Info needed: Streetable CMP truck

[Lang]

There are lots of anecdotal figures on what people can and can not do with tires, gear ratios and engine speeds. There is nothing subjective about it - just straight maths.

The subjective bit is when one bloke thinks it OK to sit on 3,000rpm all day long while another is horrified if it goes over 2,500rpm on an over run down hill. More than worries about engines exploding is the fact revs-is-fuel.

There are lots of calculators on line. I find this one pretty quick and easy. Their 3rd gear number is just standard 1:1

https://spicerparts.com/calculators/...rpm-calculator


This tells the story, even with a modern efficient design quite clearly every extra rev is costing you money.

[Hanno Spoelstra]

Originally Posted by Lang

There are lots of anecdotal figures on what people can and can not do with tires, gear ratios and engine speeds. There is nothing subjective about it - just straight maths.

The subjective bit is when one bloke thinks it OK to sit on 3,000rpm all day long while another is horrified if it goes over 2,500rpm on an over run down hill. More than worries about engines exploding is the fact revs-is-fuel.

There are lots of calculators on line. I find this one pretty quick and easy. Their 3rd gear number is just standard 1:1

https://spicerparts.com/calculators/...rpm-calculator

This tells the story, even with a modern efficient design quite clearly every extra rev is costing you money.

Thanks Lang, helpful indeed to calculate rpms at a given speed.

For a C15, cruising at a speed of 80 km/h, with a 6.16 to 1 ratio rear differential and 844 mm diameter tyres, the engine will rev at 3096 rpm.

[Lang]

Even for a Chevrolet 3,000rpm cruise is getting right up there. I think a Ford or Dodge engine would be hurting pretty badly at that pace for long periods.

Maybe you could look at some sort of overdrive. Either a change to a modern 5 speed gearbox or a bolt-on unit. I think a higher diff ratio would probably be better as you Europeans do not seem to take your vehicles bush-bashing and the unnecessary crawler first gear is just along for the ride.

[Hanno Spoelstra]

Originally Posted by Lang

Even for a Chevrolet 3,000rpm cruise is getting right up there. I think a Ford or Dodge engine would be hurting pretty badly at that pace for long periods.

Maybe you could look at some sort of overdrive. Either a change to a modern 5 speed gearbox or a bolt-on unit. I think a higher diff ratio would probably be better as you Europeans do not seem to take your vehicles bush-bashing and the unnecessary crawler first gear is just along for the ride.

Yeah, a 5-speed conversion like you did seems the way to go. On the other hand, the engine I bought is fitted with a fully rebuilt SM420 4-speed gearbox already, so maybe an overdrive/ diff swap would be a better option? We'll see later.

There isn't a lot of bush left in Western Europe to bash through - this is about as rough as it gets
https://youtu.be/rAo71QQRqpE?si=-ua7mUj2BOwazXvp

[Lang]

Still fun to get them off sealed roads into their natural environment.

Here is another 60mph chart with a wider range of differential ratios.

[Bob Carriere]

....with the newer 235........???????

Consider yourself lucky you got a FULLY synchronized 4 speed...... no more grinding......

Not sure how accurate my NOS speedo is but top I can get at approx 3000rpm is 44 mph...... and that is with my 261...... at which point I have both hands on the steering wheel........

My 10:50 tires on 16 rims gives me 532 inches of rotation per mile....
.......tire is 38 inch tall.

On January first we will be taking it out of the catacomb for the traditional spin in the back field..... fist time since 1990 that we have absolutely NO snow on the ground.....lucky if the ground is frozen solid....

Cheers