Differential Ratios
- Thread starter BBennett
- Start date
Chuffer
CCCUK regional rep
When my 1980 C3`s diff expired I decided to stick with 3.08 : 1 . My car has a TH 350 3 speed auto and lock up converter Despite the car having pretentions for street / strip performance mods by the previous owner and modded exhaust system , I wanted the car to be more relaxed ( if a C3 ever is ?? ) at cruising speeds . As Ross said you will pay the price of more noise with a 3.55 .
Thanks to all who added comments about replacing the 2.87 with a 3.55. I have been trying to do the maths to see how much extra engine speed would result in swapping these over and so far failed. My transmission is the 700R4 with the overdrive top and at 2000rpm the speed is approx 80 mph . The speedo is not that accurate.My information suggests that the overdrive ratio is 2 . The extra acceleration would be nice but if the increase in rpm and fuel consumption is excessive it is probably not worth doing. If anyone can calculate the extra engine speed I would appreciate it but I will try again to get the grey matter working.
Roscobbc
Moderator
If it helps - a C3 Vette with stock diameter tyres (so approx 27" tall) - a 3.08 rear end and 'direct' top gear (so 4th in a stock manual and 3rd in a non-overdrive auto or lock-up manual) should be around 2625/2650 rpm at 70 mph (I believe)
Chuffer
CCCUK regional rep
That`s about right for mine give or take a bit .
teamzr1
Supporting vendor
I will try it this way
This is using the 700R4 gear ratios but using a 3.55 rear diff gear with max 6,000 RPMs (bit lower in 3rd/4th gears)
So first gear can get to 44 MPH (as shown below)
2nd - 80 MPH
3rd - 123 MPH and
4th, if going 37,000 feet distance - 141 MPH (stock engine)
Changes to above depends on elevation, weather and road surface conditions
You can see of course that first gear produces the most torque
Any tranny and/or gear changes and wheel and/or tire sizes will cause incorrect speedo values
and corrected by either gear at end of speedo cable at tranny and/or inner gear in tranny
kits are sold to do this
This is using the 700R4 gear ratios but using a 3.55 rear diff gear with max 6,000 RPMs (bit lower in 3rd/4th gears)
So first gear can get to 44 MPH (as shown below)
2nd - 80 MPH
3rd - 123 MPH and
4th, if going 37,000 feet distance - 141 MPH (stock engine)
Changes to above depends on elevation, weather and road surface conditions
You can see of course that first gear produces the most torque
Any tranny and/or gear changes and wheel and/or tire sizes will cause incorrect speedo values
and corrected by either gear at end of speedo cable at tranny and/or inner gear in tranny
kits are sold to do this
Forrest Gump
CCCUK regional rep
By my calculation, 70mph in overdrive will be 2150rpm with a 3.55 diff ratio and your gearbox ratios.
That is still a very docile engine speed and definitely wouldn't be an issue to me if the pay-off is better acceleration in the lower gears.
My car is a 4 speed manual and I actually replaced the 3.55 diff for a 3.08 in a bid to slightly lower the cruising revs and make all the gears a bit taller without losing too much acceleration. (this original diff was about to explode as well which necessitated a change) My revs at 70mph are 2650 - just slightly more pleasant than they were with the 3.55. Only a tremec 5-speed conversion (or an overdrive add-on box) would give me the luxuriously low revs you would be turning at 70mph! The primitive 4 speed manual gearboxes are great fun to drive but on the motorway I do envy the low revs you 4-spd auto drivers enjoy.
That is still a very docile engine speed and definitely wouldn't be an issue to me if the pay-off is better acceleration in the lower gears.
My car is a 4 speed manual and I actually replaced the 3.55 diff for a 3.08 in a bid to slightly lower the cruising revs and make all the gears a bit taller without losing too much acceleration. (this original diff was about to explode as well which necessitated a change) My revs at 70mph are 2650 - just slightly more pleasant than they were with the 3.55. Only a tremec 5-speed conversion (or an overdrive add-on box) would give me the luxuriously low revs you would be turning at 70mph! The primitive 4 speed manual gearboxes are great fun to drive but on the motorway I do envy the low revs you 4-spd auto drivers enjoy.
Roscobbc
Moderator
I know conventional logic says that (and taking all parameters in to account) that an engine running at continous cruise at, say 1800 or 2000rpm will be more economical than same engine, same speed running at 2650rpm.......it would be great to 'real world' results before and after from someone who has done the conversion (and presuming the engine tune was sensibly within it torque band at 1800rpm - or even 2000rpm).....a 'nagging' thought is that the higher revving engine with less of a load on it would perhaps have a slightly smaller throttle opening and perhaps less of a fueling requirement?