Tire Size --> MPH & ODOM??

[bonsaiwino]

Hi again gents, So a while back I converted our '79 Spider to 15" tires. Now, I'm wondering how that changes my speedo & odo readings?? Is there quick & dirty formula to figure out how fast & far I'm going? Many thanks, Don

[divace73]

Hi again gents, So a while back I converted our '79 Spider to 15" tires. Now, I'm wondering how that changes my speedo & odo readings?? Is there quick & dirty formula to figure out how fast & far I'm going? Many thanks, Don

If the over all diameter is roughly the same then the speedo and odo would be similar, best way is to compare it with a GPS unit which are far more accurate that any of the Veglia (Vague liar ) gauges

Some speedo calibration companies are able to put a box in-line with the speedo cable that changes the ratio accordinly

[76was124]

The GPS suggestion sounds like the easiest /accurate way.

If you really want to play with the math. I believe it would be a straight ratio of the two wheel circumferences. Circumference = 2 x pi x radius. Pi being approximately 3.1415

So if the speedometer with new tires shows 60mph, the real speed would be 60 x the new tire circumference / old tire circumference.

You could wrap a piece of rope around the circumference of the old and new tires. Mark the lengths then measure each with a tape measure to get a good estimate of circumferences, then use their ratio times the speedometer value. Remember, bigger outside diameter tires will show a slower speed than you are actually driving.

[paintdudeluke]

measure the inflated diameter of the old and the new determine the % of difference. My 185 13s are nearly the same as my 205 50s.

[Dawgme85]

What size are your 15" tires? 195-55x15 are almost identical to the original 165-80x13 tires. You can also go to http://tire-size-conversion.com/tire-size-comparison/ to do a direct comparison.

[chrisfiat]

You could wrap a piece of rope around the circumference of the old and new tires. Mark the lengths then measure each with a tape measure to get a good estimate of circumferences, then use their ratio times the speedometer value. Remember, bigger outside diameter tires will show a slower speed than you are actually driving.

or you could just use the tape measure

[seabeelt]

There was a tool of sorts on Minafori. I will post if if I can find the link.
R/

[phaetn]

Great link to a very useful chart, Dawgme85!! Much quicker and easier than measuring yourself and calculating!

If one must do it "old school", another way to measure is to make a chalk mark on the tire at its lowest point, also make a corresponding mark on the ground, then rotate tire a full revolution, mark ground again and check distance between the points. No wrapping around the wheel required.

Bigger tire circumference means lower speedo reading than actual velocity because a greater distance is travelled per revolution compared to the Speedo's original calibration.

My experience on motorbikes and cars has been that larger diameter wheels potentially improve handling and braking because of increased tire contact patch, but it depends on if other components are up to it because of the increased rotational mass. You need powerful enough brakes to actually slow/stop the bigger wheels, and ride height can also be affected so there's a new CoG if the overall circumference of the tire is greater or smaller than previously.

Of course, a greater tire circumference negatively affects acceleration, but lowers RPM for a given speed and gear, too, so it's easier on bearings. Then there's the side benefit of higher top speed if the engine has enough power to get there. Everything is a trade off.

I suppose the best of all worlds is a bigger wheel with a low profile tire so circumference is close to the same but gives the benefit of bigger contact patch... Looks like some of you are right in that sweet spot.

[courtenay]

Here you go - http://www.tacomaworld.com/forum/tireca ... -265-75r16
Just plug the numbers in.

[bonsaiwino]

After plugging in my original tire size & the replacements (165x80x13 & 195x55x15) into both tire comparison links, the speed is nearly identical. You guys & your resources never cease to amaze me. And here I was hoping that 70 mph night translate to 85mph! All the best, Don

[toplessexpat]

Always worth being aware of the "minor" variances though. You don't want to be cheerfully bowling along at an indicated 65mph in a 60mph, but discover courtesy of the local law enforcement that you're actually doing 70mph.

[76was124]

or you could just use the tape measure

Yeah that's what I originally typed, but figured one of the tires was still on the car so string bends better and doesn't care if you drive over it