Outer Limits 4x4 Board

At 100km/hour what RPM do you sit at on 31" tyres ?

and if possible, with 33" tyres?

I am considering ending the search for a factory 5sp mk/mq and just bite the bullet - I want to start the fourbying already


edit: sorry, I am asking with respect to the L28 engine.

Don't know with 31's or 33's but with 35's i sit on about 3000 with 4.6:1 diffs, so unless you had lower ratio 4.1 etc diffs - not sure what L28 has, you'd be revving with 31's!

Not that hard to work out though - 4th gear is 1:1 so
100 km/hr = 1.66666 ks/minute
=166666.66cm/minute

tyre rolling diametre(cm)/166666.66
=axle rpm
axle rpm * diff ratio
= tailshaft and therefore engine rpm (in 4th gear)

should work

cheers

The revs/speedo reading wont change regardless of what tyre size you run! Your actual speed will though.

DR Frankenstine wrote:The revs/speedo reading wont change regardless of what tyre size you run! Your actual speed will though.

arr, what?

it will change, the rolling diameter changes the gearing (much like changing the gearbox or diff ratios).

ignoring that, look at what you said? - if the speed is increased at the same amount of revs, then obviously to do 100 you will require less revs.

foad wrote:

DR Frankenstine wrote:The revs/speedo reading wont change regardless of what tyre size you run! Your actual speed will though.

arr, what?

it will change, the rolling diameter changes the gearing (much like changing the gearbox or diff ratios).

ignoring that, look at what you said? - if the speed is increased at the same amount of revs, then obviously to do 100 you will require less revs.

He is correct. Your overall speed may change, but the speed at any given RPM will not change on your speedo, due to the speedo being driven from the gearbox. You will be just left with an innacurate speedo, you then have to make other changes to get the speedo to become accurate again which few people bother doing.

oh i see what your saying.

the actual speed and RPM does change, which is what I want.

speedo's can be recalibrated

They sit at between 2500 to 3000 RPM at 100 to 120 kms.........
depending on how well timed and tuned it is.

screwy

Screwy_ScrewBall wrote:They sit at between 2500 to 3000 RPM at 100 to 120 kms.........
depending on how well timed and tuned it is.

screwy

Sorry jeff but timing and tuning wont make any difference to Speedo/rev reading either

DR Frankenstine wrote:

Screwy_ScrewBall wrote:They sit at between 2500 to 3000 RPM at 100 to 120 kms.........
depending on how well timed and tuned it is.

screwy

Sorry jeff but timing and tuning wont make any difference to Speedo/rev reading either

Some of the L28s ive seen cruise at 2500 and run rough through the gears and some i cruise at 3000 and fly through the gears....
Its gotta be something to do with tuning even if its just electronic ignition or watever... If a car is not tuned properly it wont run as well........

My GQ cruised at 2200 at 100kms now it cruises at 1800 RPM and all i did was change points and condensor and adjusted the carbi a touch......

screwy

nah tuning can't make a difference, but some might have different diff ratios - which would explain why some do different RPM at the same speed.

well dunno wat the go was with the GQ. The points were absolutely burnt, i showed a mechanic mate and he said " cant beleive it was running ".....
It droped revs largly and increased fuel economy about 40kms per tank of LPG.... So naturally i assumed the same would be the case for any car....

Well the 2 L28s ive had one had 4.3s and one had 4.6s and the RPM was as i said above.
Most L28s are 4.6 which would mean about 2800 RPM around the 100km to 110 km mark....

screwy

actually that sorta makes sense

Patroler wrote:Not that hard to work out though - 4th gear is 1:1 so
100 km/hr = 1.66666 ks/minute
=166666.66cm/minute

tyre rolling diametre(cm)/166666.66
=axle rpm
axle rpm * diff ratio
= tailshaft and therefore engine rpm (in 4th gear)

should work

You mixed it around a little.

final equation should be

Patroler wrote:166666.66/tyre circumference(cm)
=axle rpm
axle rpm * diff ratio
= tailshaft and therefore engine rpm (in 4th gear)

Ahhh thats better - after i had a look i started wondering what a rolling diameter was and all.
d'oh
ta

the way a motor runs CAN'T! have anything to do with rev's/speed/speedo reading its all to do with gearing, right up till it leaves the gearbox. Then the speed will change depending on diff ratio( if its been changed from std ) Tyre size!!! Think about it. In 4th gear assuming ratio is 1:1 motor does 1 rev gearbox output shaft does 1 rev tailshaft does 1 rev Due to modification or other factors it all goes to shite from there. no amount of tuning will change that.

True, the only way it could be different is if clutch slipped, or speedo/tacho is dodgy, wheels and crank are mechanically linked.

but if you think about a car with and without a load

you might be doing 3000rpm on the tacho, but put your clutch in and the tacho will read 3600rpm

or whatever (thats just an example).

if the car is running better (more power/torque) and against a load (100km/hour wind resistance) then it should actually require less rpm to keep the car at a particular speed.

i know waht you are saying, the engine must turn a particular amount of RPM, and be geared to the wheels, but what he said does also make sense in a way.

Nah, the reason it will rev up when you put the clutch in is because you have removed the load from the motor, you're giving it more throttle than required to maintain the same revs when you remove the load - same when you go up a hill with cruise control it opens the throttle and accelerates to maintain the same revs and speed

If the car was running better - more power/torque then you might require a smaller throttle opening to keep the car at the same speed, revs would only change as speed changed - unless you clutch it.
Example:
My mk shorty needs about 90-95% throttle to cruise at 110ks, if i put the clutch in and removed the load frome the motor it would rev to the s#%thouse and hit the governor - because its almost at full throttle.
My ute (chev and twin gas research carbs) needs about 25% throttle to hold 110ks, if i put the clutch in it'd just rev up a bit more like in your example - less throttle.
I know they're different cars but if i put the same tyres on both then the engine revs would be identical if they were at the same speed and in 4th (1:1) - I put this in because they have different boxes but both have 1:1 4th

DR Frankenstine wrote:the way a motor runs CAN'T! have anything to do with rev's/speed/speedo reading its all to do with gearing, right up till it leaves the gearbox. Then the speed will change depending on diff ratio( if its been changed from std ) Tyre size!!! Think about it. In 4th gear assuming ratio is 1:1 motor does 1 rev gearbox output shaft does 1 rev tailshaft does 1 rev Due to modification or other factors it all goes to shite from there. no amount of tuning will change that.

100% accurate.

The speedo ONLY gives a "value" of speed that is related to the gearing of the original factory setup....
Changing diff ratios, tyre sizes WILL NOT & CAN NOT alter that "value". The speedo only shows what the given value is at a certain amount of engine revs going throught the gearbox at any given time...as to what happens after the gearbox is totally irevelevant to that value

If you alter the diff ratio or tyre size then the speedo will be out of calibration in regards to the factory setup. Go to the cop shop and get one of the boys to grab a hand held radar gun and come for a drive with you..... they point the radar gun at a fixed object ( say a building) and this will record your speed, you then have a comparision to go by.

On 35"s and using a MK 5 speed box & using 4.6 diffs my speedo must show 90kph to be actually doing 100kph actual speed.

As for the bullshyte about engine tune ....what drugs are you on and where's my share ???

Kingy

Bit of a bump, but I found it useful and someone else might actually too if they stumble through this thread again (I did via the bible surprisingly enough)

http://www.4lo.com/calc/gearratio.htm

Decent online calculator for calculating gear/diff/tyre sizes and whatever.

It's accurate enough, with my current setup I do about 3100rpm @ 100km/hr, with 33's that will drop to 2800rpm, and with 35's that drops to 2600rpm.

cheers