My friend just turned 16 and has baught himself an Isuzu with oversized tires.

Runs very well and everything works except the windshield whiper moter but with the oversized tires the speedo is out.

Is there a way to correct this without having to take it to a shop?

Thanks.

Put on sensible tyres? That would improve everything from handling, acceleration, ride comfort (damping matched to original specification wheels), and speedo calibration.

Otherwise no - you're probably going to have to either buy a custom inline gearbox (inline with the speedo cable), or send the speedo unit away for recalibration...

Thanks for the help.

BTW: his tires arn't exessivly oversized, just enough to make the speedo useless.

http://i3.photobucket.com/albums/y84/pigeon2244/IMG_3323.jpg

'Just'? They don't fit in the wheel arches

The easiest way to get round the problem is to get someone with a car with an accurate speedo to drive at set speeds and read what the speedo says. If you know what the original tire radius was you can quickly work it out as well.

wait wait?

What happend to the speedo now?

I don't get it.

Does it say that your driving faster than normal on the speedo or???

perhaps someone can clear this up for me?

Quote from Takumi_lfs :

wait wait?

What happend to the speedo now?

Larger tires = larger circumference = greater distance covered per wheel rotation = speedo under reads

Now I get it, But not enough to explain it.

hmm

So becasue of the tyre size it actually turns faster/slower than the speedo which can't be fixed?

E: unless you change the tires again?

Thats better than my situation, my speedo fails everytime I drive into the town center. The same place everytime it fails and won't go back on properly until I turn the car off and on. It happens to be an area full of speed cameras too, so I usually end up doing 20mph until I get to a set of lights and turn the car off and on again. Its going into the garage this week to find the problem.

Quote from Takumi_lfs :

Now I get it, But not enough to explain it.

hmm

So becasue of the tyre size it actually turns faster/slower than the speedo which can't be fixed?

He has larger tires, which means that to travel the same speed, the wheel does not have to rotate by as many degrees. This means that the driveshaft is not spinning as quickly, so the speedometer thinks the car is travelling at a slower speed than it really is.

Picture a marble, and a soccer ball. To move the marble one inch you have to roll it a full rotation, whereas to move a soccer ball requires rolling it only maybe 1/60th of a rotation or so. Of course, it's not that big of a difference with his car, but big enough to give a 3-5 mph slower reading.

Quote from Stang70Fastback :

He has larger tires, which means that to travel the same speed, the wheel does not have to rotate by as many degrees. This means that the driveshaft is not spinning as quickly, so the speedometer thinks the car is travelling at a slower speed than it really is.

Picture a marble, and a soccer ball. To move the marble one inch you have to roll it a full rotation, whereas to move a soccer ball requires rolling it only maybe 1/60th of a rotation or so. Of course, it's not that big of a difference with his car, but big enough to give a 3-5 mph slower reading.

Thanks, Added in my brains.

It's really great to learn from people over the net

And how do they actually recalibrate it? I mean like on LFS?

Turn full left and right, but on a car drive full speed then stop?

Quote from Takumi_lfs :

And how do they actually recalibrate it? I mean like on LFS?

Turn full left and right, but on a car drive full speed then stop?

If only:

"Yes officer?"

"You DO know you were doing 30 over the limit."

"Oh, I'm sorry - I think I forgot to calibrate it this morning."

"Oh, alright then - carry on. Don't let it happen again!"

theres several ways to do it, you could stick the car on a rolling road or something similar if you wish, some Rolling roads can tell how fast the rollers are being spun at, so, when it reachs 30, observe the speedo, lets say it was 30 on the rollers, 25 on the speed, and so on.

Next time, where the general discussion usually centers around cars, note that you are talking about a 4x4 to avoid comments such as Tristan's. Though his comment is completely relative for an answer about a car, those tires are completely sensible on a 4x4 that is going to go off-road.

Find out what the tire diameter/circumference is stock, then just calculate how much oversize those ones are. "Calibrate" the speedometer in your head. Pre-Tacoma Toyota trucks were generally sold with 28 inch tires stock and the speedometer was calibrated as such. Putting 31 inch tires on them left them approximately 10% oversize. I always found my speedometer to be approximately 10% off by speedgun signs at road construction, other people's cars, and being pulled over by police for doing 71 mph when my speedometer honestly read 65.

10% is quite an insignificant amount anyways when you are talking such low speeds. The difference between 65 and 71 doesn't mean much of anything on our highways in the US. You won't get a ticket for doing 71.

Thanks for all the info.
We found the sticker on the inside on the door which has the stock tire specs. And we knew what his new tires were so we just did the math and his speedo is 14.6% to slow.

For everyone who is curious, here was the math:

Stock tire = 225/75 - 15

New tire = 295/75 - 15

I then found a calculator on the internet to do the calculations and his speedo is 14.6% to slow, so when he is driving 100 km/h he is acutally going 114.6 km/h.

And what does the speedo read when he's actually going 100km/h? You might find the two errors (tyre circumference and inherent speedo error) cancel each other out to a certain extent, or be miles off to the point where you could factor in more improvement and get the speedo more accurate than original.

his speedo would read right around 85 or 86 km/h...

Once again...do the math:

What is 14.6% of 100

Then subtract that number from 100 and the result would be what the speedo reads.

Quote from Matt0snap :

his speedo would read right around 85 or 86 km/h...

Once again...do the math:

What is 14.6% of 100

Then subtract that number from 100 and the result would be what the speedo reads.

14,6% of 100 is 14,6

100-14,6 = 85,4

Sorry, took it as a challenge

No, that only works if the speedo was 100% accurate on original tyres. Speedos are NEVER accurate. Hence asking what the speedo reads when his TRUE speed is 100km/h on the big tyres.

Probably the only way to do that is to use GPS or a measured mile.

yah.... but would the speedo really be that unaccurate with stock tires? I am sure it isn't 14% out, but if it is like 2 to 6% out it shouldn't really be that big of a problem.

Quote from Takumi_lfs :

14,6% of 100 is 14,6

100-14,6 = 85,4

Sorry, took it as a challenge

CORRECT, one point for you

10% is not uncommon for a speedo to be out, even on a complete standard car. They generally range for 5% (rare), through 9% (common), up to 15% (really quite rare up here). Digital speedos are generally even worse, which is ironic as the moronic youths that love them believe that as it's digital it must be correct.

Therefore a READING of 100km/h might really be 91km/h. But if you fit bit wheels then a READING of 100km/h will be more like 104km/h - i.e. the errors have cancelled out, and now the speed under reads - which is dangerous (and risky regarding radar guns/GATSOs).

Hence, if he drives his car, as it is now, at 30, 60, 100 and 120kmh on the speedo, and makes a note of what the speed REALLY is (using GPS for instance, which is accurate enough) then he can have the speedo recalibrated accurately, and be better off than ever before.

Obviously you don't really grasp the mechanical side of speedos, but I do, so perhaps you should stop trying to misunderstand and correct me.

I know what you are saying........ Now think of what I am said..I wasn't disagreeing with you. I was just asking how often speedos are out on standard cars...

Then I said if the speedo was only of 5%, then it wouldn't be a huge problem.

So pretend the speedo is 100% accurate. I've calulated the new tire size and figured out the new speeds. Now pretend the speedo is 5% to slow, he would only be going 5 km/h MAX over the limit..which isn't a huge deal.

Now you just said a speedo could be as much as 10% out, which could be a problem.

Quote from tristancliffe :

10% is not uncommon for a speedo to be out, even on a complete standard car. They generally range for 5% (rare), through 9% (common), up to 15% (really quite rare up here).

Wow. I guess we've been lucky with our speedos. The past three cars my family has owned ('89 Pontiac TransSport, Audi A6, and Subaru Outback) have never been off by more than 2 mph at 60... which is around a 3% error.

Quote from Stang70Fastback :

have never been off by more than 2 mph at 60... which is around a 3% error.

How did you measure that? Because GPS isn't always accurate, because the only GPS I've ever used was a cheap $200 one and it only updated the speed every two seconds.

Quote from wheel4hummer :

How did you measure that? Because GPS isn't always accurate, because the only GPS I've ever used was a cheap $200 one and it only updated the speed every two seconds.

Yes, it was GPS, which is actually very accurate if you've got more than three satellites and a flat open road