Correcting for tire size
 

Despite a bunch of searching, I can't find a clear answer--

How do you correct for non-stock-sized tires? My xB came with 185/60/15's, and I have 195/60/15s. This is mostly because the only tires that I could find in 185s were garbage Walmart tires, which my car came with. The tires I've had since the beginning of June have a circumfrence 2% greater than stock. Is this even worth correcting for?

If so, how do I do it? Multiply my miles as shown on the odo by 1.02?

Needless to say, avoiding math was why I went to school for biology. I think in researching this, I've confused myself further.

You just need to DIVIDE your miles by 1.02. Whether it's worth correcting is up to you, but I do it and my correction is only 1.1%.

Hi,

I think the most accurate way to do this is to drive a measured mile, or a measured 10 miles at 60mph -- and time it with a stopwatch, and adjust the numbers based on the 1minute per mile.

Thanks, guys. I should borrow a GPS to double-check my odometer accuracy anyway. That 2% figure is based on the miata.net tire size calculator.

I'm doing the same thing to my car soon...205/60's to 215/60's which is a 2% difference. This does make a difference in your mileage calculations, and since we're all working for every additional mpg, I will definitely compute the change.

I just did the corrections on my fuel log, and noted them. It took my actual mpg down slightly, but that just means I need to be more motivated to be efficient. I'm still doing really well the last couple of tanks, though, so I can't complain.

In happier efficiency news, I talked my girlfriend out of driving 50 miles each way to Erie, PA to see a movie. That's just shy of four gallons in her car!

Hold the horses!! You went from 185/60s to 195/60s correct?...i.e., taller tires. If so then your odometer is reading 2% too LOW. You need to multiply your odometer readings by 1.02 then divide by the gallons used. You're calculated FE in your log should go up not down. If the tire is taller, you travel further for every "mile" read by your odometer as compared to your old tires. Taller tires give you MORE effective overdrive which in most (not all) cases will increase your mileage. This is one major reason I'm going to taller tires next week.

See, that's what I originally thought, then I started second-guessing myself. It's easy enough to change the logs again, though.

The worst part is, I actually had this straight at one point, because I used to have a truck with somewhat oversized tires, so I actually sat and figured it out way back then (2001 or so).

Edit:
Correct corrections added to fuel log and my spreadsheet.

This boosts my actual FE up quite a bit!

whokilledthejams -

Doesn't it feel good? It's like a taller-size-tire dividend when I enter my fuel log.

I'm coming up on my GPS correction check. I try to do it every 6 months on the assumption that the tires should be getting smaller over time.

CarloSW2

Here's another tire size calculator, this one you can compare four tire sizes to the stock size. Looks like I will be getting a better size tire for my Hypermiling.

http://www.1010tires.com/TireSizeCal...bmit&reset=yes

Sorry about that. I wasn't looking at your numbers close enough. Oops.:o

i just made excel do the work for me. i enter miles (as shown on trip OD), total cost, gallons and date. excel calculates corrected miles, mpg (from corrected miles), cost per mile, and cost per gallon (just so i don't have to enter it manually). then i use that info for my EM fuel log.

oh yeah, i also have it calculate avg mpg, total miles traveled, total gallons used, and total cost.

if anyone needs any excel help, i'd be glad to post up my excel log and my formulas

run along a measured mile and reset your trip odometer at the begging and read it at the end of a mile. That will be how far off you are. 1.2 is running faster...smaller tires .95 running slower...bigger tires. your will get almost an exact measure of the percent off. I do whatever I can to get mine near 1.0, the computer bases so many tuning parameters on speed.

What does it take to get a car's speedo recalibrated? I have smaller tires than stock, and I think my stock tires weren't even well calibrated. I'm actually about 5% off, i think.

AFAIK on old cars the speedometer pinion would get replaced. On newer cars I would guess that the dealership can change a setting in the ECM.

Some modern cars and trucks (particularly trucks) that have aftermarket ECU-tuning devices available can recalibrate for tire size changes. Of course, the catch is that this does not actually apply to most people, especially not here. Odds are, you pretty much have to live with an inaccurate speedo/odo. Consult your relevant 'tuning enthusiast' communities where applicable.

Interestingly, on the xB, my non-stock tires make the speedo more accurate, while making the odo read low. The stock size tires work for the odo, but make the speedo about 2% low. It's really bizarre that Toyota did that, on top of the fact the Japanese version of the car uses a yet another tire size (185/60/15, I think).