Correcting for tire size

[extragoode]

Originally Posted by whokilledthejams

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!

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

[Tony Raine]

Originally Posted by whokilledthejams

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!

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

[klrv6]

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.

[MazdaMatt]

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.

[wikityler]

Originally Posted by MazdaMatt

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.

[whokilledthejams]

Originally Posted by MazdaMatt

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.

Originally Posted by wikityler

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).