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Old 05-27-2010, 03:47 PM
350pano 350pano is offline
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hi frisco, thanks very much for your reply.

I have implemented plan A, and am now saving myself 5 cents a litre by using 91 octance gas. I have asked several people about this, and everyone, including the dyno chap said it would run like $^% on a lower octane. They were wrong. So excellent suggestion thanks.

I have priced a second hand 700r4, and thats clearly not going to happen. Just over $2k, more if its ackward to fit.

The reason I mentioned the diff is that mine has been on the way out for the last 7 years, so I thought if I have to change it I might go for 2.78:1 diff. Am I right in assuming the revs at 100ks would come down by 300rpm? I understand what you mean about losing some performance but does the fact the engine produces torque down low on the rev range mean it would find it reasonably easy to turn a lower ratio diff?

At present its reving at 2350 rpm at 100ks. I assume the reason it is not 3080 rpms is the different rear wheel size, 17" rather than 14" or 13". I figure if the revs come down to around 2000 that surely has to make a milage difference?

Oh and i do have a 4 cyl jappa for work etc, this is my weekend car, meant for long trips up country, so any savings in gas means I have more to spend on beer when I get there :-)

Thanks again for your help
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Actually just found this post below somewhere. My rear tyres are 24" which seems to me to indicate it already has a 2.78 diff instead of the 3.08 I had thought.

Submissions by Jacks:
For what it's worth. Below is a formula to use that might save you $$$$
To get M.P.H. divide K.P.H. by 1.6 example (96KPH=60MPH)
At 60 MPH, x Axle ratio x 336/Tyre Diameter = R.P.M.@60 M.P.H
THEREFORE 60 X 2.18 X 336 / 24 = 1831 R.P.M.
RATIO: Equals tail shaft turns, to one revolution of wheel.
DIAMETER = Measure tyre height from top to ground with tape.
Listed below, examples of R.P.M @ 60 M.P.H (96K.P.H)



Diff Ratio vs Tyre Diameter Diff Ratio
24" 26" 28" 30" 32" 34" 36" 38" 40" 42"
2.18 1831 1690 1570 1465 1373 1293 1221 1157 1099 1046
2.50 2100 1938 1800 1680 1575 1482 1400 1326 1269 1200
2.74 2301 2124 1973 1841 1726 1625 1534 1454 1381 1351
3.08 2587 2388 2218 2070 1940 1826 1725 1634 1552 1478
3.23 2713 2504 2326 2170 2035 1951 1809 1714 1628 1550
3.50 2940 2714 2520 2350 2205 2075 1960 1857 1764 1680
3.73 3133 2892 2686 2507 2349 2212 2089 1979 1880 1790
3.90 3276 3024 2808 2621 2457 2312 2184 2069 1966 1872
4.10 3444 3179 2952 2755 2583 2431 2296 2175 2068 1698
4.56 3830 3536 3283 3064 2873 2704 2554 2419 2298 2189
4.88 4099 3784 3513 3279 3074 2894 2733 2589 2460 2342

Might be something for all to use.



It will give you a guide as to what your Revs will be.
Knowing Diff Ratio & Tyre Size, at a given speed.
Without taking your vehicle for a run & no tachometer required. Hope it helps

Last edited by 350pano; 05-27-2010 at 04:02 PM.
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