tire size, speedo/odo adjustment, and fuel economy

[dlb]

tom mentioned something in another thread that got me thinking about speedo error due to larger tires: i find that my speedo is off by about 8% by running 185/80R13s, which i think are roughly the same diameter as the 195/70R14 that some of the guys on here are running. i was just thinking about how that error would affect my fuel economy math--for every 100 km i clock on the odometer, i should add another 8 km. does anyone else take this into account when calculating mileage?

[hberdan]

That's a really good question.
http://www.tiresizecalculator.info/
My speedo, with stock 175/70R13s, always reads 5mph too fast--If I am traveling 55, it reads 60, 25, it reads 30, etc. Very consistently. Do I use an algorithm to figure out actual % of difference, as the wheels are turning slower than the speedo indicates?
I dont, as I assume that the odometer is still measuring actual distances traveled. Is this correct?

[dlb]

great link, thank you! i was pretty darn close, here's what it said about 175/70R13 vs.185/80R13:

Original tire
Sidewall: 122.50 mm
Radius: 287.60 mm
Diameter: 575.20 mm
Circumf: 1,807.04 mm
Revs/km: 553.39
New Tire
Sidewall: 148.00 mm
Radius: 313.10 mm
Diameter: 626.20 mm
Circumf: 1,967.27 mm
Revs/km: 508.32

Speedometer reading with new tire is 8.87% too slow.
When your speedometer is reading 100kph you are actually traveling 108.14kph.

[arbskynxnex]

Not with Red, but I did when I had my truck. It had oversized tires that gave about a 10% error on the speedo and ergo the odo. When I did calculations without accounting for the error my mpg was 13, but when I corrected for the error (making it the actual mpg) it bumped it to about 14.5 mpg.

[takza]

Good idea to check actual distance covered? You can use Google Earth to determine exact distance on a straight length of highway and then check vs your odo. You could use a GPS? Might be able to use 10 miles worth of mile markers on a highway?

[arbskynxnex]

Mile markers would be the best to check against, especially if you have a speedo test zone nearby, those should be the most accurate. There is distortion on maps (digital or physical) from converting a round surface to a flat representation so depending where you are in comparison to the map's datum your distances on the map will change from what is on the ground. I use GPS for archaeology work and it is not as accurate as people like to think, actually its pretty good if you have the several thousand dollar units in a clearing on a clear sunny day, but if you don't meet one of those three criteria you can get some pretty inaccurate readings.

[dlb]

just finished a tank of gas and calculated my mileage taking the tire size into account. came to 12.9 km/L, or 30.3 miles/gallon, and that's mainly made up of short 6.5 km trips to work and back so i'm pretty pleased. like some peeps, i've had one or two mysterious tanks that got as much as 35 miles/gallon but god only knows how the stars aligned to create that miracle--hwy driving, wind at my back, ambient temps, lots of downhill, altitude...

[deejay1272]

just finished a tank of gas and calculated my mileage taking the tire size into account. came to 12.9 km/L, or 30.3 miles/gallon, and that's mainly made up of short 6.5 km trips to work and back so i'm pretty pleased. like some peeps, i've had one or two mysterious tanks that got as much as 35 miles/gallon but god only knows how the stars aligned to create that miracle--hwy driving, wind at my back, ambient temps, lots of downhill, altitude...

Nice! I recently achieved my best MPG ever. I hit 33.9 MPG on a recent trip to Mount Rainier in WA. Strangely though, on my way back to Portland (100% freeway driving), I saw the first half of my tank disappear in around 100 miles - suggesting as low as ~22-24 MPG. Both cases, I was driving very conservatively, but at an average speed of closer to around 60 on the way to the mountain and about 70 on the way back. I'm looking into why the two cases were so disparate.

If I can be in the 29-33 MPG range all the time, then I'll be very happy...

[Petros]

the half mark on the fuel meter is not even close to half way do not use it. I get about 300 miles to the tank, a bit more on long trips, the half way mark is usually at about 115 or 120 miles.

The speedometer and odometer do not seem to be related, and nor can the error be attributed to tire size. Most of my Tercels the speedo was right on, my current oversized tires have it going about 1 mph slower than the speedo shows (per my GPS), but the standard size tires did not. Larger tires should error in the other way. And the same year Tercel4wd I pick-up in NM has a very large error, about 5 mph at 60 mph. But it has stock tires. So it may have to do with the speedo itself, or even the amount of wear on the plastic speedo gear in the trans. I would clock the odometer separately from landmarks on the hwy (or a speedo test section on the hwy).

[deejay1272]

I would clock the odometer separately from landmarks on the hwy (or a speedo test section on the hwy).

So this may seem like a ridiculous question, but I don't really understand how one is supposed to use the hwy speedo check sections. I never had these growin' up in Ohio!

[dlb]

the mile/km markers are on the sides of the highway and are posted every mile/km. what petros and takza are suggesting is taking note the next time you drive on a hwy of your mile/km when you pass one and then checking again when you pass, say, a tenth marker. so if your odo is at 100.0 miles when you pass a '80' marker, your odo should be at 110.0 when you pass the '90' marker. it'll be more accurate the further you measure it so try to keep track on a long trip. i'll do this next time myself. as user arbskynxnex points out, GPS may be just as inaccurate as our speedos/odos so i think you have to check against multiple sources if you want a more accurate picture.

[dlb]

i've tested this a few times now and my odo is definitely off, according to the km markers. i consistently find my odo reads that i've traveled .9 km when i've actually traveled 1 full km. this is actually even more out than the calculation from the aforementioned website stated, and i will now add 10 km for every 90 km that the odo reads i have traveled when i calculate my mileage.

neat!

[splatterdog]

I have 195/70/14's and I see about a 10% error accross the whole range. To stay out of trouble I add 3mph in the 30's, 4mph in the 40's,etc. GPS and the speed check trailers used by the cops were used in my calibration.

[danzo]

The last few days I've been rolling on Toyo 205/45-16's on the front and have noticed that the speedo reading is a bit less than one mph higher than the GPS mph at highway speeds. The tires are worn down to the wearbars so I believe the speedo would be really accurate (i.e. same as GPS) if the tires were new and had more tread/larger diameter.

So according to Tire Rack the diameter for this size is about at 23", give or take .3" by tire make/model. This gives a circumference of 72.26". TR also says this tire should make 900 revolutions to cover a mile, but it would cover 5419', or 1.026 miles, so this diameter is a bit off.

Assuming that a T4wd with a 4.11 (can't speak for the 3.73s) will have a fairly accurate speedo when the drivewheels turn at 900 revs/mile, then a tire with a circumference of 70.4" would do the trick. This gives a tire diameter of 22.4" for a theoretically kinda accurate speedo.

Purchase accordingly, or live on the wild side and get the tires with the coolest looking tread design.

[Petros]

I have seen different Tercel4wd with the same size tires have very different speedo errors, up to 10 percent or more. So from one car to the other I do not know if there is an "rule" you can apply to your particular car. Must differences in speedo, cable slop perhaps, wear on gear in trans, or who knows what else.