ideal RPM for a 2.5 diesel 5speed MT *100kph cruising?

**ash0279** · July 6th, 2012 12:17 AM

mods. feel free to merge where this should belong. anyways, i am planning on getting a Crosswind next year. all my driving experience come from 1.6L - 2.0L petrol engines all 5 speed MT... hence, my baseline RPM is 2.8k~3k RPM at cruising speed regardless of vehicle load.

i posted same question in Isuzu cars talk but i got same 2.8k~3k RPM answer from a 2012 Sportivo owner,, and i am not fully convinced because i believe, diesel engines have higher compression ratio hence lower RPM compared to petrol engines running at same CRUISING SPEED

**Picard** · July 6th, 2012 04:09 AM

It's true that diesel engines have lower maximum rpms compared to petrol engines. But your cruising rpm based on a specific speed and selected gear will depend on the gear ratio of the vehicle and not the type of fuel it uses.

**ash0279** · July 6th, 2012 06:04 AM

I see. so, I googled some info specific to 4JA1 gearboxes. Hope someone can derive answer of my orig question based on below supplement info:

===============
Application: ISUZU - 4JA1 ( TFR54 )
4JA1
Gear Ratio:I 5.77; II 2.903; III 1.856; IV 1.000; V 0.812; Reverse 5.556
Central Distance 88mm; Torque 230N.m; Weight 35kg
===============

**Picard** · July 6th, 2012 07:06 AM

Google speed/rpm calculator.

**niky** · July 6th, 2012 09:33 AM

Unfortunately, you can't change the gearing (not easily, at least), so whatever you do, the Crosswind will still be doing over 3000 rpm at 100 km/h.

Best economy for the Crosswind is at the lowest speed in the highest gear you can maintain without pressing harder on the pedal. Which would be around 50 km/h, but that's illegal on the expressway. Best realistic cruising speed is around 60-70 km/h, which should net you about 16-18 km/l on the NLEX and 14-16 on the hilly SLEX.

**Ry_Tower** · July 6th, 2012 10:20 AM

idela rpm for most old school diesel is 1,800-2,200rpm. Change gears in flat surfaces as early as 1,800rpm-2,000rpm.

**ash0279** · July 6th, 2012 10:12 PM

*niky & ry_tower: appreciate your inputs. i understand better now.. it would be a great paradigm shift, switching to the old-school diesel like 4ja1.. i guess win win situation nalang siguro, because i'll be needing soon a bigger car that's durable and lesser maintenance w/ lesser electronic components + the ability to carry heavy loads during long drives.