Quick Speedo/Revs per mile question

[RollingThunder]

I just bought a new wheel/tire setup and need to figure out my revs per mile to resynchronize my speedometer. Does the speedometer take its reading from the front or rear tires? And what is a reliable calculator for revs per mile? Each one I've found gives me a different number. I'm running 275/40/18 and 285/40/18.

 

[ArizonaGT]

Reading is taken from the rear.

Look up the revs/mile on the tire manufacturer's website; if you are lazy, my own calculator says 748 for a 285/40/18.

 

[RollingThunder]

Manufacturer's website says 770. That seems like a huge difference from 748. I guess I should just go with what the manufacturer says?

 

[Dread53]

Put in whatever number you think is closest and find a 60 mph highway with good mile markers. It should take 60 seconds to cover one mile at 60 indicated. If it doesn't, adjust the number until it does. Or do the same thing with a GPS in the car and compare the two numbers. That's the way I set mine.

 

[JeremyH]

Its more accurate to measure the actually rear tire diameter and use that for the formula. Just using the tire size and not the actually diameter as it sits inflated with loaded suspension will be 10-20 revpm difference due to tire manufacturing difference/sizing.

I used the tire size formula to set my revpm initially and i was still 6-7mph off when cruising at 70mph.

So i measured the tire diameter in inches and used that in the formula and now my speedo is within 1-2mph of vgps all the way up to 80mph.

 

[Vapour Trails]

Its more accurate to measure the actually rear tire diameter and use that for the formula. Just using the tire size and not the actually diameter as it sits inflated with loaded suspension will be 10-20 revpm difference due to tire manufacturing difference/sizing.

I used the tire size formula to set my revpm initially and i was still 6-7mph off when cruising at 70mph.

So i measured the tire diameter in inches and used that in the formula and now my speedo is within 1-2mph of vgps all the way up to 80mph.

The key is to use a rev/mile calculator that takes into account deflection of the tire under load. Generally they add about 3% to the calculated value.

For instance, theoretically a 235/55/17 should be 742 revs/mile if it did not deform.

742*1.03=765
I have confirmed that this is the correct setting using a GPS to monitor speed.

 

[69Mach1-409]

I started with the base reading from the manufacturer, found a back country road, held her steady at 50mph, stopped, and adjusted it until it was in between the Pioneer GPS HU in my car & my 'rents Garmin portable GPS.

*I know GPS units aren't 100% accurate, but they were reading within 1.5mph of each other and I set the car's actual speed reading to fall in between the two.

 

[RollingThunder]

I don't have access to a GPS unit, but there are alot of the radar signs that display your speed around here. Does anyone know if these are accurate enough to use as my reference in place of a GPS?

 

[69Mach1-409]

I don't have access to a GPS unit, but there are alot of the radar signs that display your speed around here. Does anyone know if these are accurate enough to use as my reference in place of a GPS?

No idea.. it would depend if they were calibrated correctly I'd guess.

Could you have a friend with a 'new' vehicle drive at a set speed and you pace them.. adjusting as needed?