my car is chewing thru petrol on HWY

[JDMJNKY]

what would be a better diff ratio for low revs cruising. ive got a ser 1 manual t18 centre
T50 ae82 4age standard ecu.

[Hen is a total nutcase]

From the top of my head you'd have a 3.9 diff ratio (depending on tyres). Mine sits at about 3500 in 5th at 110 (drove for most of a year with no speedo) with a 3.9 on 195/50 15s.

I think you may be able to find a 3.7 for a T series. But I'm not 100%.

In any case, that isn't the reason you're sucking down juice. There must be something else wrong. Mrs rallystranza's 3.9 diffed AE86 4AGE gets about 7-8 litres/100km on the freeway (I'm sure he'll correct me if I have that wrong).

Check the usual things like error codes, temp sensor and AFRs to see what's happening.

Hen