gohawks

Okay I have figured out that my 1988 4Runner came w/ the tire size 225/75R15 (correct me if I'm wrong).

Currently I have 30x9.5R15 size tires on the Runner.

My question is how much does the difference between these two tires change my speedometer/odometer?

I have tried using the various calculators online but due to my ineptitude with math I can't figure it out exactly.

Any help would be appreciated. I want to get a better idea of what my fuel mileage actually is.

Thanks.

JHupp

The best way to do this would be to first convert your standard measurement to a metric one, most tire calculators are metric, the conversion can be found here, http://www.redrock4x4.com/tech/tire_...ethod=standard So your standard tire is an aproximate 241/79/15, next I would go here, http://www.miata.net/garage/tirecalcold.html and once you plug in your values, it will actually show your tire differences and how much your speedo will be off by. I would round your standard tire size to a 235/80/15 ot 245/80/15 so your speedo will be about 6.2% too slow or when it reads 60 mph. you will be traveling about 64 mph. Of course the more fun way is to solve it out using good old physics, but I thought it would be easier to cheat and use the websites.

gohawks

Thanks for the quick reply. So is my mileage off 6.2% as well?

eric-the-red

Yes, you actual mileage will be 6.2% higher than what you measure with the odometer.

Flamedx4

Jeez.
Go to a tire chart like the ones at BFG or at Tire rack and look up tire sizes. Get the diameter. (doing the math to get the theoretical metric size doesn't help much, and is most often inaccurate.) 225s are roughly 28 inches tall. No need to figure it to 3 decimal points.

With a simple calculator you can find the percentage you are off if that's what you want to know. 28 + 7% = 30 Your speedo is low by 7% So 40mph indicated= 43 actual. 65mph indicated = 69 actual.
Don't forget this when you figure your gas mileage. Take the miles on the odometer and add 7%, then divide by the gallons.

If you want to know what size gears would be best for a diffferent tire size, you can use the same percentage method, but cross-multiplication is the really useful tool.
Lets say you want to run 33s.
4.1 * 33 = 135.3 135.3/ 28 = 4.83 (so 4.88 would be the best choice.)

gohawks

Geez,Lost me already, but I'll take your word for it that I'm 7% off, I can figure the mileage out from there.

Thanks for the help

tc

My speedo reads 70MPH, the GPS reads 71 MPH after going from 31's to 33's (6% difference) - obviously, the speedo wasn't too accurate to start with.

I would not assume that the speedo was right from the factory. Find out exactly how much it is off with either a stopwatch or GPS. To do it with a stopwatch, get a buddy, and time how long it takes to go 5 miles (using the freeway mile posts on a flat, straight section using the cruise control), and figure it out from there.

Convert the time into hours by dividing the seconds by 60, then adding THAT result to the minutes, and dividing THAT summation by 60. Divide 5 (or however many miles you go) by that final result. EXAMPLE:

It takes you 4 min 35 sec to go 5 miles.
35/60 = 0.5833
4+0.5833 = 4.5833
4.583/60 = 0.07639
5/0.07639 = 65.5 MPH

ebelen1

I've used the charts and they say I'm about 11% off. I drove w/ a buddy at 55, 60 and 65 mph and I was 10% off. Just got a GPS and I'm somewhere between 8 - 10% off depending on my speed. For example:

1. At 40 mph on the speedo, I'm really going 44 on the GPS;
2. At 50 on the speedo, I'm really going 54 -55 on the GPS; but
3. When I'm going 70 on the speedo, I'm only going 76.

Pretty weird as it's not consistent but the GPS unit is the consistent factor.

JHupp

If I am not mistaken, which I very well could be, the odometer, and speedo, are off more at higher speeds than they are on lower speeds. so, at 1 mph, there is a .3 mph difference. I havent done any serious checking on the charts, but here in jersey we have displays on the highways that tell you how fast you are going, and tell you to stop speeding, my odo reads 60, the sign says about 64 to 65, and the chart says I should be of by 6 mph at 60 mph with my tires. I think they may be a little high, to be on the safe side, but were pretty accurate with my somehwat precise test.

farmerj

If you multiply 10 MPH by .062 you get .62 MPH difference

If you Multiply 60 MPH by .062 you get 3.72 MPH difference.

So you can't really say you are 3 MPH or 6 MPH off. It depends on how fast you are going.

At any point you will be ~6% off in MPH.

JHupp

good, so I was right with the faster/slower thing

Flamedx4

Sorry, wasn't referring to you, was referring to that 241/79/15 stuff.

JHupp

yeah, like I said the tire falls between a 235/80/15 ot 245/80/15, advised that it was an estimate only, ohh, and the speedo is 7% off, depending on how fast you drive, the odo will be off, but its hard to say, it will be under your actual mileage which is good for resale, but not by much

Flamedx4

Good point. My truck had 37s on it, then 35s for at least 2 years with stock gears. Assuming stock tires prior to that, looking at the mileage on the tires reciepts that I got with the truck, I would guess that I actually have 235,700 miles instead of the 218,900 that is showing... Ick.

JHupp

not a bad deal at all, if only I could just stay at 62k, for another 3 years, would be niiice