEM speedo/ODO be calibrated to 155R-12's when OEM tires for the US were 145R-12's(steelies) and 165-70-R12's(alloys)?

[htchbck]

How would the OEM speedo/ODO be calibrated to 155R-12's when OEM tires for the US were 145R-12's(steelies) and 165-70-R12's(alloys)?

Because many speedos aren't calibrated that well, especially over time. I've seen all kinds of variations in festiva speedos. Technically the ODO, being direct drive, shouldn't change over time. I've had between -2% and +3% error in the odos of various cars, with stock size tires inflated to mfgr specs. My Miata with factory tires reads a little under 5% high on the speedo, about 3% high on the ODO. I don't think they calibrate them too well. I actually have one size bigger on my MX6 (195/70R14 vs 185/70R14 stock) and it still reads a little on the high side. ODO is dead on now though.

[Pu241]

htchbck,
I completely agree!
Everyone of mine when they had OEM tires was off one way or the other.
If you look close or disassemble a speedo/ODO I beleive it says "1025 rpm = 1 mile."
So I assume that's what they in fact are "calibrated" to.

[htchbck]

If you look close or disassemble a speedo/ODO I beleive it says "1025 rpm = 1 mile."
So I assume that's what they in fact are "calibrated" to.

Yep, 1025 is correct. I found those markings a long time ago when I was pioneering the 140MPH protege speedo swap. Thats how I was able to determine that using the 'ge speedo wouldn't change the calibration. There may be different gear ratios on actual speedo head inside the transmissions that account for differences in size, but after the cable leaves the trans, all the ODOs (early 90s MX6, protege, 323, and festiva) are all calibrated the same. Since the speedos themselves are not direct drive, there can be a lot more fluctuation in them though.

Edit: Come to think of it, the Miata one is a 1025 too lol. I was considering using it to gain the beautiful green lighted needles Sadly, I never found a (cheap) spare Miata cluster to pirate for parts lol.