[leoblack9]

Vehicle Specs:
Isuzu dmax diesel 3.0L 5-speed manual with a 4x2 non-lsd differential. The 4jj1 engine has 150hp and peak torque around 1800~2200rpm. Great low end torque and most economical diesel truck in its class.

Vehicle Usage:
Delivery truck for work so it hits the highway once a week. City driving otherwise. Never tows. If not driving on the highway its used in the city of Manila. Heavy traffic occasionally.

Current Tires:
Factory tires were Bridgestone Dueler H/T 684II 255/60/18s (30") but i downsized it to Kumho Crugen HT51 245/70/16s (29.5") which is the smallest OEM recommended tire on the door sticker.

Ratios:
1st Gear: 4.332
2nd Gear: 2.560
3rd Gear: 1.492
4th Gear: 1.000
5th Gear: 0.791
Reverse: 3.947
Final Drive: 3.416

Putting this data in to grimjeepers' gear ratio calculator i get:

Road RPMs at given speed in kilometers per hour
.......15kph....60kph.....100kph
1.....1571......6284.......10473
2.....928........3713.......6189
3.....541........2164.......3607
4.....363........1451.......2418
5.....287........1147.......1912

I hit 15km/l or 35mpg on the instant fuel economy reading with a light foot on freeway speeds while cruising at 100km/h at around 1900rpm (but gps reading shows at 90-92km/h).

I'm thinking of upsizing a bit on my next tire change to maybe 31 or 30.5 inch tires. Maybe to get exact 100km/h reading on the gps and lessen cruising RPMs from the slightly larger tires. The Isuzu has massive low end torque so I'm sure it would scoff at the tire difference.

Following options are Otani RK1000 10ply 245/75/ R16 & Westlake SL309 6PR - 31x10.5 R15. Obviously the caveats are these are light truck tires and most likely be heavier than the Crugens I have. There are no passenger / suv tires available for the new sizes I'm aiming at--all truck or mud tires.

So should I take the risk or stay on the same oem size?