Working out percentages

Submitted: Sunday, Sep 08, 2019 at 20:39
ThreadID: 139043 Views:999 Replies:6 FollowUps:8
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)
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Reply By: IvanTheTerrible - Sunday, Sep 08, 2019 at 20:51

Sunday, Sep 08, 2019 at 20:51
I thought that the 8.25, 9.25 and 10 was the width and the 20 was the diameter. With all the tyres the same diameter the speed would be the same
AnswerID: 627622

Follow Up By: GerryG - Sunday, Sep 08, 2019 at 21:05

Sunday, Sep 08, 2019 at 21:05
Hi Ivan
With the old system, the higher the first number, the higher the tyre diameter.
8.25x20 have a dia. of 966mm, 9.00 have a dia. of 1019mm and 10.00 have a dia. of 1074mm, hence the difference in rolling diameters. Actual figures depend on the tyre.
The 20 bit is the size of the rim or if you like, the size, in inches of the hole across the tyre, which in my case wont be changing.
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Follow Up By: Member - johnat - Sunday, Sep 08, 2019 at 21:12

Sunday, Sep 08, 2019 at 21:12
The difference in circumference (which is what will alter with a larger diameter) is also dependent on the profile - or the distance between tyre outer and wheel.

You'll also need to make an adjustment to the "indicated" speed on the speedo, since the changed diameter will apter the circumference and thus alter the indicated speed.

I suspect that you'd be better off changing the gearing by altering the internal gearing in the box. Assuming, of course, that the speedo indication is not related to the gearbox (which is usually the case).
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Reply By: Allan B (Sunshine Coast) - Sunday, Sep 08, 2019 at 21:07

Sunday, Sep 08, 2019 at 21:07
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Hi Gerry,

Based upon the figures you have provided ....

9.00s would be 2186rpm at 80kh and 2376rpm at 90kh.
10.00s would be 2078rpm at 80kh and 2259rpm at 90kh
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Allan

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Follow Up By: GerryG - Sunday, Sep 08, 2019 at 21:12

Sunday, Sep 08, 2019 at 21:12
Thanks Allan. Those figures look right. Now, how did you do it?
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Follow Up By: Allan B (Sunshine Coast) - Sunday, Sep 08, 2019 at 21:29

Sunday, Sep 08, 2019 at 21:29
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For the 9.00 tyre and 80k/h......
divide the 8.25's 2300rpm by 343 and multiply be 326 = 2186rpm.

Same procedure for the other sizes and speeds.



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Allan

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Follow Up By: RMD - Sunday, Sep 08, 2019 at 22:35

Sunday, Sep 08, 2019 at 22:35
OR
divide 326 by 343 = 0.95 0.95 x 2300 =2186. just the other end of the equations for each case.
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Follow Up By: Allan B (Sunshine Coast) - Monday, Sep 09, 2019 at 08:46

Monday, Sep 09, 2019 at 08:46
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Gerry,

Perhaps I should have answered you in "percentages" and using your given relative "revs per kilometre" figures....
8.25 tyre at 343 rev/km is nominal 100%
9.00 tyre at 326 rev/km is 326 divided by 343 = 0.95 = 95% (rounded)
10.00 tyre at 310 rev/km is 310 divided by 343 = 0.90 = 90% (rounded)

So, apply those percentages to your tyre & speed figure provides changed rpm's.





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Allan

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Reply By: Gone Bush (WA) - Sunday, Sep 08, 2019 at 21:57

Sunday, Sep 08, 2019 at 21:57
This might help:

EO Tyre Size Calculator
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Reply By: qldcamper - Monday, Sep 09, 2019 at 07:02

Monday, Sep 09, 2019 at 07:02
Consider also the extra stress on the brakes.
Drum brakes suffer considerably. Just raising the point not argueing it.
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Reply By: GerryG - Monday, Sep 09, 2019 at 07:43

Monday, Sep 09, 2019 at 07:43
Thanks everyone, you all have something to offer. As I said I've put bigger tyres on various vehicles before without too much thought about the maths or implications of effect on brakes, speedo readings and body clearance. Guess I'm getting to the cautious stage of life!
The EO Tyre calculator was interesting with it's capacity to convert imperial tyres to metric.
It was the mathematics I was interested in, so thanks Allan and RMD.
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Follow Up By: duck - Monday, Sep 09, 2019 at 10:01

Monday, Sep 09, 2019 at 10:01
GerryG
I have a Isuzu NPS300 that should have 8.5Rx 17.5 121/120L & has the warrior conversion 285/70R 19.5 it has the speedo conversion in-line (bolts to the box & cable screws into it) but a simple cheap gps heads up display is more acuate
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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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