Need Someone to Check My Math

[drkanglr]

Okay, so I got a new truck w/ some 32" tires. The speedometer/odometer are not calibrated, and I'm trying to figure the difference out. I think that I have it, I just need someone to check my math.

Constants: Stock tires 265/70R15, 705 revolutions per mile (according to Goodyear specs)
New tires 32"/11.5"/15", 658 revolutions per mile (Goodyear specs)

5280 feet per mile

Speedometer first:

705/1 = 658/x so:

705x = 658 so:

x = 0.93333

55mph/0.93 = 59.14, or about 60 mph. So I need to drive 5 mph slower than my target speed.

Now the odometer:

705/5280 = 658/x, so:

705x = 3,474,240; so:

x = 4928 (to check math here, 4928/5280 = 0.93333)

So, if I divide 1 mile by 0.93333, I get 1.07. That should equal the true distance traveled by the larger tires, correct? So if the odometer says there are 38,000 miles (assuming the larger tires were put on w/ no miles) then the real mileage would be about 40,714 miles.

Let me know if that's right. I'm trying not to spend cash on getting a recalibration done, so if I know how fast I need to drive to avoid being pulled, and I know what factor to multiply my trip meter by to get true gas mileage, that'd be great. Thanks, I know there are some folks here that are smarter than me that can support or disprove my work.

[matthepepe]

not 100% sure about the math either, but having a brother thats a cop, the reason they have a cushion for pulling people over some cops its 5 mph and some its up to 10 mph depending on the city and roads traveled, they give you those few mph because its very possible the speedometer isnt calibrated and the person has no idea they are speeding and for the fact they have better things to do then write a speeding ticket for 3 mph over ;)

[Talon]

Your math looks right to me. Keep in mind though that your 5mph buffer isn't constant. It will vary up and down depending on speed. Using 5mph sounds like a reasonable all around factor to use though. Recalibration, I'm not sure how intensive that is, shouldn't be that hard or expensive. I know you can do it with Hypertechs Power Programmers yourself by plugging it into your diagnostic port under your dash. You might be able to have any shop or repair facility do that the next time you take it in for an oil change or any other kind of work. Did you have the tires/wheels changed at a store, Goodyear for example, if so, they probably should've done the recalibration for you. Might be worth checking out. Nothing more annoying than always having to add 5. LOL

[matthepepe]

your original tires specs were ... 265/70R15
what are they on the new tire.... i dont know how to figure that out w/ the stuff you posted.

[drkanglr]

The new specs aren't measured the same. They're 32 x 11.5 x 15. That was one of my problems, I found a few calculators on the net, but they all needed the measurements w/ aspect ratios and "mm" measurement like the stock tires.

Thanks for double checking for me!

[Presto333]

from wut i can see ur math look right, but like talon said, the amount will differ a bit depending on how fast ur going, but like matt said they arent going to pull u oer for a couple miles per hour so 5mph is a good number...

[Sloffie]

i put 32's on my cherokee this summer, and i would say that it is about a 5 mph difference at lower speeds but the rate is definatly higher at highway speed, more like around 15 mph, depending on what vehicle i there should be a gear you can put in to equal out the new gearing. to find out your real difference just drive behind a friend of yours at a several specified speeds.
But thats not why cops keep pulling me over, make sure the new tires dont come out of the fender wells more than a couple of inches, they love bustin me for that one. to find out your real difference just drive behind a friend of yours at a several specified speeds.

[Presto333]

that reminds me, u can drive next to friend and write wut ur speed is at for different amounts of rpm's (lol, i saw that one on pimp my ride) might be a little better than trying to add a certain amount to ur speed...

[Bigal1]

I think your math is right. Here's how I would do it:

Assume the speedometer is correctly calibrated to tire revolutions (is that correct? I thought it measured engine rpm...)

Old tires:
705 revolutions per indicated mile

New tires
658 revolutions per actual mile

55 (indicated miles)/hour times [(705 revs/(indicated mile) /(658 revs/(actual mile))] gives you 59 mph and some change.
Doing it the way I have done it above you can do dimensional analysis to see that it works dimensionally as the final units will be in actual miles/hour

Use the same ratio for converting indicated miles on the odometer to actual miles.
(solution left for the student... :yawn: )

[Emmit]

that reminds me, u can drive next to friend and write wut ur speed is at for different amounts of rpm's (lol, i saw that one on pimp my ride) might be a little better than trying to add a certain amount to ur speed...
an old 4x4 magazine actually suggested this when going to larger than stock tires.

I put larger than stock tires on my truck and wanted to get my speedometer calibrated. I was told that it wasn't really possible/feasible for such a small increase in tire size (I only went up and out an inch) as the cog wheels are in increments of 8mph.

So instead for about the first month or so I drove with a GPS on my dash and used it for speed. I still do often on the interstate, but in town I leave it at home now.

For an increase in 1 inch it's about 3mph at 60-70mph.

[C.Carles-AOG]

Try this site:
http://www.miata.net/garage/tirecalc.html

[drkanglr]

Try this site:
http://www.miata.net/garage/tirecalc.html

That's similar to some of the speedometer calculators that I found online. The problem is that both tire sizes have to measured the same way. i.e. old tire: 265/70R15 but my new tires are measured as: 32/11.5/15 and that option is not there. Thanks, though. I found a place in town that'll do it for $60. I'm going to try to get the dealer to do it, since technically the odometer has a false reading on it. We'll see.

[Emmit]

That's similar to some of the speedometer calculators that I found online. The problem is that both tire sizes have to measured the same way. i.e. old tire: 265/70R15 but my new tires are measured as: 32/11.5/15 and that option is not there. Thanks, though. I found a place in town that'll do it for $60. I'm going to try to get the dealer to do it, since technically the odometer has a false reading on it. We'll see.
Dealer will likely tell you no, since the false reading is not a manufacturing defect but caused by modifications made to the vehicle.

[matthepepe]

aren't the old specs in mm .. if you just change the new specs which appear to be in inches you should be able to use the calculators online.

[drkanglr]

It's mm, but the second number isn't a direct number, it's an aspect ratio that is devised using the hight/width of the tire or some crazy thing like that. I don't remember exactly how it's figured out.

[El Pirata]

Forget the hard way of figuring it out, get a radar gun and check your speed while going down the road from fixed objects.

[russ]

Way too much work...
Its a simple ratio problem.

[Bunker 51]

I had the same problem with my jeep i put 40" tires on it and i couldnt calculate the math so i thought o well i'll just go as fast as all the other cars on the road

[thedeem0n]

Buy a TI-83 Plus...I'm currently going to school to be an Electrical Engineer, & my Father is a Master Electrical Engineer (meaning he's got a masters degree) with 35+ Years Field Experience doing Research/Development, Surface Level and Surface Mount Troubleshooting, & a whole bunch more §§§§... & We both use TI-83 Plus Graphing Calculators and we use Microsoft Windows Calculator set to Scientific to Calculate Binary.....