300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by cib24 »

I know that no one has ever done this, but would it be possible to put in a longer 6th gear ratio to drop the RPMs below 3,000 when cruising at 70-80 mph?
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by Dr. FrankenRex »

warpc0il wrote:
Wed Oct 18, 2017 2:19 pm
Cruise control is less fuel efficient than a light right foot, as it can only react to things like inclines rather than anticipate.
It is indeed, however it is much more effective at keeping me below 65 than my right foot is! :lol:
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by Ainmhidh »

My 300+ miles (in an R3) was warmed up, then filled in Swindon before driving to Bristol. Next day was on to Market Drayton in Shropshire, then back to Milton Keynes. Lots of slow motorway miles and still chickened out 24 miles (and about 2 gallons of fuel left) short 'cos of the naggy fill light! :-)
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by Dr. FrankenRex »

That's also a point, filling up when it is warm will deffo make a difference - how much so I don't know, but I'd probably rule out a 400 mile run unless it's already warm. 325 in an R3 should be doable, though?
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by PeteH »

cib24 wrote:
Wed Oct 18, 2017 2:33 pm
I know that no one has ever done this, but would it be possible to put in a longer 6th gear ratio to drop the RPMs below 3,000 when cruising at 70-80 mph?
It's possible. At 75 mph, the revs are:

s1: 3670
R3: 3660
R3 with s1 diff: 3410
R3 with s1 auto diff: 3300
R3 with RX-7 auto diff: 2990
R3 with RX-7 auto diff and 245/45 tyres: 2820

I know there are plenty of people on here who would argue that an RX-8 would produce worse mpg if asked to pull such high gearing. Personally I don't agree, and I think you could make the car "relatively" economical. But it certainly wouldn't feel like a sports car any more....

(Also please note that the last option is sufficient for 240mph at 9000rpm :shock: )
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by cib24 »

Yeah, I agree that cruising at lower RPMs will improve mpg but I am not talking about changing the diff, just the 6th gear ratio so 1st through 5th would be the same as any other car.

Because my car has a lower 5th gear despite the higher 4.3 diff, I have noticed I average about 2-3 mpg better at 70-75 mph than I ever did in my RX-8 as my RPMS are lower than the S1 RX-8 when traveling in the top gear at the same speed. Hence, why it would be interesting to try a lower 6th gear in a RX-8 and see if it makes a difference.

For example, the FD came with different 5th gear ratios depending on the spec and where it was sold (same transmission as 192 box):

70mph 4.1 4.3 4.44
0.719 2737 2870 2964
0.762 2900 3042 3141
0.806 3068 3217 3322

80mph 4.1 4.3 4.44
0.719 3128 3280 3387
0.762 3315 3476 3589
0.806 3506 3677 3797

On the 5th gear ratios -

.719 - Stock US FD 5th gear. 4.1 diff.

.762 - JDM Type RS, RZ and Spirit R had this with the 4.3 diff.

.806 - Most JDM transmissions have this 5th gear ratio with the 4.1 diff. This ratio did not come with the 4.3 diff.
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by qwakers »

hit the 300 mile club yesterday, 301.1 miles, 11.8l
no picture, sorry.
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by ChrisHolmes »

AndyBrad wrote:
Wed Oct 18, 2017 10:53 am
managed to get 250 on my R3. new oil filter and plugs on v power.

How the hell are you guys hitting 300
70mph on the motorway Cumbernauld to Cheltenham simples
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by warpc0il »

AndyBrad wrote:
Wed Oct 18, 2017 10:53 am
How the hell are you guys hitting 300?
Start with a double-DIN adaptor and then you're ready to fit this... ;)
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by ChrisHolmes »

I am running 245/35x18 tyres so my speedo effectively reads 10% fast. Does that mean the odometer also counts faster?
Conversely the engine needs more reves to achieve the true 70mph cruising speed.
PeteH, Warpcoil, cib24 can you work out the impact on the mileage and hence mpg calc?
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by wiggles »

Does the Fule light start flashing when it gets really low? or just stay on solid? chickened out whenever it has come on so far.
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by warpc0il »

ChrisHolmes wrote:
Thu Oct 19, 2017 10:49 pm
I am running 245/35x18 tyres so my speedo effectively reads 10% fast. Does that mean the odometer also counts faster?
Conversely the engine needs more reves to achieve the true 70mph cruising speed.
PeteH, Warpcoil, cib24 can you work out the impact on the mileage and hence mpg calc?
Those tyres are only 4.6% smaller than standard, so at an indicated 70mph you're only doing 66.8mph.
The same will apply to the odometer, so for every 100 miles you clock up, you've only actually travelled 95.4
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by warpc0il »

wiggles wrote:
Thu Oct 19, 2017 10:53 pm
Does the Fule light start flashing when it gets really low? or just stay on solid? chickened out whenever it has come on so far.
The fuel light isn't designed to flash though it can flicker when the level is approaching the reserve zone, especially if you're throwing the car around corners.

By the time the light is on solid you should have about 10L left in the tank, or about 6L accessible by the pump.
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by ChrisHolmes »

warpc0il wrote:
Thu Oct 19, 2017 11:03 pm
ChrisHolmes wrote:
Thu Oct 19, 2017 10:49 pm
I am running 245/35x18 tyres so my speedo effectively reads 10% fast. Does that mean the odometer also counts faster?
Conversely the engine needs more reves to achieve the true 70mph cruising speed.
PeteH, Warpcoil, cib24 can you work out the impact on the mileage and hence mpg calc?
Those tyres are only 4.6% smaller than standard, so at an indicated 70mph you're only doing 66.8mph.
The same will apply to the odometer, so for every 100 miles you clock up, you've only actually travelled 95.4
Puzzling! Is that the circumference of the tyre that is 4.6% less as the speedo is definitely 10% out versus GPS speeds on my camera and phone and TomTom. At 70 on the speedo I am doing 63 GPS and at 77 speedo I am doing 70 GPS (Still 20mph per 1000rpm which does my head in)
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by warpc0il »

The car speedo will always over-read when new, for a couple of reasons
- they must be within +10%/-0%, that is they must never under-read and can be up to 10% over
- as the tyres wear so the calibration would change anyway
- drivers like to see bigger numbers on the dash

GPS devices sample their location and then calculate speed based on the time and the straightline distance between the sample points.

Although your phone or satnav appears to be showing a continuous reading, it's actually updating based on these join-the-dots calculations.

Unless you're driving along a perfectly straight road, the GPS will show a lower speed than that required to follow the curves and bends in the road.
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by PeteH »

Sounds spot on. Almost all cars are designed with Speedos that are between 5 and 7% optimistic. The Construction and Use regs state that Speedos can be up to 10% optimistic, but can never be pessimistic. This is to avoid people accidentally speeding. So designing about 6% optimism is the easiest way to guarantee meeting the rules.

So your "extra" 5.4% of speedo optimism is entirely normal.

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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by ChrisHolmes »

Many thanks gents, good info
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by Dr. FrankenRex »

warpc0il wrote:
Fri Oct 20, 2017 12:05 am
GPS devices sample their location and then calculate speed based on the time and the straightline distance between the sample points.

Although your phone or satnav appears to be showing a continuous reading, it's actually updating based on these join-the-dots calculations.

Unless you're driving along a perfectly straight road, the GPS will show a lower speed than that required to follow the curves and bends in the road.
One further thing to add, is that the GPS speedos rarely take the inclination of a road in to account, as it's like they are viewing it from above. As such if you maintain 60mph along a perfectly flat road, and then begin an ascent or descent the GPS speed will go down as your speed between the two points it's sampling will have decreased 'as the crow flies'
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by PeteH »

Dan.NJ72 wrote:
Fri Oct 20, 2017 10:00 am
One further thing to add, is that the GPS speedos rarely take the inclination of a road in to account, as it's like they are viewing it from above. As such if you maintain 60mph along a perfectly flat road, and then begin an ascent or descent the GPS speed will go down as your speed between the two points it's sampling will have decreased 'as the crow flies'
I've heard this said, and I suspect it is true (altitude is not that well defined in gps world). But how much error? Time to bring out the trusty calculator.....

On a 1 in 10 hill (quite a significant A-road gradient), the gps speedo will under-read by 0.5%. In other words, if your readout is to the nearest mph then it probably won't change the reading at all!

Even up a 1 in 3 hill (the steepest gradient in the UK), the gps speedo will only under-read by 5.4%. Since you will be in first gear at this point, it probably doesn't matter!
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by warpc0il »

All these factors are cumulative, all adding to the differences between the GPS displayed speed (slower), the actual road speed, and the speedo displayed speed (faster).
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by Dr. FrankenRex »

I disagree on the numbers...

If you take vector calculations in to affect, assuming a perfectly straight piece of road and a perfect incline, a 1 in 1 (50% gradient) hill would half the indicated speed via the GPS would it not?

As such it then varies based on the gradient uniformly:

Incline vs percentage difference between actual speed and GPS detected speed

1:1 - 50%
1:2 - 33%
1:3 - 25%
1:4 - 20%
1:5 - 18%
1:6 - 14.3%
and so on...

Feel free to correct me, it's been YEARS since I've done vector mathematics

i.e. travelling up a 1:1 hill at 60 mph for an hour would result in 30 miles of 'as the crow flies' travel?
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by warpc0il »

Oh, and the revs per speedo mph will always be the same in any given gear, regardless of the tyre size fitted, as the speedo gets its input from the ABS sensors, so the only way that would change is if you replaced the diff.

Anyone fancy developing a doppler radar to fit under the car and measure against the road surface?
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by warpc0il »

Calling Pythagoras to the white courtesy phone...
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by Dr. FrankenRex »

My calcs are based on the assumption that GPS speedos discount altitude 100% - which I don't know about.
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Re: 300 Mile Club (S1 RX8s) or 325 mile club for S2 RX8s

Post by PeteH »

Do I have to Dave.....?

Ps. Far more interesting is the "speed over ground" sensor. These are actually common place (we use them on the F1 car). They don't use radar, they use a laser optical recognition process. It's rather complex, but it essentially looks at the rate of movements of the road texture. They are super accurate, and the only practical method of calculating true car speed. They actually measure speed in two directions (longitudinal and lateral), and some simple maths gives us the "slip angle" of the car, which is also rather important for us. (We also have GPS, but we don't use it much, except under very specific conditions).