tyre puzzle

The 2007 Hi Rider ranger was originally fitted with 235/75/15 tyres which have a circumference of 90.7ins.
The previous owner replaced these with some 18in CSA rims (probably off a Rodeo) fitted with 255/60/18 tyres having a circumference of 94.4ins.
The result is that the engine feels as if it isn't turning over fast enough when towing a a lightly loaded dual axle trailer at 90kph (2000 rpm in 5th)
I use 4th if carting a decent load.
Because I'm towing a lot at the moment I put the original 15in steel wheels back on but, surprise - the rpm's at 90kph are the same - 2000.
I expected them to increase. Why not, if the circumference has decreased ?
I've just spent a lot of dosh and am about to fit some 16in Ford allloys with 235/70/16 tyres (circumference 91.0 ins, which is very close to the original 15in rims 0 but it looks like I'm going to be disappointed.

The 2.5 l Ranger runs 215/70/15's (84.4 ins) and the gearing feels just right.
Putting bigger wheels on Rangers to up circumference and therefore the gearing is not a good idea and just makes them feel intractable. I suspect it does nothing for the fuel economy either.
Any answers?
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Reply By: Member - Charlie M (SA) - Monday, Apr 30, 2012 at 22:17

Monday, Apr 30, 2012 at 22:17
Hi
Changing the tyres will not change speedo or revs, as that is related to gearing on speedo drive.
With a GPS it will show true speed of both sizes.
With the 18inch wheels on you would be going faster than speedo is showing on GPS if speedo was right speed on 15inch wheels.
But in actual fact speedo may have been slow on GPS with original wheels and might bee correct speed for 18inch wheels.
On the other car it may have a different gearing to suit the lower tyres, but still be right on GPS speed.
Hope this helps
Charlie
AnswerID: 484674

Follow Up By: werewasi - Monday, Apr 30, 2012 at 22:47

Monday, Apr 30, 2012 at 22:47
Tks Charlie,
You got it when you said " Changing the tyres will not change speedo or revs, as that is related to gearing on speedo drive. "
I remember that with the 18in wheels the GPS showed 102 kph at 100 on the speedo. What I should have done today, was take a GPS reading with the 15in wheels and it would probably have shown an 'over' speedo reading when doing a true 100kph.
It just feels so much better with the smaller wheels on that I cant believe that only a 3.2% change in the rolling circumference can make such a difference.
That's why I was perplexed when the rpm's seemed not to have altered.
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FollowupID: 759944

Follow Up By: nickoff - Tuesday, May 01, 2012 at 14:57

Tuesday, May 01, 2012 at 14:57
My thoughts on changing tyre sizes.

I think what you really need to monitor changes to the tyre sizes on your vehicle are,
GPS unit for true speed,
A Scangauge or similar to monitor fuel usage on the fly,
And an exhaust gas temperature gauge to see how hard your vehicle is working.

Using the Speedo and Tacho only will give you the same reading no matter what the tyre size fitted as their pickup of from the engine and drive train, before the tyre.

The GPS will give you an accurate speed reading with each tyre size fitted. This is independent of the vehicle’s own speedo.

The Scangauge can monitor the fuel use and shows if the gain from the tyre size is worth the extra speed, or if you are using more fuel to maintain extra speed. Even though the Scangauge works from the vehicles computer, it will register the different fuel volume usage under different conditions.

Measuring and monitoring fuel flow and usage over a given journey will show if there is any improvement in economy.

The Exhaust Gas temperature gauge can show if you are overworking the engine and/or causing premature wear by changing tyre sizes.
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FollowupID: 759990

Reply By: The Bantam - Monday, Apr 30, 2012 at 23:10

Monday, Apr 30, 2012 at 23:10
AHH remember if you increase your diameter by 1 you increase your curcumference by 3.14.

Changing tyre diameter can drasicly change how a vehicle performs....remember when crusing in higher gears the actual amount of power used is quite small....particularly with weight on small changes in geraing can make all the difference to how the car travels and the fuel economy.

Remember the vehicle will always perform best across the board with close to standard diameter tyres

235/75R15s are very close in diamter to the standard tyres fitted to most 4 cylinder 4wd utes.

remember in most states now you are only allowd a 15mm diameter increase over the standard tyres.

cheers
AnswerID: 484677

Follow Up By: werewasi - Tuesday, May 01, 2012 at 10:39

Tuesday, May 01, 2012 at 10:39
Tks Bantam,
Well he good news now is that it seems that the new rims (16in) with a 70 profile are so close to the original wheel circumference that engine performance will be optimal.
I am ditching the 15in steelies because they are just so heavy. I'd like to hear if someone has a good tip for lifting wheels up on to the studs without risking stuffing the threads. I have a residual rotator cuff injury and that sort of lifting is bad news.
18 in wheels can go back to Gumtree.
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FollowupID: 759972

Follow Up By: The Bantam - Tuesday, May 01, 2012 at 10:56

Tuesday, May 01, 2012 at 10:56
The solution is not to lift the wheel onto the studs.......line the wheel and the hub so the studs more or less line up and then adjust the jack.

in steel wheels once you get the first stud thru the hole wind the jack up and it will lift the wheel off the ground...push and its on.

with alloy wheels if you are fussy, you might have to fiddle a little more....but again you only need 1 wheel nut on and only a couple of threads.... then jack up and it will lift the wheel off the ground where the remainder are easy.

As for the 16 inch rims... most of the light 4wd utes originally came with 205R16s or equavalent on them in the base model...which are arround 82 profile

If you play width against profile you should be able to get very close to the original tyre diameter.

cheers
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FollowupID: 759975

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