Best Gear Ratios / Rear Axle Combination

[123GT-AMAZON]

ok

same size wheels correct fact

regardless of how fast the wheels are turning as that is ! speed related

60mph is the same as 60mph fact

ok now the science part

prop is same size yes fact now if you changed the size of wheels then yes i would agree the wheels would be going faster than stock but its like a push bike regardless what gear you are in you do said speed what ever gear your peddalin like a goddin in 1st , 2nd 3rd,4th and so on so using that in comparison if your gearbox prop end is going at said speed its turning the prop same cycles correct yes so maybe yes the axle ratio is turing at a better gearing but you are doing 60mph regardless i tried this out on my xr3i changing wheels and that was the only thing that changed wheels size and tyre size your wheels still turn at the same rotation speed do they not ??

and you feel like the car is doing a better speed through the gears and geared better yes i agree but thats about all and the very small amount it could possibly change it would take up the slack in speedo anyway so no major loss if anything its trued up the speedo worst case as you have a 3-5mph most times faster than you actually are doing but lets face it it aint gonna make much diffrence to write home about so not really worth worrying

kind regards
robert

[Burdekin]

Mph is miles (distance) per hour (time). Therefore if you take my example of a diff ratio of 3:1 and 4:1 for 1 turn of the prop the wheels turn 3 and 4 times. This equates to travelling 3 times the circumference of the wheel vs 4 times the circumference of the wheel. This means that for every rotation of the prop the car is travelling 33% further. So that would mean the speedo would be 33% out if you change the diff from 3:1 to 4:1. At 60mph with a 3:1 diff and then changed to 4:1, the speedo would be 20mph out.

It's the same thing about the wheel size, larger wheels go further per rev due to the larger circumference.

[Underdrive]

Khe Sanh, I almost agree with you except with say a 4:1 axle, the prop turns the pinion 4 times to turn the crownwheel once. Robert would have been right if the speedo gear was measuring the number of turns the crownwheel of the diff makes but it is the output of the gearbox (prop) and therefore has to be affected by the diff ratio, as you said. You obviously can't trust the speedo gear in the gearbox unless it is matched with the diff ratio that originally went with it.

[Saner]

Interesting discussion, I had assumed that I would need to adjust my speedo.

Anyone able to shed any light on my problem?

[Colonel Froth]

Assuming the first online calculator I found is accurate, the M40 with a 1:1 4th gear, a 4.56:1 diff, and 25" tyre diameter would give 70mph at 4290rpm. Changing to 4.1:1 will yield 70mph at 3857rpm. A speedo calibrated to work with the old diff would show 70mph with the new diff when you're actually doing 77.9mph so it's a big enough difference to get your pants pulled down by the police. I don't know how many teeth the speedo drive gear that you need would have though, or which model you might get one from.

[Colonel Froth]

If you don't end up finding out what the right gear is you could always keep the one you have, get the numbers for the speedo re-cut from off-white self-adhesive vinyl, and replace the lines on the drum to match the 'new' speeds generated by the 4.1 diff. If yours is the 120mph speedometer and it tells the correct speed with the 4.56 diff, you would (roughly) need to replace 120mph with 130mph and space the rest of the numbers out evenly.