diff

**uterus** · 25-08-2012, 11:06 AM

Hi-can anyone suggest the best diff for economy ,currently running hz ute original salisbury diff with 5 speed celica 308 slight cam 650 double pumper torqure manifold.now running at around 2500 revs at 100 ks would like to get down to around 2000 revs if poss,just a cruiser not looking to be quick of the mark just normal driving.
cheers

**HK1837** · 25-08-2012, 02:03 PM

Easiest is work out what you have now, and multiply the ratio by 0.8 to find out what you need.

Without knowing your tyre size, 2500rpm at 0.85:1 5th gear equates to about 3000rpm in 4th gear, so if you have say 235/60/14 tyres my guess is you have a 3.36. Multiply by .8 and you have 2.78. But you'll have to do your own sums. Work out the tyres circumference in metres, convert 100kM/h to metres/min and work out how many revolutions the tyre will do in one minute, multiply this ny the diff ratio and this gives you tailshaft revs. Multiply this by the gear ratio (1 for 4th and 0.85 for 5th) to get engine revs. Or work backwards

**agr071** · 25-08-2012, 02:06 PM

For what it's worth, i'd run 3.08 ratio. If it was auto i'd have 2.78.

**ozbox** · 25-08-2012, 03:02 PM

the best diff for economy would be a 2.60 ratio but it would be a bit sluggish off the line depending on what toyota ratioed box your running...personal chice for the me with a celica 5 would be a 3.08

**adam perth** · 25-08-2012, 08:08 PM

go the 3.08

**HK1837** · 25-08-2012, 08:25 PM

2.78 would be OK with 245/50/14 but I agree that 3.08 would be better. 2500 at 100kph is pretty good so I'd stick with the diff you have and spend the money on a Quadrajet and Edelbrock Performer. You may find that it uses more fuel at 2000rpm with that manifold and if the cam is too big. A HEI dizzy with vac advance will help too.

**Vombil** · 25-08-2012, 10:14 PM

i just went from a 2.78 (i think) to a 3.55 in my 308 wih supra 5 speed. first is now for towing or tree stump removal! The 2.78 was too tall. I could barely get it in 5th on the freeway at 100k's. I didnt need 4th ever around town. Much nippier now but i reckon 3.08 would probably be a good middle ground too

**adam perth** · 26-08-2012, 10:55 AM

vombil, where is your 2.78 now?
i need one for my 350/T400 HQ.

**HK1837** · 27-08-2012, 08:45 AM

I used to run a 2.78:1 in my LX Torana behind an iron case Celica (0.85 5th) with 265/50/14 and 253/60/14 tyres. It was fine at freeway speeds and it was only a 202. It had a 30/70 cam, Yella terra head, 390 4BBL Holley and extractors. 5th gear was perfect at 100km/h. It was a bit lighter though at aroung 1350kg. When I put an A9X diff in it it was 3.08 and better, but even with the 2.78 I used to pull a ski-boat with it, and because of the Celica's tall 1st gear it'd easily pull the boat up slippery ramps.