Working out percentages
Submitted: Sunday, Sep 08, 2019 at 20:39
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GerryG
I never was really good at maths, but I know some one out there will be able to help.
I have a 4WD mobile
home with 8.25x20 tyres. (It's built on a Hino truck).
I feel It is under geared. At 80 kph it's doing 2300rpm. At 90 kph ,- 2500 revs. It starts off in 2nd gear with-out hesitation and feels like it could use a sixth gear. (It's five speed)
I need new tyres so I thought I'd use the opportunity to rectify, to some extent, the above issue.
So! I have two options (sensible ones anyway). a. fit 9.00x20 tyres and b. fit 10.00x20 tyres.
My problem? I want to work out the theoretical difference in the expected rev drop.
Now! 8.25s have a 343 revs per km count.
9.00s have a 326 revs/klm, and 10.00s have a 310 revs/klm.
What would be the theoretical engine revs for each of these new tyres, at 80 and 90 kph resp?
Hope it's not too confusing.(Like a lot of us, I've fitted up larger tyres to other vehicles. but never really done the maths. I also understand about braking, tyre clearance etc)
Reply By: Ron N - Monday, Sep 09, 2019 at 18:19
Monday, Sep 09, 2019 at 18:19
Gerry, remember that as you raise your gearing higher, the harder the engine has to work, to drive the vehicle along at any set speed.
Trucks are normally fairly low-geared to improve their grade-climbing ability with sizeable loading, or full loading.
The advantage of gearboxes with more gears is that they often have a higher overdrive ratio, or sometimes, even overdrive on the top two gears.
As a result, it may be worthwhile investigating fitting a later 6 speed gearbox.
There are new 6 speed Hino gearboxes for sale on the 'net, taken from new trucks.
I'd suggest you have a chat to a parts or service manager at Vernon and Vazey, to discuss the best way of reducing engine RPM's at highway speeds.
Bigger diameter tyres may not be the best way of doing this.
Hino Gearboxes - Vernon & Vazey
Cheers, Ron.
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Follow Up By: Member - Neil T6 - Wednesday, Sep 11, 2019 at 20:29
Wednesday, Sep 11, 2019 at 20:29
I go along 100% with Ron N on this topic. May be more expensive in the first instance but gives a much better result overall.Might depend on how long the owner intends to keep the vehicle?
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