Speedo accuracy

[kev500sa]

Hope someone can help me on this:

How accurate is the Pajero speedo? ( NH 3.0l V6)

1. I have 31" tyres on.
2. At 120km/h on the speedo I am doing 109Km/h on the GPS. (should be other way around)
3. Does that mean I have 5.29 diffs in? (possible, vehicle was rebuilt by others)
4. How can I identify 5.29 vs 4.875 externally?

Thanks in advance.

Kevin.

[Drujba]

I also have 31's, original diff ratios 4,875, but my speedo reads 100 km/h when the gps says 106 km/h. So with bigger tires than standard, your speedo should read less speed than actual.
But, if you assume that the diff gears have been changed, you can calculate your actual speed, if you know your gear box ratios:

Speed km/h = angular speed of the wheel (omega) X wheel radius in meters x 3.6

omega = wheel rpm X Pi / 30 = [Engine RPM / (5th gear ratio X diff ratio)] X Pi / 30

the radius in meters of the 31" tire is 31X25,4/2000 = 0,3937, but if you have mud tires is better to actually measure the diameter, as the actual size may be different from the inscription on the tire.
So, if you know your 5th gear ratio, and the tire radius in meters, the above equation becomes a very simple one, you should just read your RPM together with the speed read by the GPS on a straight highway line, and than you can find the diff gear ratio:

diff ratio = (engine RPM X Pi X wheel radius in meters X 3,6) / ( 5th gear ratio X GPS Speed km/h X 30)

I hope i did not gave you any headaches

[Drujba]

Just for fun, I made a system of equations, and I finally came to a very simple formula:

If you assume that the tachometer in your car was originally reading the speed with very high accuracy, and that your diff gears have been changed, your actual gear ratio = the original gear ratio X (Speed GPS/Speed tachometer)

So if your original gear ratio was 4.875, than your actual gear ratio should be 4.875 X 1,1009 = 5,366.

As this gear ratio does not exist in Pajero, and the tachometers are everything but accurate, and the speed read is influenced by the pressure in the tire, road surface, etc, you can assume that yes, you may have 5,285 gears in your diffs. But if you want to be sure, try the equation in the above post

And, the good news is that you can easily go for 33's or even 35's with 5,29 in your diffs. It depends on what you want to do with your car.

[caxtrol]

And now, how I can measure the difference in kilometers between the normal size of the tires and the new size, because the speedo in kilometers will be differente ???

Thanks

CAXTROL

Just for fun, I made a system of equations, and I finally came to a very simple formula:

If you assume that the tachometer in your car was originally reading the speed with very high accuracy, and that your diff gears have been changed, your actual gear ratio = the original gear ratio X (Speed GPS/Speed tachometer)

So if your original gear ratio was 4.875, than your actual gear ratio should be 4.875 X 1,1009 = 5,366.

As this gear ratio does not exist in Pajero, and the tachometers are everything but accurate, and the speed read is influenced by the pressure in the tire, road surface, etc, you can assume that yes, you may have 5,285 gears in your diffs. But if you want to be sure, try the equation in the above post

And, the good news is that you can easily go for 33's or even 35's with 5,29 in your diffs. It depends on what you want to do with your car.

[NJV6]

That error will just be standard speedo error. Its reading high so you won't get a speeding ticket

4.875 might stil be on a tag on the front and back diffs.

[Drujba]

That error will just be standard speedo error

The speedo error will occur on original size tires, and would be approx 5 to 6 %. But he drives on 31"s, which is 5% more radius than standard, so that speedo error will be counter-balanced, even more the speedometer will read less than actual speed measured by the GPS, as I already experienced in my case.
Still, he gets 10% more reading on the speedo compared with the GPS, and he has bigger tires than standard. This means that he might have the diffs exchanged with some lower ones.

@caxtrol: The tire size in this case will not influence the calculation, because both measurements are done in the same time, while driving on same size tires, once from reading the speedo and once from reading the GPS. Never mind if the tires are original size or not. The radius will be divided with itself therefore will be simplified in the formula. The only influence comes from the accuracy of the speedometer, therefore by assuming that the car had originally 4,875 gears, the result was 5.366 which is not actually existing. But the closest value is 5.285, so there are pretty big chances that he has these gears. But if his car had 4.63 or 4.2 gears, the result will be different.
If you want to know the speedometer error, just drive on fairly new standard tires, inflated at the standard pressure, on a straight tarmac highway, while being in the same gear (5th or 4th) and read the speedometer in parallel with the GPS. Make more readings, let's say a set of five parallel readings. Than divide the speedometer reading to the GPS speed, and you will get a set of values like 1,049 ; 1,053 ;1,057 etc., which means 4,9%; 5,3% ; 5,7% and so on... The speedo error would be the average of these values, and it would be more accurate if you will make more parallel observations.

@kevin - the best way to find your final gear ratio is to use the equation in my first post, an accurate GPS speed reading, to measure the actual tire diameter with a ruler and to know the gear ratio of your 5th gear or 4th if automatic (this you may find in the owner's manual). You will have to inflate the tires at the same pressure, and to make sure you have the same size and type of tires on the drive axle.
If you are not sure how to do it, just measure your tires diameter as described above, search your owner's manual for the ratio of your 5th gear, read the GPS speed in 5th gear when you have 3000 RPM and 4000 RPM, and come back here with the values, and I will calculate your gear ratio.

[NJV6]

That error will just be standard speedo error

The speedo error will occur on original size tires, and would be approx 5 to 6 %. But he drives on 31"s, which is 5% more radius than standard,

Mine came on 31's. 31's can measure from 29.5 to 31 when new, look at the tyre stats.

Many new cars have up to 10% error.

Kev, finding you gear ratio is simple with no maths. Smply jack up one rear wheel (mark it with chalk at the lowest point to the ground) and count how many times you turn the driveshaft (yip, you'l have to crawl under) to make the wheel do two full rotations.

[Drujba]

... Smply jack up one rear wheel (mark it with chalk at the lowest point to the ground) and count how many times you turn the driveshaft (yip, you'l have to crawl under) to make the wheel do two full rotations.

this is also possible, but as the gear ratios are so close, you may get a result in between the two ratios, and than it will be hard to interpret. For better accuracy, you can count the drive shaft revolutions for more wheel full rotations, 5, 10...

[NJV6]

this is also possible, but as the gear ratios are so close, you may get a result in between the two ratios, and than it will be hard to interpret. For better accuracy, you can count the drive shaft revolutions for more wheel full rotations, 5, 10...

With one being less than 5 rotations (4 and 9/tenths - 4.875) and one being more (5 and 3/tenths - 5.285) it should be pretty simple.

Glen

[kev500sa]

Thanks for the replies guys,

I have had a suspicion that the diff ratios were not standard since I got the Pajero 'bout 3 yrs ago. When I got it it had OE tyres on, ie 29". We were driving in convoy once and the following vehicle complained that I was driving very slowly - we had agreed to drive at 120km/h but he maintained that I was driving at 100km/h. I then changed to 31" tyres and got a GPS - It was strange to me that even though I had bigger tyres the speedo was still over reading. So I recorded the revs vs speed (GPS) and the closest final drive ratio I could find was 5.29. Not a train smash.....However I am worried that the front diff is still the original 4.875 (as per the VIN plate). This would create too much wind-up in the drivetrain.

I thought that there may be a simple visual check to confirm the front diff ratio, looks like I will have to jack it up and turn the driveshaft to confirm....I can't do that on the rear one - LSD.

All of the above is due to me upgrading the motor and putting in a body/suspension lift so I can run 33". With 33" tyres the speedo should be within 5%. Just The revs will be a bit on the high side when cruising.

Thanks again for your replies.

Kevin.

[Drujba]

... All of the above is due to me upgrading the motor and putting in a body/suspension lift so I can run 33". With 33" tyres the speedo should be within 5%. Just The revs will be a bit on the high side when cruising.

Thanks again for your replies.

Kevin.

If your original ratio was 4,875 and now you have 5,285, the 33"s will almost behave as the standard 29,6" tires. You will also notice that the gap between the speedo and gps reading will be substantially reduced.

This is what I am trying to do now myself, change my 4,875 diffs with 5,285 in order to upgrade to 33"s. My problem however is the weak engine, therefore I will need lower gears for 11,5% bigger tires.

Your V6 is 150 or 180 HP ?

[kev500sa]

The old motor was the 109kw 3.0l V6, after the mods I am doing now I should have a 3.8l V6 giving 160kw and 330Nm, possibly more. So with the 5.29 diff......

Cheers.

[Drujba]

The old motor was the 109kw 3.0l V6, after the mods I am doing now I should have a 3.8l V6 giving 160kw and 330Nm, possibly more. So with the 5.29 diff......

Cheers.

ohhh, give us some details. You got a rebuild kit, other crankshaft, other rods, what are you doing exactly? Turbo also? Compressor?
With this power and 5.29 diff you can easily go for 35's... By the way, what tranny you have on that rig?

[NJV6]

Kev, even with a LSD in the back you can still turn one wheel. Try it for the front and the back.

If you are going to a 3.8 there is no point (in my humble opinion) going to a 5.29 ratio when you have plenty of torque to drive it and the 5.29 is a smaller diff. The 3.8's torque is nice and low in the rev range so original diffs + bigger tyres would be a great choice.

The speedo is a simple and cheap fix later.

[kev500sa]

OK, been looking at doing the motor rebuild for a long time. Found a mechanic who races Pajeros, he called in his tuner and they have come up with this spec:
Re-bore block, new crank, new pistons and conrods to give 3.8l. Heads and block skimmed to give 10.5:1 compression. Reprofiled cams, custom built extractors, free flow exhaust. K&N airfilter, inlet ported, 60mm TB, Piggy back ECU and a dyno tune. Cost about $4000. Doing a similar build they got 160Kw and 330Nm on the back wheels.

I have a manual tranny.

While the motor was out had the gearbox redone, new clutch. He has completed the body lift, put in Dobinsons rear springs, lifted the suspension - new ball joints (top and bottom) - free wheel hubs, new front wheel bearings.

Doing a complete mechanical rebuild. 270000km on the clock. (I think it has done more). Before the rebuild had no power, smoking, compression test showed 8.1:1.

The Pajero should be finished by mid Feb. He better finish, got a 2500km trip planned.

Cheers.