GPS speed cant be relied on; according to GPS with seven sats in site, t'other day, my house was walking around the block...... would have been convenient if I had wanted a pack of fags, but unfortunately, I didn't ACTUALLY see the tobaconists pass my window......
Relationship between speed and speedo reading is also a straight liniar one, and they rarely go from reading high to reading low, they will either under read or they will over read, but very, very rarely both.......
Unless you have a Land-rover, in which case they can do both at the same time, curtecy of the un-keyed speedo drive on the gear-box out-put shaft that can slip and grab, making the needle waver around a nominal that rarely has an co-incidence with that which you are actually travelling at......
235/70's are, I think the 'standard' size for later Disco's and are an 'equivilence' for the older 205R16, so that shouldn't influence things much.
Rangie doesn't have a fifth gear, as its an auto, and it wont go that slow in top (4th) becouse the Torque concverter wont lock up until 5000rom, which is 55pmh, but IF you do the maths, that works out at just under 1,500rpm in top for 40mph.
1,100 RPM sounds a little low, but 4th on the Rangie is 1:1 as it would be in the Disco, which has 5th adding 25%, so it's not THAT farr off what might be expected.
If you check the manuals, either Haynes or the RAVE, will give the reduction ratios in each gear, and if you multiply them out you can work out how fast you should go at any given rpm in any given gear........
Maths goes like this:
Engine RPM x Gearbox Reduction = output shaft speed
Gearbox output shaft speed x Transfer Box Reduction Ratio = Prop-Shaft speed
Peop Shaft Speed times Differential Reduction Ratio = Wheel Speed
Wheel Speed times Circumference of wheel = Road Speed
Where the Circumference of wheel = Pi (3.142) x Combined Diameter of Wheel = Tyre
In the case of a 235/70R16, the wheel diameter is 16" or 406mm
Side wall height is 70% of 235, or 235 x 0.70 = 164.5mm
But there's a side wall at the top and the bottom of the wheel, so you need to multiply that by two, to get 329mm to add to the wheel diameter, 406mm, to get the roling diameter of the wheel, 735mm. Multiply that by Pi, and you get 2.3m per revolution.
Which is the hard bit done for you; just remember the ratio's in the boks are reduction ratio's, so the number you get out should be smaller than the number you put in.... if not, you need to divide, rather than times, as the ratio will have been quoted 'backwards'
Eg: Series Diffs have a ratio often quoted as 4.7 , Coiler Diffs quoted as 3.5, but that's short hand for 4.7:1 or 3.5:1, the bottom number of a fraction. If you multiplied the input revs by aone of those ratio's number you'd get out would be bigger, rather than smaller, so what you have to do is divide, rather than multiply.......
