roninb18

if i'm going to put bigger tires on the truck i might buy....what else do i need to do?...does it throw off the speedo and odometer??....change gearing?....i'm thinking 31's or 32's, probably all terrain ta's

White SR5

Throws off speedometer, not odometer though. You will lose some acceleration if you don't regear too, but it's not a lot. Now I'm not sure on this one, but I think as a general rule, for every 1" bigger tires than stock, you can add .5mph for every 10mph that you go. For example, I went from 31" to 33". That's a 2" gain. So for every 10mph I can tack on 1 more mph. If it reads 10mph, it's closer to 11mph. If it reads 50 mph, it's closer to 55mph. If it reads 100mph, it's closer to 110mph, etc. Don't quote me on that though, I'm not sure.

Zach

Tacoma Dude

That math sounds about right to me (from what limited experience I have with the tire calculator).

But I wasn't aware that it would not affect the odometer? If it affects the speedometer shouldn't we expect to see a difference with the odom. too?

Would you run into any odometer differences if you went to a smaller tire?

Darren

Yes, the odometer is affected as well.

AverysDad

my understanding is that the odometer would be affected but i think the effect is very negligable..........

i think the extra distance per revolution of a bigger tire would probably only make a difference of a few hundred miles after 100k miles were clicked by on the odometer....but im not a math major so correct me if im wrong

:-)

Tacoma Dude

Well I would assume that if with one inch you have .5 mph per every 10 mph difference to equal 60 mph and actual speed of 63 mph then likewise after driving 60 miles you would have actually traveled 63, correct?

Doing some quick work on a calculator that means after your odometer clicks on to 100K miles you would actually have traveled 105K

Darren

The difference is the same percentage difference in the tire size.

For me, going about 300 miles on the trip/odometer equals ~322 actual miles driven. That definitely sounds like it will add up over time to me. Actually, that's quite a bit over just the 300 miles.

I use the 300 mile example as that is about 1 tank of gas.

rowan

i was just working on the math for this actually, because im trying to find out what i am actually getting for gas mileage.

it took me a while and im not sure if this is right but... from going to 225/75R15 (about 28.3s) to 32's, i go about 60 miles further for every 300 that i go. (was working with 1 tank of gas like darren)

the % thing comes out a little differently for me. about 20 miles less. my math might be wrong though. http://www.miata.net/garage/tirecalc.html is a good tire calc. and it will tell you how much your speedo is off by. you have to know the metric sizes of your tires though, which i do not...

White SR5

Well, I was thinking of it a bit differently. The speedomoter measures the speed you are travelling at with stock sized tires. When you get bigger tires, the tire circumference increases, so in one rotation of the tire you get a bit more distanced travelled. The full 360º rotations for a stock tire and the bigger tire are the same, but the distance travelled is not. Now the odometer is also measuring the miles travelled by the stock tire. You have the same number of full 360º rotations, only the distance travelled isn't the same. If you think of it in rotations, it will not be off. In other words, if you were to put the stock tires back on when the big ones wore out, it would be as if you had never put the bigger tires on there in the first place -- the mileage would be accurate. But if you are trying to figure out the # of miles you are travelling, then yeah it would be off. But hey, think of it this way -- when (if) you sell the truck, it will appear to have less miles on it than it actually does!

Zach

keisur

it's not a straight line curve relationship. the difference increases the faster you go.

here's a good calculator to use: http://www.4lo.com/calc/gearspeedo.htm

White SR5

Hehe, hey Keisur, I used your calculator. If the original stock tires were 30 inches and I put on 90 inch tires and the speedomoter read 10,000mph, I would actually be going 30,000mph!

Zach

keisur

Is that your next mod? if so, you may need to regear.

kyle_22r

hey, anybody know how much it would be to get your speedometer recalibrated, especially on an old truck like mine?

keisur

here: http://www.dakotadigital.com/Detail....rtNumber=SGI-5

And a write-up on a Taco install to boot: http://128.83.80.200/taco/speedo.html

However, I'm not sure it would be the same on yours, I mean, yours is older so it may just be that you have to adjust a gear or speedo cable or something. anyway, you can at least get some good info from those sites.

you can also do an internet search for 'toyota speedometer recal' or even ask some of the TTORA guys.

Flygtenstein

After several tire changes and one gear change, I would say the time and effort spent fixing the speedometer is not worth it.

With 28's and 4.56's, stock, my speedo was right on as well as teh odometer.

With 31's and 4.56's, my speedo read a little low, like a couple percent. The odometer was off almost a tenth.

With 33's and 5.29's, the speedo and odometer are right on.

Go as fast as the traffic around you, or drop 200 bucks resolving the "problem."

rowan

not to get off topic... but Flygtenstein, are you over at 4x4wire? think i saw you a couple times.. but i am just now getting my password back. im rowan there as well.

white- that was about what was going through my head before i started doing the calculations. you try figureing out how much my mileage is off by. it gets confusing.

225/75R15's to 32s. i went 300 miles on my odometer with 15.8 gallons. whats my actual gas mileage?

JSharp

When you go to a different tire size your speedo and odo will be off by a fixed percentage, the same percentage as the differences in tire size.

Example -

265/70 X 16 = 30.7" diameter.
265/75 X 16 = 31.6" diameter.

31.6/30.7 = 1.029

Assuming things were perfect before you changed tires -

At 60mph indicated your true speed would be 60*1.029 = 61.76mph

1 mile shown on the odometer would mean you've really traveled 1.029 miles.

Whatever mulitplier you come up with for your change can be used in any speed or mileage calculation or gear ratio calculation. No magic involved.

Most of the time it's not enough to worry about but if you make large changes over the long term it matters...

Robinhood150

Get a GPS.

rowan

have one. don't use it often in my truck though

Flygtenstein

Yeah, I am that guy from Wisconsin with that truck that keeps popping up in Colorado, Utah, Arizona and such.

GPS is a good resolution to the issue, costs about the same as fixing the speedo and it will work with all possible tire/gearing combinations.