Optimizing tire size for light truck. 70/30 city/highway driving
 

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?

Stay away from LT tires (LT tires come in Load Ranges = Ply Ratings). That's the wrong way to go for rolling resistance. LT tires are designed more for load carrying capacity than fuel economy. Stick with P type tires (Standard Load/Extra Load).

Most likely a slightly taller tire won't be so bad at all, considering the low-end torque of that engine is quite good, as you won't be towing. Also considering it won't go hardcore off-road, maybe a slight increase to ground clearance which would be provided by taller tires won't be so relevant. On a sidenote, isn't there any taller rear-end which could be swapped, to allow a similar performance with smaller tires which would decrease riding height to improve aerodynamics while on highway?

thanks for chiming in CapriRacer. So LT tires are out. The only next upgrades I have are either 245/75/16 or 265/70/16. I would prefer to stay as thin as stock as possible with the slight increase in sidewall height but 245/75s are not really popular here as P-tires.. only as truck or a/t tires. Its easier to find P-tires for the 265s but would that have some penalty for going wider?

Not that I know in the local tuning scene. Knowing Thailand and its drag racing scene for the D-max I'm pretty sure there's something like this there.

Doesn't the D-Max, like many other trucks from Japanese manufacturers (yet most of the production is concentrated in Thailand and South Africa nowadays), have both low-stance and high-stance from the factory? According to the tire sizes you mentioned, sounds like yours has the high stance. So, in order to keep the engine revving the same on both low-stance and high-stance models, the differential is often taller for the low-stance to compensate for the lower tires.

Oh! I understand now what you meant. The low stance models are the crew cab units. They often have 5 lugs and have LT215/70R15 112S 6PR 6.5Jx15 ET41. I am not sure if their gears are drop-in on my unit since the low-ride & hi-ride models have different hubs, wheel offsets and may have different rear axle assembles altogether.

Do you have other options if you change the wheels? That seems like a lot of tire.

Might be worth taking a look.

stock wheels are 16x7 ET33. Other options I can go is 15x7 ET30 or similar offset. I don't want to go bigger wheels for cost and more expensive tire options--heck I think 16s are already the sweet spot.

I've asked around, cRiPpLe_rOoStEr, and the cost to upgrade or replace diff gears are more expensive than what it's worth. So i'm back here again.

So far I've expounded my tire options:

29.5"
245/70 r16s - stock size
265/70 r15s - same diemeter (0.1") than my size 245/70 r16s but less wheel and more tire. More highway terrain options but its 20mm wider.

30"
255/70 r16s - slightly wider than my 245/70 r16 but brings up the total diameter from 29.5" to 30"

30.5"
245/75 r16s - my target tire size but all options are A/T or light truck tires with 6 ply rating.
265/70 r16s - lots of highway terrain options, next practical choice. but 20mm wider. No need to change wheels.

31"
31x10.5 r15s - same problems as 245/75, expensive

The taller sidewall tends to also be good for comfort.